Identify the musical key of a song by selecting the chords used in the progression.
Song keys are usually decided by ear: a melody lands on a home note and everything else bends toward it. This tool takes the other route. You enter the chords of a song, and it finds the keys whose seven diatonic chords contain every chord you list. If all of your chords belong to C major's family, C major or its relative minor, A minor, is the answer. Most users are surprised to learn a key and its relative minor share the exact same chord set, so chords alone never settle the question completely. The melody's final note, not the tool, has the last word.
A concrete input and expected output from the current implementation.
Input
C - F - G - C
Expected output
Key: C major. Every chord you entered (C, F, G) is diatonic to C major, and the closing G to C is a perfect cadence (V to I). A minor, the relative minor, shares these chords but loses the tonic weight of the first and last C.
F and G rule out the neighboring keys: F major's scale contains Bb, so its chords could not include a G major triad, and G major's scale contains F#, so it could not include an F major triad. Only C major contains all three, and the closing V to I confirms the tonic.
Every key has seven diatonic triads, built by stacking thirds on each note of its scale: C major gives C, Dm, Em, F, G, Am and Bdim. The tool holds the chords you enter against each key's set and keeps the keys that contain all of them. Only the chord symbol matters, so you need no voicings or fingerings, and enharmonic spellings (C# versus Db) count as the same pitch.
Every major key shares its full chord set with its relative minor, so C major and A minor tie on content. Position breaks the tie: progressions usually begin or end on the chord that feels like home. A list starting and ending on Am reads as A minor; one starting and ending on C reads as C major. The tool follows that logic, up to the limit of chord-only analysis.
Yes. Am, F, C and G are all diatonic to both A minor and C major, and the two keys cannot be separated by chord content alone. The tool chose A minor because the progression starts and ends on Am, which is how a listener would hear it. If your melody ends on A, A minor is right; if it ends on C, C major is.
Not usually. E7 is not among C major's seven diatonic chords, so the strict match fails, but E7 is the dominant of A minor, and E7 to Am is the standard cadence into the relative minor. Such secondary dominants are common and do not change the key. Keep the key the tool returns and hear the E7 as a temporary pull toward A minor, then back.
The example and behavioral notes were checked against the browser implementation. Standards and primary references below define the relevant format, formula, or platform behavior.
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