Will that screen recording fit in your email attachment limit? How much phone storage does an hour of 4K eat? Can your upload finish before the meeting? All of these questions reduce to one piece of arithmetic that takes ten seconds once you know it. Video file size is not mysterious — it is almost entirely determined by bitrate multiplied by duration.
Bitrate is the data spent per second of video, usually quoted in megabits per second (Mbps). Since file sizes use bytes, the only trap is the factor of eight between bits and bytes.
The formula
File size in megabytes equals bitrate in Mbps, times duration in seconds, divided by 8. A 10 Mbps recording lasting 60 seconds is 10 times 60 divided by 8, or 75 MB. For longer content, a friendlier form: size in gigabytes is roughly bitrate in Mbps times duration in hours times 0.45. So a two-hour movie at 5 Mbps is about 4.5 GB, and an hour of 45 Mbps 4K camera footage is around 20 GB. Audio tracks add a little — typically 0.1 to 0.3 Mbps — which usually disappears into rounding.
Typical bitrates to plug in
Resolution alone does not set the bitrate; the encoder settings do. Still, common real-world figures cluster usefully: standard-definition streaming sits around 1 to 2 Mbps, 1080p streaming around 5 to 8 Mbps, and 4K streaming around 15 to 25 Mbps. Cameras recording locally are far more generous, since they are not fighting for bandwidth: phones commonly write 1080p at 15 to 20 Mbps and 4K at 45 to 100 Mbps depending on settings. This is why an hour of phone footage dwarfs an hour of the same resolution streamed from a service.
Why estimates drift
Two effects nudge reality away from the formula. Variable bitrate encoding spends more data on complex scenes — confetti, water, rapid motion — and less on static ones, so the quoted bitrate is an average and busy content runs larger. And decimal-versus-binary unit reporting means your operating system may display the same file as a smaller number of “gigabytes” than the math predicted, purely because it counts in units of 1024. Neither effect usually exceeds 10 to 15 percent, which is fine for planning.
Keep the one-liner in your pocket — Mbps times seconds divided by 8 gives megabytes — and you will never again be ambushed by a full storage warning halfway through recording something important.
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