BPM and Key Finder + Delay and Reverb Time Calculator
Detect the tempo and musical key of any audio file, then calculate tempo-synced delay and reverb times for every note value. Runs entirely in your browser. Free, private, no signup.
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Red Mile Records, Calgary
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Book a SessionHow to Find the BPM and Key of a Song
BPM stands for beats per minute: the speed of a track measured by counting the number of beats that occur in 60 seconds. Key is the tonal center of the music, the root note and scale (major or minor) that a song is built around. Knowing both is essential for DJs planning harmonic mixes, producers matching samples to a session, and mixing engineers setting tempo-synced effects.
Our BPM and key finder analyzes your audio file using the Web Audio API and a spectral analysis algorithm, entirely inside your browser. Your file is never uploaded to a server. The tool decodes the audio, runs an onset-detection function to estimate tempo, and uses a chroma-based pitch-class profile correlated against the Krumhansl-Kessler key profiles to determine the most likely musical key.
Detected BPM can land at half or double the felt tempo, especially on tracks with heavy syncopation or sparse percussion. Detected key can return the relative major or minor. Always confirm results by ear before committing to a mix decision.
How to Calculate Delay and Reverb Time from BPM
The formula is simple: divide 60,000 by the BPM to get the duration of one quarter note in milliseconds. Scale that value by the note subdivision you want. A half note is twice the quarter-note value, an eighth note is half, a dotted value adds 50%, and a triplet multiplies by 2/3.
For delay effects, setting the delay time to a note value that matches the song tempo keeps the repeats rhythmically locked to the beat instead of smearing across it. Common starting points: a dotted eighth note for rhythmic slap delays, a quarter note for vocal throws, and a sixteenth note for tight slapback.
For reverb, engineers use these values to set predelay and decay times that breathe with the track rather than fighting it. The predelay (the gap before the reverb tail begins) is typically set to a short note value like a 1/16 or 1/32, giving the dry signal separation before the tail. The decay time (how long the tail rings) is often set to a longer value like a dotted eighth or quarter note, then adjusted by ear so the reverb clears before the next phrase. We cover the mixing side in depth in our 5 pro mixing secrets guide and in our loudness guide for Calgary artists.
Why Use a BPM Finder and Delay Calculator Together?
Most producers and engineers use separate tools to detect the BPM, then manually enter it into a delay calculator. This tool connects the two: detect the BPM and key of your audio file, then send that BPM directly into the delay and reverb calculator with one click. The workflow saves time and removes the copy-paste step that introduces errors, especially when you are working across multiple sessions or analyzing reference tracks.
Frequently Asked Questions
Is this BPM and key finder accurate?
The results are estimates based on spectral and onset analysis. In testing, accuracy is comparable to other free tools on the market. BPM detection works best on tracks with a clear, steady beat. Key detection uses the Krumhansl-Kessler pitch-class profile method. Results can vary depending on audio quality, tempo changes, and mix complexity. Always confirm by ear.
Does the tool upload my audio file?
No. Analysis runs entirely in your browser using the Web Audio API. Your audio file never leaves your device. Nothing is uploaded, stored, or sent to any server. This is the same privacy model used by Tunebat and LANDR's finder tools.
What audio formats are supported?
MP3, WAV, M4A, AAC, OGG, FLAC, and any format your browser can decode. Chrome, Firefox, Safari, and Edge all support the most common audio formats.
How do I calculate delay time from BPM?
Divide 60,000 by the BPM. That gives you the quarter note in milliseconds. Multiply by 2 for a half note, divide by 2 for an eighth note. For dotted values, multiply the base by 1.5. For triplets, multiply by 2/3. Our calculator does this automatically for every standard note value.
What is reverb predelay and how do I set it?
Predelay is the gap in milliseconds between the dry signal and the start of the reverb tail. It creates separation so the vocal or instrument stays clear while the reverb adds space behind it. A common starting point is the 1/16 or 1/32 note delay time for the current BPM. Use the calculator above to find the value, set it in your reverb plugin, then adjust by ear.
Why does the BPM show at half or double the tempo I expected?
BPM detection algorithms count rhythmic events per minute, and some patterns can be interpreted at multiple metric levels. A track that feels like 140 BPM to a listener can register as 70 or 280 depending on where the strongest beat pattern falls. The tool shows the primary detection plus the half and double alternatives so you can pick the one that matches what you hear.
Can I use this for DJing and harmonic mixing?
Yes. Detect the BPM and key of tracks in your library to match tempos and plan harmonic transitions. For Camelot wheel compatibility, convert the detected key to its Camelot code (many charts are available online) and mix tracks that share the same code or adjacent codes.
Is this tool free?
Yes. Both the BPM/key finder and the delay/reverb calculator are completely free, with no signup, no paywall, and no usage limits. Built by Red Mile Records, a professional recording and mixing studio in Calgary, Alberta.