A 15-second Reel at 128 BPM is eight bars: put tempo in the prompt

At 128 BPM in 4/4 time, one bar lasts 1.875 seconds, so eight bars last exactly 15 seconds, which fits a 15-second Reel. Sume's music prompt takes tempo as a number, so you can write "128 BPM" and a named moment such as "drop at 0:07" into the brief. Treat both as directions: the docs call the axes creative direction, not guaranteed output values.

The prompt guidance is in the Music 1.0 docs and the Music Router docs (read 2026-10-06).

What is the arithmetic?

A beat is 60 divided by the BPM seconds long, and a bar in 4/4 is four beats.

What goes in the prompt?

Music 1.0 and the Music Router have no duration field and reject duration and duration_seconds. Length is steered in the prompt: the docs suggest writing a length such as "a 2-minute track" or section markers like [0:00-0:30] Intro: .... For a Reel, write something like: "Cocky synthwave, 128 BPM, A minor. Pulsing bass, gated snare. Hook for the first 7 seconds, full drop at 0:07, hard stop at 0:15. Instrumental, no vocals."

How do I check it landed?

Provider-reported tempo and lyrics are model captions, not measurements. Listen to the file, and if it runs long, trim and fade it in a Timeline render with soundtrack.fade_out_seconds, up to 10 seconds. A track that is too short can set soundtrack.loop.