BPM & KEY

Manual

1. What BPM & KEY does

   BPM & KEY analyzes an audio file to automatically detect its tempo (BPM) and musical key — useful for tagging tracks, checking a file before loading it into a DAW, or picking tracks for a DJ set.

2. How to use it

   Drop an audio file onto the drop zone or click to choose one. Once analysis finishes, the BPM, key, and a confidence score appear in the Readout. Analysis runs in a background Web Worker, so the page stays responsive while it works.

3. How BPM detection works

   First, it measures how much the sound's frequency content changes from moment to moment (onset strength). Then it looks for a repeating pattern in that signal using autocorrelation, searching a 60–200 BPM range for the best-fitting tempo. A mild preference toward moderate tempos (around 120 BPM) helps cut down on the half/double tempo mistakes described below.

4. How key detection works

   It analyzes the frequency content of the audio and tallies how much energy falls near each of the 12 pitch classes (C, C#, D…), producing a fingerprint-like profile called a chroma vector. Higher frequencies — which are more likely to be overtones rather than the fundamental note — are weighted down to reduce their influence.

   That chroma profile is then compared against 24 reference key patterns (12 major and 12 minor, based on the Krumhansl-Schmuckler key profiles), and the closest match is reported as the key.

5. Accuracy notes

   BPM detection can mistake a tempo for its half or double (for example, reading a 128 BPM track as 64 or 256). When the detected value falls outside the typical range for most tracks (roughly 70–160 BPM), a "possible half/double tempo" note is shown. Key detection can be less accurate for sparse or slow ballads, or tracks with complex harmony.

6. Search range at a glance

   Here's the range the analyzer searches, and when the half/double note appears.

   Item	Range / condition
   BPM search range	60–200 BPM
   Half/double note shown when	Detected value falls outside 70–160 BPM
   Key candidates	24 (12 pitch classes × major/minor)

7. Use case: picking tracks and matching keys for a DJ set

   Knowing the BPM and key of your library ahead of time makes it easier to line up tracks with a similar tempo, or pick harmonically compatible keys to mix.

8. Use case: tagging samples before loading them into a DAW

   Checking BPM and key before dragging a sample or loop into your DAW helps you decide whether it fits the project's tempo, and makes it easier to record in a filename or tag.

9. How it works: in-browser processing and privacy

   Analysis runs in a Web Worker inside your browser; the file is never uploaded anywhere. Decoding uses the Web Audio API.

10. FAQ

    The result doesn't match the track.

    Check the half/double tempo note first. Beyond that, some tracks change tempo or modulate key partway through, and the analysis produces a single value based on the track's overall tendency, so it may not match every section.

    Which file formats are supported?

    Any common audio format your browser can decode — WAV, MP3, FLAC, AAC, OGG, and more.

    How long does analysis take?

    It depends on the track length and your device, but a few seconds to around ten is typical. Analysis runs in a background Web Worker, so the page doesn't freeze while it works.

    What does the confidence score mean?

    A 0–1 score for how clear the autocorrelation peak was during BPM detection. A higher value means a stronger, clearer repeating beat was found at the detected tempo.

    Any tips for better key detection accuracy?

    Tracks with a clear harmonic section — an intro or chorus with chords, for example — tend to give more accurate results than sections that are mostly vocals or percussion alone.