Quick answer: the key of a song is the chord (and note) that feels like home. To find it, look at the first and last chords, check which key contains all of the chords, and listen for a five chord falling back to home. If the chords are C, G, Am and F and the song starts and ends on C, the key is C major. If you have the recording, or you already know the chords, our free key detector does all of this for you: drop the song in, or type the chords, and it tells you the key and how sure it is.
What “the key” means
A key is the set of seven notes a song is built around, and the chord that sounds like home is its tonic. In the key of C major those notes are C D E F G A B, the white keys of a piano, and the home chord is C. Every other chord in the song is measured from it: that is what Roman numerals like I, IV and V mean.
Finding the key is useful because it tells you which chords will fit, lets you transpose the song to a comfortable range and shows you which scale to solo with.
Way 1: look at the first and last chords
Most songs begin and end on the tonic, or very near it. Write down the chords of the song and look at the ends:
- C G Am F … C → likely C major.
- Am F C G … Am → likely A minor.
It is not a rule: songs that fade out, start on a different chord or end on a question mark break it. Treat it as the first clue, then confirm it with the next ways.
Way 2: find the one key that contains all your chords
List the distinct chords and look for the key whose seven chords include every one of them. This is where the circle of fifths helps: a key is seven neighbours on the circle, and its chords sit together in one frame. Try it with the interactive circle of fifths.
For C, G, Am and F:
| Key | Does it contain C, G, Am and F? |
|---|---|
| C major (C Dm Em F G Am B°) | Yes, all four |
| G major (G Am Bm C D Em F♯°) | No, F is missing |
| F major (F Gm Am B♭ C Dm E°) | No, it has Gm, not G |
One key contains all four: C major. If one chord does not fit, it is usually a borrowed chord, not a new key. A B♭ in a C major song, for example, is borrowed from F major or from C minor.
Way 3: listen for the five chord falling home
The strongest sign of a key is a V → I move: the chord a fifth above home (the five chord, often with a seventh) resolving to home. G → C settles C major. D7 → G settles G major. If you hear a chord that wants to fall to another one, the second is probably home.
Way 4: find home by ear
No chords? Play or hum the song and look for the note that sounds like the end: the one you could stop on and feel finished. Then:
- Find that note on your instrument. It is the tonic.
- Play a major chord and a minor chord on it, with the song in your head. Whichever one sounds right tells you the key is major or minor.
- Check the chords you hear against the key’s seven chords.
The last note of the melody is very often the tonic, which makes it a good place to start.
The relative minor trap
Every major key shares its seven chords with a minor key three half steps lower: C major and A minor both use C, Dm, Em, F, G, Am and B°. So the same four chords can be two different keys:
- C G Am F starts on C: C major.
- Am F C G starts on Am: A minor.
When the chords alone do not decide, where the progression starts and where it rests do. That is exactly what our key detector weighs when you give it chords, and why it calls close ones “close calls” instead of guessing. From a recording it can only hear which notes ring most, so it also shows the relative key.
Finding the key from sheet music: the key signature
If you have a score, the key signature (the sharps or flats at the start of the staff) narrows it to two keys, a major and its relative minor.
- Sharps: the last sharp is a half step below the major key. Sharps F♯ C♯ G♯ → the last is G♯ → A major.
- Flats: the second-to-last flat is the major key. Flats B♭ E♭ A♭ → E♭ major. One flat is the exception: F major.
- No sharps or flats: C major or A minor.
Then look at the last note and the first chord to choose between the major key and its relative minor. The circle of fifths chart lists every key signature, and you can print it.
What if some chords do not fit?
Almost every song borrows a chord or two from a neighbouring key. A chord outside the key is not a mistake: it is colour. If most of the chords fit one key and one or two do not, the key is the one that fits most, and the rest are borrowed. Given chords, the key detector marks them “outside” and tells you which key each one comes from.
Quick checklist
- Write the distinct chords down.
- Note the first and last chord.
- Find the key that contains them all (allow one or two borrowed).
- Listen for a V → I resolution.
- If it could be major or relative minor, trust where it starts and rests.
Skip the checklist and drop the song, or paste your chords, into the key detector: it finds the key, shows the ones that also fit, and lets you hear it.
Further reading: Circle of fifths explained · How to transpose chords · Music theory basics for songwriters