How to Determine Fretboard Radius

Fretboard radius describes the transverse curvature of the guitar’s playing surface. A smaller numerical radius means a more pronounced curve; a larger radius means a flatter surface.

For bridge setup, the published fretboard radius is only the starting point. The effective fret-top geometry, a possible compound radius, the intended string gauges and the bridge’s own saddle architecture determine the final string-radius requirement.

Radius Fundamentals

Why Fretboard Radius Matters

Fretboard radius influences:

• String-height relationships
• Bridge saddle setup
• Bending clearance
• Chording feel
• Available action
• Replacement bridge selection
• Nut and saddle geometry

The strings should follow a transverse curve that works with the effective playing surface of the neck.

When the bridge string radius is too curved:

• The middle strings sit too high relative to the outer strings.
• The outer strings may reach their lowest usable action before the middle strings.
• The action can feel unnecessarily uneven.

When the bridge string radius is too flat:

• The middle strings sit too low relative to the outer strings.
• The middle strings may buzz or choke first.
• The outer strings may remain unnecessarily high.

Radius is one part of the setup.

Neck relief, fret condition, individual action and playing style remain equally important.

Four Different Radius Measurements

The word radius can refer to several different parts of the guitar.

These measurements should not be treated as automatically identical.

Fretboard Radius

The curvature of the wooden fingerboard surface.

This is normally the radius stated in the guitar manufacturer's specifications.

Fret-Top Radius

The curve formed across the crowns of the installed frets.

This is the effective transverse playing surface beneath the strings.

It may differ slightly from the nominal wooden-board radius because of:

• Fret height
• Fret leveling
• Uneven fret wear
• Previous refret work
• Incorrect fret installation
• Deliberate fretwork

Saddle Radius

The curve formed by the heights or contact geometry of the bridge saddles.

Depending on the manufacturer, a published saddle radius may describe:

• Saddle crown positions
• Nominal string-contact positions
• An unnotched saddle arrangement
• A radius intended for a particular fretboard range

Always check how the manufacturer defines the value.

String Radius

The curve formed across the installed strings at a specific point.

For setup purposes, the relevant measurement is generally the curve along the undersides of the strings because action is measured from:

• Top of fret
• To underside of string

The radius across the string tops is affected by the different string diameters and is not the same measurement.

Nominal Radius vs. Actual Playing Geometry

A manufacturer's published radius is an important starting point, but it does not confirm the complete current setup.

The effective geometry may have changed through:

• Fret leveling
• Refretting
• Saddle replacement
• Saddle-slot filing
• Installed shims
• Bridge replacement
• Uneven wear
• Previous setup work

For a bridge setup, the actual fret-top and under-string geometry matter more than an assumed model-family specification.

Common Radius Conversions

To convert inches to millimeters:

Millimeters = inches × 25.4

To convert millimeters to inches:

Inches = millimeters ÷ 25.4

Current Manufacturer Examples

Current production instruments demonstrate the wide range of available specifications.

Examples include:

• Gibson models with a 12-inch fingerboard radius
• Fender models with 7.25-inch, 9.5-inch, 12-inch or compound-radius fingerboards
• Ibanez models with a 400 mmR fingerboard
• Charvel models with a 12-to-16-inch compound radius

These are product examples, not universal brand standards.

Manufacturers may use several radii across:

• Different models
• Production years
• Series
• Factories
• Signature instruments

Always check the exact instrument.

Measuring the Neck

Start with the Official Specification

Before measuring, look for:

• Official product page
• Technical drawing
• Owner's manual
• Archived manufacturer specification
• Exact model designation
• Production year
• Serial-number information where relevant

Third-party listings can contain copied or generalized specifications.

Use them only when an authoritative source is unavailable.

Required Tools

Useful tools include:

• Radius gauge set
• String radius gauges
• Notched radius gauges
• Accurate straightedge
• String-action gauge
• Feeler gauges
• Digital caliper
• Good lighting
• Magnification where necessary

A digital caliper does not measure a curved surface directly, but it can be used for chord-and-depth calculations.

