How to find the BPM of a song, and why tools disagree

To find the BPM of a song, upload it to a tempo detector and then check the answer against how the song feels, because the number you get back is sometimes half or double the one you wanted. Our free BPM finder does the detection: no signup, whole song, and it shows the half and double alternatives next to the main figure. Below is how to use it, how to check the result in ten seconds, and how often it is wrong.

How to find the BPM of a song, step by step

1. Open the BPM finder and drop in your file. It takes MP3, WAV, FLAC, M4A, OGG, AAC and AIFF, up to 10 minutes and 120 MB. Without an account you can check 20 songs a day.
2. Wait a few seconds. Unlike our preview, it analyses the whole song, not a 60-second clip.
3. Read the result. You get a tempo in beats per minute, rounded to a whole number, with the half and double time beside it (a result of 128 comes with 64 and 256). The key is shown too, when the model is confident enough to give one. See the key finder for what that means.
4. Check it against the song, as described next, and pick the neighbour that matches.

Check the number in ten seconds

A tempo is beats per minute, so count beats for exactly one minute, or count for 15 seconds and multiply by four. Tap your foot to the pulse you would dance or nod to, not to every hi-hat. If you count about 87 and the finder says 174, the song is at 87 in the way a person feels it, and 174 is the same music counted twice as fast. Neither is a mistake in the arithmetic: both describe the same audio. Which one you want depends on what you are doing, so we show both and leave the choice to you.

The same idea works backwards for mixing and practice. Halving or doubling a tempo keeps the beats lined up, so a 70 BPM track and a 140 BPM track can sit together in a mix. A 70 and a 100 cannot, without stretching one of them.

Why BPM finders disagree by half or double

A detector listens for a repeating pulse, and a groove contains several: the kick, the snare, the off-beat hi-hat. Each is a valid pulse, at a different speed, and the detector has to choose. Choose the one a listener would tap and it is right. Choose the one twice as fast and it is off by an octave. We wrote up the details in most BPM detection errors are octave errors; the short version is in the table below.

How often it is right

Tempo detection in StemX is Beat This! (Foscarin, Schlüter and Widmer, CPJKU), a pre-trained beat tracker. We scored the exact version we ship on GTZAN, a dataset it was not trained on, using the standard rule that an answer counts as right if it is within 4% of the annotated tempo. It was right on 81.9% of 998 tracks (95% CI 79.4–84.1%).

Of the 181 tracks it missed, 114 came back at half, double or triple the real tempo. Count those as right and the figure is 93.3%, which is why showing the neighbours is useful: for most misses, the right answer is on the card. Only 67 tracks, about 7%, were wrong in a way you cannot fix by halving or doubling.

Limits

• About 1 track in 5 gets a number outside 4% of the annotation, and the lower end of the interval is just under 80%. Do not feed the result straight into automatic beat-matching without listening.
• GTZAN is ten genres of 30-second clips. Your track may behave differently, especially with live drummers, tempo changes or very sparse intros. The finder reports one tempo for the whole song.
• A song that speeds up or slows down has no single correct BPM; the figure is a summary, not a map of the song.
• If you plan to publish or perform something built from a track, use one you own or have the rights to use. Analysing a file does not change who owns it.

Where the number is useful next

Every full split in our vocal remover also shows the song's tempo and key, and its results player has a click track that follows the detected beat grid, handy for practising along to an instrumental. If you only want the tempo, the BPM finder is the quicker route.

Model: Beat This! by Francesco Foscarin, Jan Schlüter and Gerhard Widmer (github.com/CPJKU/beat_this).