THE FRET LAYOUT GUIDE
One scale.
Thirty-six possible checks.
A useful fret table starts with a defined reference, a stated musical model and a clear distinction between arithmetic and a finished instrument.
Define the length before calculating
Enter a theoretical, uncompensated scale length. Coordinate zero is the nut’s ideal vibrating take-off reference. A distance in the table is measured along that scale from zero to the centreline of a theoretical fret. It is not measured from the back of a nut, the edge of a headstock or the end of an untrimmed fingerboard blank. Those physical features can have different locations.
The calculator has no instrument database. It does not guess a manufacturer’s nominal scale from a model name, copy the measured position of an existing bridge or adjust for a compensated nut. If you are studying an existing instrument, define what your measurement represents before using it as an input. The numbers remain a mathematical layout for the length you actually entered.
Why the spaces shrink
In twelve-tone equal temperament, an octave contains twelve equal frequency ratios. Each semitone multiplies frequency by the twelfth root of two, approximately 1.059463. For an ideal flexible string with constant wave speed, fundamental frequency is inversely proportional to vibrating length. A higher note therefore uses a shorter ideal length.
The UNSW Music Acoustics FAQ explains the equal semitone ratios. Its companion page on strings, standing waves and harmonics gives the ideal string relationship and explains several real-string complications. Combining those two relationships gives the model used here:
remaining length = L × 2−n/12
distance from nut = L × (1 − 2−n/12)
Here L is the entered scale and n is the fret number. The gap before fret n is its from-nut distance minus the preceding from-nut distance. Fret zero is the nut reference at zero. The calculator evaluates the exponential directly from L for every row. It does not repeatedly divide a progressively rounded length by 18 or a shortened decimal fret factor.
A 650 mm scale, worked through
The octave anchors
At fret 12, 2−12/12 is one half. The remaining ideal length is 325 mm, and the distance from the nut reference is also 325 mm.
At fret 24, the remaining length is one quarter of 650 mm: 162.5 mm. Its from-nut position is 650 − 162.5 = 487.5 mm.
At fret 36, one eighth remains: 81.25 mm. Its from-nut position is 568.75 mm. Those three anchors are useful arithmetic checks; they are not bridge compensation instructions.
The first fret on this same scale is approximately 36.4817 mm from the nut reference. The second is approximately 70.9158 mm from the reference, so the second gap is about 34.4341 mm. The gap is already smaller than the first. That pattern continues: positions increase toward the theoretical end while the distance between adjacent frets decreases.
For marking a layout, use the from-nut column as the common reference and treat the previous-gap column as a cross-check. Adding many rounded gaps can create cumulative error. Stewart-MacDonald’s fret-scale ruler instructions discuss the even-tempered spacing factor and practical problems caused by accumulated layout error. The calculation here avoids that numerical accumulation, but it cannot remove error in measurement, marking, printing or cutting.
An inch example without a hidden scale change
Enter 25.5 with inches selected. One inch is exactly 25.4 mm, so the entered length represents 647.7 mm. The theoretical 12th-fret position is 12.75 inches, or 323.85 mm. The 24th-fret position is 19.125 inches, or 485.775 mm. Selecting another unit changes the display rather than replacing the physical length with a rounded version.
A direct edit has a different meaning. If you switch from 650 mm to inches and then type 25.5, you have entered a new length of 647.7 mm. Fretmark keeps an internal physical value during unit changes, but an intentional new entry replaces that value. Exported input details help make the distinction visible.
Choose the rows you need
The interface accepts a whole number from 1 through 36. That is a practical table limit for this tool, not a statement that an instrument must have any particular fret count. With fewer than twelve frets selected, the 12th-fret summary is marked outside the selected table. It does not silently add another row to your requested layout.
Positive scale lengths up to 10,000 mm are supported. A zero length, a negative length, a missing number, a fractional fret count or a number outside the supported range is rejected. The old result is cleared, exports become unavailable and the relevant field is marked. Restore the example to return to a known 650 mm, 24-fret state. Tiny positive numbers that cannot produce distinct finite positions are also rejected rather than shown as a convincing but collapsed layout.
What each export contains
The text report and CSV contain the original theoretical scale in the selected unit, the number of rows and each calculated position, preceding gap and ideal remaining length. Their calculation values are exported without the table’s display rounding. That does not make the physical inputs more accurate; it simply prevents the presentation format from becoming an extra arithmetic error.
The on-page diagram is a proportional preview. It has no physical size guarantee on a screen. The SVG strip is different: its coordinates and declared dimensions are in millimetres, and it includes a labelled reference bar. The strip has no fingerboard outline, kerf, slot depth or toolpath. It is a one-dimensional set of theoretical centrelines with a reference origin.
Opening an SVG in a design app, placing it in another document or selecting a printer’s fit-to-page option can change its scale. Check the labelled bar and several known positions with an appropriate physical reference before relying on a print. A declared SVG width does not prove a printer produced that width. Extremely short entered scales can crowd the lines and labels; the numeric table remains the primary reference.
The limits matter to the result
Real strings have bending stiffness and are stretched when pressed to a fingerboard. String construction, action, fretting force and setup can affect intonation. The UNSW explanation describes why a real instrument’s bridge-to-12th-fret distance can exceed its nut-to-12th-fret distance. Fretmark does not calculate those corrections and does not recommend where to install a saddle or bridge.
The tool covers a single straight scale with twelve-tone equal temperament. It does not produce a multiscale or fan-fret layout, a diatonic pattern, microtonal frets, partial frets, a tension estimate or a string-gauge recommendation. It also cannot determine whether an instrument blank, neck joint or existing fretboard is suitable for the layout. Those are separate design and setup questions.
Use the result as transparent geometric arithmetic with stated assumptions. A mathematically correct table is useful for learning, checking and preparing a reference; it is not a certificate that a built instrument will play in tune. The calculator remains free, and all entries and generated exports stay in your browser unless you choose to share a file or email it yourself.