A file size limit is the maximum a service will accept per upload. Simple in principle, and the source of a surprising amount of wasted effort, because platforms disagree about what they are measuring.
Three distinctions that decide whether you fit:
Before or after the platform's own processing. Some services measure what you hand them; some measure what they store after re-encoding. Discord's mobile clients changed in August 2026 to measure before compression, matching how desktop always worked — which means a file that previously squeaked through on a phone may now be refused.
The file, or the message carrying it. Email is the notorious case. Attachments are base64-encoded in transit, inflating them by around 37%, and providers measure the encoded message. Gmail's documented 25 MB therefore corresponds to a raw file of roughly 18 MB, and a video compressed to exactly 25 MB bounces.
Per file, or per account. Slack caps individual uploads generously and counts everything against a shared workspace pool. Fitting the first limit and exhausting the second are different problems.
Two habits follow. Target below the cap, not at it — a target-size encode lands within a couple of percent either way, and landing 2% over a hard ceiling is a failed upload. And check the number against the platform's own current documentation, because these move: Discord's free tier has been 25 MB, then 10 MB, and is 20 MB as of August 2026, and plenty of guides still quote whichever one they were written against.