Measuring the Fretboard or the Frets

First decide which surface must be checked.

To Confirm the Wooden Fretboard Radius

The gauge must contact the fingerboard surface itself.

This may require:

• Strings removed
• A notched gauge that clears the frets
• Measurement between suitable frets
• An unfinished or unfretted fingerboard

Bindings, rolled edges and inlays can interfere with full-width contact.

To Check the Effective Playing Surface

Place the gauge across the fret crowns.

This measures the fret-top radius rather than the bare fingerboard.

For bridge setup, this can be more useful because the strings play over the frets, not directly over the wood.

Measuring with a Radius Gauge

With the strings removed or moved aside:

1. Select a likely gauge.
2. Place it across the surface without pressing.
3. Hold it perpendicular to the neck centerline.
4. Observe the contact points and light gaps.
5. Try the next smaller and larger radii.
6. Identify the closest stable fit.
7. Repeat the measurement at several fret positions.

Do not force a gauge against the frets or board.

Pressure can hide the gaps that identify an incorrect radius.

Reading the Gauge

When the gauge is too curved for the surface:

• It tends to contact near the center.
• Gaps remain toward the outer areas.

When the gauge is too flat:

• It tends to contact toward the outer areas.
• A gap remains near the center.

A correctly matched gauge follows the usable surface without rocking or showing a systematic center or edge gap.

Small localized gaps may indicate:

• Uneven fret height
• Fret wear
• A damaged fret
• Irregular fretboard work

Do not interpret one isolated high or low fret as the radius of the entire neck.

Measure at Several Positions

Check the radius at positions such as:

• Near the first fret
• Around the seventh to ninth fret
• Around the 12th fret
• Near the highest frets

This helps distinguish:

• Constant radius
• Compound radius
• Irregular fretwork
• Local wear
• Previous leveling errors

A single measurement near the nut cannot identify the complete geometry of a compound-radius neck.

Fretboard Radius vs. Neck Profile

Do not confuse fretboard radius with neck profile.

Fretboard radius describes the curvature across the playing surface.

Neck profile describes the shape of the rear of the neck, such as:

• C
• D
• U
• V
• Asymmetrical

The two specifications are independent.

Compound Radius

What Is a Compound Radius?

A compound-radius fingerboard becomes progressively flatter from the nut toward the body.

For example, a neck may change from:

• 9.5 inches near the nut
• To 14 inches near the highest frets

or:

• 12 inches near the nut
• To 16 inches near the highest frets

The smaller starting radius provides greater curvature in the lower-fret area.

The larger ending radius provides a flatter upper-fret surface.

A correctly produced progressive radius forms a conical or cone-like playing surface rather than a simple cylinder.

Constant Radius vs. Compound Radius

Constant Radius

The nominal transverse radius remains the same along the fingerboard.

Examples:

• 9.5 inches along the complete board
• 12 inches along the complete board
• 400 mmR along the complete board

Compound Radius

The transverse radius changes gradually along the board.

Examples:

• 9.5 to 14 inches
• 10 to 16 inches
• 12 to 16 inches

Do not assume that every changing-radius neck follows one universal progression.

The rate of change depends on:

• Nut width
• End-of-board width
• String spread
• Scale length
• Manufacturer design
• Specified starting and ending radii

Why Compound Radius Affects Bridge Setup

The strings continue beyond the last fret toward the wider bridge spacing.

On a correctly projected compound geometry, the required string radius at the bridge can therefore be flatter than the radius measured at the final fret.

Do not automatically set the bridge to:

• The nut radius
• The average of the two published radii
• The radius at the 12th fret
• The terminal fretboard radius

Use the bridge-radius recommendation for the exact neck where available.

