The ratio comes from your two BPM values
The page divides target BPM by current BPM and applies that multiplier to local playback. A smaller target slows the song; a larger target speeds it up. The displayed multiplier makes the conversion inspectable instead of hiding it behind a vague slider.
Song tempo changer and pitch preservation are separate choices
With Preserve pitch selected, the browser compensates pitch while playback speed changes. Turn it off to hear tape-style speed change, where tempo and pitch move together.
Measure first when the source BPM is unknown
Use the local BPM finder for a recording or the tap tool for a live count, then return with that value. Entering a guessed source produces a mathematically consistent ratio based on the guess, not a verified song tempo.
Questions
These answers cover local playback, source values, and browser limits.
Does this upload the song?
No. The selected file becomes a temporary object URL inside this tab.
Does it permanently edit my original file?
No. It changes local preview playback and leaves the selected file untouched.
Why might a format fail?
The browser must be able to decode that codec. A filename extension alone does not guarantee browser support.
Sources used for playback behavior
The speed control uses the browser media playbackRate and preservesPitch behavior defined by the HTML media platform; the BPM ratio is shown directly on the page.
Live microphone or selected-file audio is processed locally in this browser and is not uploaded.