About this tool
Work out capo positions and sounding keys for guitar right in your browser. In one mode you pick an open chord shape and a capo fret to see which key you are really playing in; in the other you choose a target key and the tool shows the capo fret needed for each easy open shape, highlighting the simplest option. No data leaves your device.
The calculation uses pitch classes 0–11 for the open-shape roots: C=0, D=2, E=4, G=7 and A=9. The sounding key equals (shape pitch class + capo fret) mod 12. To go the other way, the required capo fret for a target key with a chosen shape is (target pitch class − shape pitch class + 12) mod 12, so a capo never sits above the eleventh fret.
For instance, playing E-shape chords with the capo on the third fret sounds in G. This is handy for matching a singer's range while still using familiar open-position fingerings, and for quickly finding the friendliest shape — C, A, G, E or D — for songs in awkward keys.