Fret Position and Spacing Calculator

A vibrating string halved in length sounds exactly one octave higher. That single physical fact, combined with the decision to divide the octave into twelve equal steps, determines every fret position on every fretted instrument ever built.

Equal division means equal ratio, so each fret must shorten the remaining string by the same proportion: the twelfth root of two, about $1.0595$. Twelve of those steps multiply out to exactly two, which is the octave. The position of any fret measured from the nut follows:

where $L$ is the scale length and $n$ the fret number. Fret 12 gives $L - L/2$, which is exactly half the scale, on every instrument regardless of size. On a 648 mm Fender scale that is 324 mm.

The consequence you can see by looking at any guitar is that spacing shrinks going up the neck. Each fret takes its cut from what remains, and what remains gets smaller every time. On that 648 mm scale the gap from fret 1 to 2 is 34.32 mm, while the gap from fret 21 to 22 is only 11.45 mm.

Fret 1 sits 36.37 mm from the nut on that scale. That number, and every other in the table, is a theoretical position in equal temperament, which is what a fretboard is built to.

Luthiers positioned frets accurately for centuries without logarithms. The workshop method was to divide the remaining string length by a constant and mark that distance past the previous fret, then repeat.

The constant is traditionally called eighteen, which is why the method is known as the rule of 18. The true value is closer to $17.817$, and the difference matters: using a round 18 accumulates error that becomes audible in the upper register, which is exactly the complaint about some historical instruments.

The two approaches are the same mathematics. Dividing the remaining length by $17.817$ each time produces the same geometric series that the exponential formula generates in one step. The constant is simply $1 / (1 - 2^{-1/12})$, worked out once and written on the wall.

Following it by hand on a 648 mm scale: 648 divided by 17.817 gives 36.37 mm for fret 1. The remaining length is now 611.63 mm, and dividing that by the same constant gives 34.32 mm to fret 2, which lands at 70.69 mm from the nut.

The historical method is worth understanding for one practical reason. It shows why errors compound: each position depends on the one before it, so a single mismarked fret displaces everything above it. That is the argument for the approach in the next section, measuring every fret from the nut rather than from its neighbour.

Scale length is the distance from nut to bridge saddle, and it sets both the physical layout and much of how an instrument feels.

Two standards dominate electric guitar. The Fender scale of 648 mm, usually written 25.5 inches, and the Gibson scale of 628 mm or 24.75 inches. The 20 mm difference sounds trivial and is not. At the same tuning and string gauge, the longer scale requires more tension to reach pitch, which makes strings feel stiffer, bend less easily and produce a brighter, more defined attack. The shorter scale feels slinkier and sounds warmer.

Fret spacing scales with it proportionally. On the Gibson scale fret 1 sits at 35.24 mm rather than 36.37 mm, and the whole board is tighter, which players with smaller hands often prefer. Classical guitars run longer still, around 650 mm, with fret 19 at 433.9 mm from the nut.

Basses extend the same logic much further, with 34 inch scales standard and short-scale instruments at 30 inches feeling dramatically different under the hand.

Multiscale or fanned instruments apply two scale lengths at once, longer on the bass side for tension and definition, shorter on the treble side for comfort. Each string effectively gets its own scale, so the fret table has to be calculated per string and the frets end up angled.

Choosing a scale is mostly about tension and reach rather than tone. Working out the string tension a given scale and gauge produces is the companion calculation, and a string tension reference makes that concrete.

The single most important habit when marking a board is to measure every fret from the nut, never from the fret before it.

Fret-to-fret measurement stacks error. A tenth of a millimetre of drift at each of twenty-two frets can accumulate to two millimetres by the top of the neck, which is enough to be audibly out of tune. Measuring cumulative distances from one fixed reference means each mark carries only its own error, and mistakes stay local instead of propagating.

That is why the table gives distance from the nut as its primary column, with fret-to-fret spacing beside it for checking rather than for laying out. Use the cumulative figures to mark, then verify a few gaps against the spacing column to catch a transcription error.

The reference point needs care. Distance is measured from the face of the nut where the string leaves it, not from the edge of the slot or the end of the fretboard. A millimetre of ambiguity there shifts every fret on the instrument.

Precision expectations are worth setting honestly. Aim for a tenth of a millimetre on the marks, which is achievable with a good rule and a sharp knife, and remember that fret slots have width: the string contacts the crown of the wire, so the slot should be cut so the crown centre lands on the mark.

One thing the table deliberately does not calculate is saddle compensation. Pressing a string down stretches it slightly sharp, so the bridge saddle sits 1 mm to 3 mm beyond the theoretical scale length. Intonation is set there, at the bridge, after the frets are in.

Measuring from the wrong part of the nut heads the list. The scale length starts at the point where the string leaves the nut face. Using the fretboard end, the edge of the nut slot or the headstock joint shifts everything, and the error is uniform enough that the instrument seems merely out of tune rather than mismeasured.

Stacking fret-to-fret measurements is the mistake that ruins otherwise careful work. It feels natural, because the spacing column is right there, and it accumulates drift that only becomes obvious in the upper register when the instrument is finished.

Forgetting saddle compensation causes a specific and very common confusion. Someone measures the distance from the nut to the twelfth fret, then from the twelfth fret to the saddle, finds the second figure is a millimetre or two longer, and concludes the guitar is built wrong. It is not: the extra length is deliberate, and it exists so a fretted note plays in tune.

Using a round 18 rather than 17.817 for the historical method introduces cumulative sharpness up the neck. If you are following the traditional shortcut, use the accurate constant.

Assuming an existing guitar matches its nominal scale is worth checking too. Measure nut to twelfth fret and double it before trusting a spec sheet, because production tolerances and refretted necks both drift. If the doubled figure disagrees with the nominal scale, the table for the measured scale is the one to use.