For example, one documented compound fingerboard changes from:

• 10 inches at the nut
• 13 inches at the 12th fret
• 14.5 inches at the 22nd fret

The specified bridge-saddle starting point for that geometry is 16 inches.

This is a product-specific example, not a universal formula.

Compound Radius Is Not Identified by Two Measurements Alone

A radius that differs between two positions may indicate a compound board, but it can also result from:

• Uneven fret leveling
• Inconsistent fret height
• Refret work
• Fret wear
• Measurement error
• A twisted neck

A proper compound radius should change progressively and consistently.

When the measurements change irregularly, inspect the fret plane rather than assuming that the neck was intentionally built that way.

Calculating Radius from Width and Center Depth

When no suitable radius gauge is available, a circular radius can be calculated from:

• Chord width
• Center depth, also called sagitta

Formula:

R = c² / (8s) + s / 2

Where:

• R = radius
• c = measured chord width
• s = center depth between the arc and the straight chord

This method requires highly accurate measurements.

The center depth is small, so a minor measurement error can produce a large radius error.

It is best suited to:

• Unfretted boards
• Machining checks
• Technical inspection
• Precisely supported parts

For routine guitar setup, a suitable radius gauge is normally more practical.

Setting String Radius

Measuring String Radius at the Bridge

Use a string radius gauge near the bridge.

The gauge should contact the undersides of the strings, not their tops.

Procedure:

1. Install the intended strings.
2. Tune the guitar.
3. Establish neck relief.
4. Set the outer-string action.
5. Place the correct gauge beneath the strings near the saddles.
6. Raise it gently until it reaches the two outer strings.
7. Check the inner strings.
8. Adjust the inner saddle heights where the bridge permits.
9. Retune and recheck.
10. Complete a playing test.

The gauge should be held:

• Perpendicular to the string direction
• Close to the bridge
• Without pushing the strings upward unnecessarily

Why Under-String Gauges Are Preferred

The guitar strings have different diameters.

A radius measured across their top surfaces includes those diameter differences.

For example:

• The top of a thick wound string sits farther above its underside.
• The top of a thin plain string sits much closer to its underside.

An under-string gauge removes the string diameters from the comparison and checks the curve that is directly related to action above the frets.

This distinction is especially important when comparing:

• Heavy bass strings
• Thin treble strings
• Mixed plain and wound strings
• Extended-range string sets

Over-String Radius Gauges

A gauge placed over the string tops measures a different geometric surface.

It may be useful for specific inspection methods, but its reading must account for:

• Individual string diameters
• Wound and plain construction
• Gauge-set composition

Do not assume that the string tops should form the same numerical radius as the fretboard.

For normal saddle-height setup, the under-string measurement is more directly relevant.

Set the Outer Strings First

A practical sequence for individually adjustable saddles is:

1. Set the low outer string to the required action.
2. Set the high outer string to the required action.
3. Place the selected radius gauge beneath the strings.
4. Let the gauge reference the two outer strings.
5. Adjust the inner strings to follow the curve.
6. Retune.
7. Measure the action of each string.
8. Make final functional corrections.

The bass and treble outer strings do not need to have identical action.

The radius gauge can be held at the slight angle created by the different bass- and treble-side heights.

Radius Is Not the Same as Action

Radius describes the transverse relationship between the strings.

Action describes the vertical clearance between each string and the fret tops.

A guitar can have:

• Correct radius with action that is too high
• Correct radius with action that is too low
• Incorrect radius with acceptable outer-string action
• Correct nominal geometry but poor playability because of uneven frets

Set and evaluate both separately.

Radius Is a Setup Starting Point

A radius gauge creates controlled geometry, but it does not replace individual action measurements.

After matching the basic curve:

• Measure each string's action.
• Play every string across the neck.
• Test bends.
• Check for buzz and choking.
• Adjust individual strings where the bridge permits.

Small functional deviations may be appropriate because:

• Bass strings vibrate through a wider path.
• Players attack different strings with different force.
• Fret condition may not be perfectly uniform.
• The player may prefer unequal bass and treble action.

