Work backwards from a target file size — like fitting under a 10MB upload limit — to find the bitrate you need to encode at.
The Audio Bitrate Calculator answers “what bitrate can I use to make this recording fit in a given file size?” — the question behind every upload limit, email attachment and storage budget. Enter the size you must stay under and the length of the audio and it returns the exact bitrate in kbps.
It also snaps to the nearest standard MP3 bitrate that fits with a safety margin for frame overhead, and tells you whether the result will be transparent for music, fine for speech or only suitable for voice notes.
For constant-bitrate audio, size = bitrate × duration ÷ 8, so bitrate = size × 8 ÷ duration. A size in MB (1,000,000 bytes) differs from MiB (1,048,576 bytes) by about 5%, so the tool lets you choose, defaulting to the stricter decimal MB. The MP3 suggestion reserves 1.5% for headers and padding.
Find the bitrate for a 10 or 25 MB attachment.
Plan episode sizes.
Meet the 16 MB WhatsApp cap.
Know the quality cost of a size target.
| Unit options | KB, MB (decimal), MiB (binary), GB |
|---|---|
| Verdict bands | ≥256 excellent · ≥128 good · ≥64 speech · ≥32 voice only |
| MP3 margin | 1.5% overhead reserved |
128 kbps MP3 is fine for most listening; 192–320 kbps for critical music; 48–64 kbps for speech.
No: 8 kbps = 1 kB/s.
Once you know the bitrate, use the Audio Cutter, Trimmer or any ToolsCrow editing tool — each lets you export as MP3, and Phase 2's Audio Compressor (coming soon) will let you set a bitrate directly.
Real-world VBR encoders can land a few percent above or below a flat bitrate calculation, so aiming slightly under a hard limit (e.g. 9.5MB for a 10MB cap) is safer.
This tool runs entirely in your browser using the Web Audio API. Nothing is uploaded, stored or shared — there is no server to send your file to, so it works offline once the page has loaded.
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