Where the numbers come from
Every fret divides the remaining string length by the twelfth root of two, because there are twelve equal semitones in an octave and each one multiplies frequency by that same amount. The distance from the nut to fret n is:
d = S − ( S ÷ 2^(n/12) )
where S is the scale length. Older workshops used the "rule of 18" — divide the remaining length by 17.817 each time — which is the same thing to three decimal places and is why you still see 17.817 in old jigs.
Getting it right in the wood
- Work from the nut every time. Cumulative measurement is the single most common way home-built necks end up unplayable above the twelfth fret.
- Cut on the correct side of the line. The slot position is the centre of the fret. Decide before you start whether your saw cuts on the line or beside it, and stay consistent.
- Slot depth beats slot width. Too shallow and the fret will not seat; too wide and it will not hold. Match the slot to the tang, not to the crown.
- Print at 100%. If you use this table as a paper template, check your printer is not scaling to fit. Measure a known dimension on the printout before you trust it.
Choosing a scale length
Longer scale means higher tension at the same pitch, which gives a tighter feel, more headroom before the string flops, and a brighter attack. Shorter scale means lower tension, easier bends and a warmer, quicker-decaying note. That is most of the practical difference between a Strat and a Les Paul before you plug either of them in.
If you are building for down-tuning, go long. A 25.5″ guitar tuned to drop C feels slack; a 27″ baritone at the same pitch feels like a normal guitar in standard. The string tension calculator will show you the exact numbers for your intended tuning before you commit.
Multiscale, in one paragraph
A fanned-fret neck gives the bass strings a longer scale and the treble strings a shorter one, so the low strings get tension and clarity while the high strings stay easy to bend. The perpendicular fret is the one that stays square; frets below it lean one way and frets above lean the other. Choosing fret 7 rather than fret 0 keeps the fan mild where your hand spends most of its time, which is why almost nobody picks the nut.
Before you cut anything: verify one calculated distance against a ruler you trust, and verify the twelfth fret sits at exactly half the scale length. Those two checks catch essentially every unit or entry error.