The Guitar Fret Spacing Calculator gives you exact fret-slot positions for any scale length, from the nut all the way to the 24th fret. It is built for luthiers, kit assemblers, and makers cutting fretboards on a CNC, laser, or 3D printer.
How it works
Western guitars use equal temperament: every fret raises the pitch by one
semitone, a frequency ratio of 2^(1/12). Since pitch is inversely proportional
to the vibrating length, each fret shortens the string by a factor of
2^(-1/12). The distance from the nut to fret n is therefore:
d(n) = scale − scale ÷ 2^(n/12)
The traditional luthier shortcut is the rule of 18, more precisely the
constant 17.817. You divide the scale length by 17.817 to find the first
fret, then divide the remaining length by 17.817 for the next, and so on. Both
methods give the same answer because 17.817 = 1 ÷ (1 − 2^(-1/12)).
A built-in sanity check
The 12th fret is one octave up, and an octave halves the string length — so the 12th fret always lands at exactly half the scale length. If your layout puts it anywhere else, the maths is off. The calculator highlights this midpoint for reference.
Example and notes
For a 25.5 in Fender scale, the first fret sits about 1.431 in from the nut, the 12th at exactly 12.75 in, and the gaps shrink steadily as you move up the neck. Cut your slots at these centrelines, then add a touch of saddle compensation when you set up the instrument so fretted notes intonate true. Measure along the centreline of the board, and double-check your unit before cutting.