Fret Position Calculator
Luthiers cut fret slots at precise positions so that each fret raises the pitch by one equal-tempered semitone. The position of any fret depends only on the scale length and the fret number, through the twelfth root of 2. Enter your scale length and the fret number, and this calculator returns the distance from the nut to that fret, the distance from that fret to the bridge, and the spacing to the next fret. It works in any unit you enter the scale length in (millimetres or inches), since the result is just a proportion of that length.
Fret position formula
distance from nut(n) = L * (1 - 1 / 2^(n / 12))
remaining to bridge = L - distance from nut
spacing to next fret = distance(n+1) - distance(n)
L is the scale length (nut to bridge saddle). The result is in the same unit as L. At n = 12 the distance is exactly L / 2, the octave.
Common scale lengths
- Fender-style electric guitar: 25.5 inches (647.7 mm).
- Gibson-style electric guitar: 24.75 inches (628.65 mm).
- Classical guitar: typically 650 mm.
- Electric bass (long scale): 34 inches (863.6 mm).
- Mandolin: about 13.875 inches (352.4 mm).
Fret position: frequently asked questions
How is fret position calculated?
Each fret raises the pitch by one equal-tempered semitone, so the vibrating length must shrink by a factor of the twelfth root of 2 per fret. The distance from the nut to fret n is the scale length times (1 minus 1 divided by 2 to the power n over 12). The 12th fret sits at exactly half the scale length, the octave.
What is the 18 rule of thumb and is it accurate?
Old luthiers divided the remaining length by about 17.817 (sometimes rounded to 18) to place each fret. That constant is the exact value 2 to the power 1/12 divided by (2 to the power 1/12 minus 1). Using 18 instead of 17.817 introduces a small but cumulative error, so this calculator uses the exact twelfth-root-of-2 rule.
Where is the 12th fret?
The 12th fret is the octave and falls at exactly half the scale length. On a 25.5 inch (647.7 mm) guitar scale, the 12th fret is 12.75 inches (323.85 mm) from the nut. Every fret position scales linearly with the scale length you enter.
Does this account for intonation compensation?
No. The theoretical positions assume an ideal string. Real strings have stiffness, so the bridge saddle is set slightly back to compensate, but the fret slots themselves are cut to the theoretical equal-tempered positions this calculator produces.
Sources and definitions
- Fret spacing follows directly from 12-tone equal temperament: each semitone scales the vibrating length by the twelfth root of 2. This is a standard mathematical definition.
- National Institute of Standards and Technology: SI units reference (length).
Reviewed by the CalculatorHub team, edited by James Graham, 19 June 2026. See our methodology.