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Fret position calculator

Fret positions from a scale length, cumulative from the nut, with the twelfth-fret midpoint check that catches an error before you cut anything.

The question this answers: “I am building to a 648mm scale. Where does every fret slot go, measured from the nut?”

Nut to saddle. Work in whatever unit you like — the answers come back in the same one — but do not mix them.

Optional, and it moves the saddle rather than the frets. Set by ear on the finished instrument.

Result

Twelfth fret
324 from the nut
Exactly half the scale length, which is the check worth doing before cutting anything: if the twelfth is not at the midpoint, something is wrong with the arithmetic or the units.
Saddle, before compensation
648 from the nut
Compensation is added to this and depends on the strings and the action. It is set by ear rather than computed.
Fret positions from the nut
1: 36.37 · 2: 70.7 · 3: 103.1 · 4: 133.68 · 5: 162.55 · 6: 189.79 · 7: 215.51 · 8: 239.79 · 9: 262.7 · 10: 284.32 · 11: 304.73 · 12: 324 · 13: 342.18 · 14: 359.35 · 15: 375.55 · 16: 390.84 · 17: 405.27 · 18: 418.9 · 19: 431.76 · 20: 443.89
Cumulative from the nut rather than fret-to-fret, because measuring each gap separately accumulates error down the board.
Show the working
  1. The constantthe twelfth root of two, from equal temperament1.059463
  2. Fret 1648 − 648 ÷ 1.05946336.37
  3. Fret 12648 − 648 ÷ 1.059463^12324

Twelfth fret: 324 from the nut. Saddle, before compensation: 648 from the nut. Fret positions from the nut: 1: 36.37 · 2: 70.7 · 3: 103.1 · 4: 133.68 · 5: 162.55 · 6: 189.79 · 7: 215.51 · 8: 239.79 · 9: 262.7 · 10: 284.32 · 11: 304.73 · 12: 324 · 13: 342.18 · 14: 359.35 · 15: 375.55 · 16: 390.84 · 17: 405.27 · 18: 418.9 · 19: 431.76 · 20: 443.89

How it works

Each fret sits at the scale length less the scale length divided by the twelfth root of two raised to the fret number. That constant is the definition of equal temperament rather than a preference, which is why this is arithmetic rather than advice. Positions are given cumulatively from the nut because measuring gap to gap accumulates error down the board.

What it assumes

  • Equal temperament. Historical and just-intonation instruments divide the octave differently and none of this applies to them.
  • A straight fret line perpendicular to the strings. Fanned-fret instruments have a different scale length per string and want this run once per side.
  • That the scale length is measured nut to saddle rather than nut to bridge pin.

Where it stops being reliable

  • It cannot tell you what scale length to build to. That is a decision about the instrument, the strings and the player’s hands.
  • Compensation is not computed. How far the saddle moves back depends on the string gauges, the action and the setup, and it is found by ear on the finished instrument rather than derived.
  • It says nothing about fret slot depth, tang size or fingerboard radius, all of which are properties of the wire and the tooling you have.

Runs entirely in your browser — nothing you type is sent anywhere. Last reviewed 2026-08-19. More on Luthiery.