Do not preserve a mathematically perfect radius at the expense of actual playability.

Measuring Action After Radius

Action should normally be measured from:

• Top of fret
• To underside of string

Use the same reference fret for all strings.

Common setup references include the 12th or 17th fret, depending on the manufacturer and procedure.

Follow the specification for the actual instrument rather than combining values from different setup systems.

Bending and Radius

A strongly curved fingerboard can require greater string clearance for wide bends, particularly in the upper frets.

During a bend, the string moves sideways toward the higher central area of the curved fret surface.

With very low action, it may contact a higher fret and choke.

A flatter upper-fret radius generally provides more bending clearance, provided that:

• Frets are level
• Relief is suitable
• Action is appropriate
• The neck is structurally sound

Radius alone does not guarantee low action or clean bends.

Check Bends After Setting Radius

After the basic setup:

1. Tune the guitar.
2. Play every string across the neck.
3. Test bends in the upper register.
4. Use the player's normal bend width.
5. Listen for buzzing, fading or choking.
6. Recheck individual string action.
7. Recheck intonation after significant height changes.

Do not assume that choking always requires a different bridge radius.

It may also be caused by:

• High fret
• Uneven fret leveling
• Insufficient action
• Incorrect relief
• Neck irregularity

Bridge-Specific Radius Adjustment

Setting Radius on Individually Adjustable Saddles

Bridges with individual saddle-height screws allow the string radius to be set directly.

Typical examples include:

• Stratocaster-style bridges
• Many hardtail bridges
• Modern individual-saddle systems
• Some Telecaster-style bridges

Procedure:

1. Complete the neck-relief adjustment.
2. Set the action of the two outer strings.
3. Select the required under-string gauge.
4. Position the gauge near the saddles.
5. Adjust the inner saddles to follow the gauge.
6. Keep each saddle mechanically stable.
7. Retune after adjustment.
8. Measure the individual action.
9. Test the instrument.
10. Set intonation last.

Where a saddle uses two height screws, both screws should remain in stable contact unless the bridge design specifies otherwise.

Avoid:

• Severely tilted saddles
• Loose height screws
• Insufficient screw engagement
• Saddles resting on adjacent saddles
• Uncontrolled changes to the intended string spacing

Setting Radius on a Tune-O-Matic Bridge

Most Tune-O-Matic bridges do not provide individual saddle-height screws.

Their basic radius is determined by:

• Bridge-body geometry
• Relative saddle heights
• Saddle crown geometry
• Saddle order
• String-slot depth
• String position on the saddles

The two bridge posts adjust overall bass- and treble-side height.

They do not independently change the radius across the six saddles.

Nominal Tune-O-Matic Radius

A Tune-O-Matic bridge may be advertised with a nominal radius such as:

• 12 inches
• 14 inches
• A usable radius range

This value must be interpreted carefully.

The actual string radius can be changed by:

• Saddle notching
• Saddle-slot depth
• Lateral string position
• Replacement saddles
• Saddle wear
• Previous filing

Schaller, for example, specifies that the effective string radius of its GTM can vary from approximately 12 to 14 inches according to where the saddles are notched.

This is specific to that bridge geometry.

It should not be generalized to every Tune-O-Matic bridge.

Conventional Saddle Slots

On a conventional saddle, the string slot establishes:

• Lateral string position
• Contact depth
• Contact edge
• Part of the final string height

An excessively deep slot lowers that string and changes the string radius.

Before modifying a saddle, confirm:

• Intended string spacing
• Fretboard-edge clearance
• Existing radius
• Required string gauge
• Contact-point direction
• Available saddle material

Deepening a slot is a permanent reduction.

Material cannot be restored by reversing the file.

Do Not Create Radius by Casual Filing

Saddle slots should not be deepened merely because one string appears high against a gauge.

