How to find the key of a song

There are four methods, and they are worth doing in this order because each one is quicker than the next and they check each other.

1. Read the key signature — if you have sheet music

The key signature narrows it to exactly two candidates: one major key and its relative minor. Three sharps means A major or F♯ minor, and nothing else.

For sharps, the last sharp is the leading tone — go up one semitone for the major key. Three sharps end on G♯, so the major key is A major.

For flats, the second-to-last flat is the major key. Four flats are B♭ E♭ A♭ D♭, so the major key is A♭ major. (For one flat, memorise it: F major.)

Then use step 2 to decide between the major key and its relative minor.

2. Look at where it ends

The last chord of a piece is the tonic far more often than not, because ending on the tonic is what makes an ending sound like one. The first chord is a weaker but still useful signal.

If the signature says "A major or F♯ minor" and the piece ends on F♯ minor, you have your answer.

3. List the chords and see which key contains them

This works with no sheet music at all, which is why it is the method most people actually need.

Write down every chord. Then find the key whose seven diatonic chords account for most of them. A song using C, F, G and Am is using four of the seven chords of C major, and no other key contains all four.

The chord that feels like home among them is the tonic. Usually it is the one the progression keeps returning to.

4. Listen for the tonic

Hum a note that sounds like a resting point against the music. Play it and hold it. If it feels settled rather than unfinished, that is very likely the tonic.

Then decide major or minor by the third above it. A major third sounds bright; a minor third sounds darker. This is the least precise method and the most useful one to develop, because it is the only one that works when you are listening rather than reading.

When the methods disagree

Disagreement is information, not failure. The usual causes:

The song is modal. A song using only the chords of C major but circling around Dm is in D Dorian, not C major. The notes belong to one key; the tonal centre is elsewhere. See modes.

It changes key. Modulation is common — a bridge or final chorus lifting to a new key. Analyse sections separately.

There are chromatic chords. A single foreign chord is usually a secondary dominant or a borrowed chord, not evidence of a different key. Look at whether it resolves back into the key you suspected.

It is genuinely ambiguous. Some music is. A progression that never commits to a tonic does not have a hidden right answer.

Major or minor, quickly

Once you have narrowed it to a relative pair:

The raised seventh is the most reliable tell in written music. A piece in A minor will keep showing G♯ as an accidental even though the signature has no sharps, because minor keys raise the seventh to build a proper dominant.