Guitar Outline Formula

by Leo Bidne

Originally published in Guild of American Luthiers Data Sheet #79, 1978



From taking several measurements of various guitars, I’ve discovered only slight differences from one to the next. These differences seem to contribute very little to the finished sound. The question is, what is the reason for making a guitar the shape that it is? What are the determining factors, besides the obvious ones? Can there be a “formula” that produces an “ideal” outline? This is what I set out to find.

What I came up with, rather independently, so closely resembles, at least in dimension, the existing standard outline of the classic guitar as introduced by Torres, that I wonder if a similar technique may have been in use in the past, in some modified form. This formula, based on the string length, may be of no use to acoustical science, but it’s as fun as a math game, and twice as surprising. Although the information below describes how to derive an outline for a classic guitar with a string length of 65CM, joining the body at the 12th fret, it can also be used, with slight modifications, for other instruments, such as the flattop, the acoustic bass, the electric guitar, etc. Once the outline is established, the depth of the sound cavity can be adjusted with the sides, helping to establish the instruments sonority, as well as adjusting the soundboard bracing.