A capo is a clamp that shortens every string by the same amount, raising the whole guitar in pitch. That means the easy open chord shapes you finger above the capo no longer sound by their open names. This tool translates between the shape you play and the chord that actually rings out.
How it works
A capo on fret n raises every note by n semitones. Because all six strings move together, the chord you finger keeps its exact interval structure: the relationships between the notes are unchanged, so a major shape stays major and a minor shape stays minor. Only the root pitch shifts.
The maths is therefore a simple twelve-note rotation. Each chord root maps to a number from 0 to 11 (C = 0, C# = 1, and so on). To find the real chord you add the capo fret and wrap around 12:
real root = (shape root + capo fret) mod 12
Reverse mode does the opposite. Given the chord you want to hear and the capo position, it subtracts the capo frets to find the shape you should finger:
shape root = (target root − capo fret + 12) mod 12
The chord quality (major, minor, 7, sus, dim, etc.) is simply carried across unchanged in both directions.
Worked example
Play a G shape with the capo on fret 2. G is pitch class 7, the capo adds 2, so the real chord is pitch class 9, which is A. Strum the familiar G, C and D shapes capo 2 and you are actually playing in the key of A — perfect for matching a singer without learning barre chords.
Tips and notes
Sharps and flats name the same pitch (A# and Bb are identical here); this tool displays the sharp spelling. Open-friendly shapes worth keeping under a capo are G, C, D, A, E, Em and Am. Above fret 7 the playable neck gets cramped and tone thins, so use the high positions sparingly. Everything is computed locally in your browser.