The apparent error may instead be caused by:

• Wrong saddle order
• Incorrect replacement saddle
• Bridge-body deformation
• Fret problem
• Wrong gauge selection
• Compound-radius projection
• Incorrect outer-string action
• Previous work on another saddle

When a fixed-radius bridge is unsuitable, replacing the saddles or bridge may be safer than filing several contact points deeply.

KMS Radius Systems

KMS V-Saddles

KMS V-saddles center the string through their geometric form rather than through individually filed conventional slots.

This supports:

• Defined centering
• Broad gauge compatibility
• Gauge changes without conventional reslotting
• Controlled contact geometry
• Preservation of the saddle surface

Different string diameters seat according to the V geometry.

The installed string set therefore contributes to the final string-height relationship.

Use the intended string gauges during the final setup.

Do not file conventional string slots into KMS V-saddles.

KMS ONE and Vintage ONE

When selecting or setting up a KMS ONE or Vintage ONE bridge, check:

• Guitar fretboard specification
• Effective fret-top radius
• Installed string gauges
• Actual string radius
• Outer-string action
• Saddle version
• Existing fret and setup condition

A nominal 12-inch Gibson-style specification is a starting point.

The actual guitar may differ because of fretwork, previous saddle work or a compound/custom neck.

The V-saddle system should be evaluated with the bridge installed and the intended strings at tension.

KMS FlowTrem2 Saddles

The FlowTrem2 uses application-specific V-saddle geometry rather than conventionally filed string slots.

For compatibility and setup, confirm:

• Exact neck radius
• Whether the neck has a constant or compound radius
• Radius required at the bridge position
• Intended string gauges
• Six- or seven-string configuration
• Existing saddle arrangement
• Current FlowTrem2 specification

The bridge should remain mechanically level according to its setup instructions.

Do not use unequal post depth as a substitute for the intended saddle-radius configuration.

Double-Locking Tremolo Radius

Double-locking tremolos may create their radius through:

• Different saddle heights
• Saddle groups
• Factory-installed shims
• Individual saddle shims
• Radius-specific saddle sets

Do not assume that all saddles are identical.

Before changing the radius, document:

• Saddle order
• Existing shim positions
• Shim thickness
• Bridge model
• Manufacturer specification

Floyd Rose Original Radius

Floyd Rose specifications vary according to the exact product and configuration.

Documented current examples include:

• 10-inch bridge radius with a factory-installed center shim
• 12-inch radius after removal of that shim
• Models supplied with a 12-inch radius
• Radius-specific saddle sets in 8, 10, 12, 14, 16 and 17 inches

The exact current product specification must therefore be checked before modifying the saddle arrangement.

Do not describe every Floyd Rose bridge as having one universal factory radius.

Floyd Rose Saddle Shims

Floyd Rose documents saddle shimming as a method of adapting the bridge radius.

A shim raises the selected saddle.

Before adding or removing shims, check:

• Exact tremolo model
• Existing factory shim
• Saddle order
• Required final radius
• Saddle screw engagement
• Saddle stability
• Intonation range

Use purpose-made components or the manufacturer's approved method.

Do not stack uncontrolled material beneath a saddle without confirming that it remains fully supported and securely clamped.

Radius-Specific Saddle Sets

Where available, a radius-specific saddle set can establish the intended geometry without individual improvised shims.

This can provide:

• Defined saddle heights
• Consistent contact
• Repeatable assembly
• Cleaner serviceability

The set must still match:

• Exact tremolo family
• String count
• Baseplate
• Saddle-mounting system
• Required string spacing

Fixed-Radius Bridges

Some bridges have no practical individual radius adjustment.

Examples can include:

• Fixed-radius Mustang-style bridges
• Certain wraparound bridges
• Roller bridges
• Preconfigured Tune-O-Matic bridges
• Acoustic saddles

Before selecting one, verify:

• Published saddle or string radius
• Manufacturer definition of that radius
• Fretboard geometry
• Bridge-position radius on a compound neck
• Available setup tolerance
• Whether professional reshaping is possible

Mounting compatibility does not guarantee radius compatibility.

Related Geometry and Diagnostics

String Gauge and Radius

String diameter affects radius measurements differently depending on the method.

Under-String Measurement

Different string diameters are largely removed from the comparison because the gauge contacts the string undersides.

Over-String Measurement

Different diameters directly affect the measured top surface.

Conventional Filed Saddles

Slot depth can change the underside height of an individual string.

V-Shaped Saddles

Different diameters can seat at different depths within the V geometry.

Always perform the final assessment with the intended string set installed.

Nut Radius and Bridge Radius

Nut geometry and bridge geometry serve different positions along the string path.

On a constant-radius neck, they may be based on the same nominal board radius.

On a compound-radius neck:

• The nut follows the more curved starting geometry.
• The bridge follows the flatter projected geometry.

Do not use the locking-nut radius as the required bridge radius without checking the complete neck design.

Pickup Pole-Piece Radius

Pickup pole pieces may form a visible arc.

That arc does not define the correct bridge string radius.

Pickup adjustment concerns:

• Magnetic balance
• Output
• String-to-string response
• Magnetic pull

Bridge radius concerns:

• Action
• Fret clearance
• Bending
• Playing geometry

Set the bridge according to the neck and frets.

Then adjust the pickups to the resulting string positions.

Verification and Replacement

Common Radius Mistakes

Avoid:

• Assuming one radius for an entire guitar brand
• Measuring only at the nut
• Confusing board radius with fret-top radius
• Confusing saddle radius with actual string radius
• Measuring string tops as though string diameters were equal
• Pressing a gauge against the surface
• Ignoring compound-radius projection
• Averaging two compound-radius values
• Setting the bridge to the nut radius on a compound neck
• Adjusting radius before outer-string action
• Filing Tune-O-Matic saddles without diagnosis
• Filing conventional slots into KMS V-saddles
• Changing Floyd Rose shims without documenting the factory setup
• Trying to correct uneven frets at the bridge
• Selecting a replacement bridge by mounting dimensions alone

Before Selecting a Replacement Bridge

Record:

• Guitar manufacturer
• Exact model
• Approximate production year
• Published fingerboard radius
• Actual fret-top radius where relevant
• Constant or compound construction
• Radius required at the bridge
• Existing bridge type
• Saddle system
• String spacing
• Intended string gauges
• Required bridge height
• Available radius adjustment
• Clear photographs

A replacement bridge must satisfy both:

• Mechanical mounting compatibility
• Playing-surface compatibility

Practical Measurement and Setup Sequence

1. Identify the exact guitar and published specification.
2. Determine whether the neck is constant or compound radius.
3. Inspect fret condition.
4. Measure across the fret tops at several positions.
5. Establish the required bridge-radius starting point.
6. Install the intended strings.
7. Set neck relief.
8. Set bass- and treble-side outer-string action.
9. Check the string radius near the bridge.
10. Adjust individual saddles or approved shims where possible.
11. Measure the action of every string.
12. Test bends and fret clearance.
13. Set intonation.
14. Perform a complete playing test.

When Professional Assistance Is Recommended

Professional setup is advisable when:

• Measurements change irregularly along the neck
• Compound-radius geometry is unknown
• Frets are uneven
• Saddles require permanent filing
• A Tune-O-Matic bridge appears to have the wrong radius
• A tremolo requires shimming
• Bends choke despite reasonable action
• The bridge cannot provide the required geometry
• The guitar has been refretted or modified
• The cause of the mismatch is unclear

Related Guides

• Tune-O-Matic Installation & Setup →
• How to Set Guitar Bridge Intonation →
• How to Measure Guitar Bridge Post Spacing →
• ABR-1 vs. Nashville Bridge →
• How to Choose the Correct Tremolo Block Height →