Work backward from a video file size budget

Separate duration, bitrate and file size

Duration says how long the video plays. Bitrate describes how much encoded data is used per second. File size is the total stored data. Resolution affects the encoding problem, but resolution alone does not determine the file size. Two exports with the same dimensions can have very different bitrates.

For a worked estimate, use a 60-second clip, 8 Mbps video and 128 kbps audio. Convert both rates to bits per second, add them, multiply by 60 and divide by eight to get bytes. The media payload is 60,960,000 bytes, or 60.96 decimal MB.

60 × (8,000,000 + 128,000) ÷ 8
= 60,960,000 bytes
= 60.96 MB
≈ 58.14 MiB

Check which size unit you are reading

A decimal megabyte is 1,000,000 bytes. A binary mebibyte is 1,048,576 bytes. The same example can therefore appear as roughly 60.96 MB or 58.14 MiB without any data disappearing. Use the unit in the upload requirement and compare it with the exact exported byte count when a limit is close.

Open the video size calculator in file-size mode to see both units. Remember that the result estimates audio and video payload. Container metadata and other tracks can add bytes to the finished file. An estimate just under a hard limit needs room for that difference.

Work backward from a 50 MB budget

Switch to video bitrate from file budget. Keep the duration at 60 seconds and audio at 128 kbps, enter 50 MB and reserve 5%. That leaves 47.5 MB for the media payload. Divide its bits by 60 seconds and subtract the audio rate. The video budget is approximately 6.2053 Mbps.

The 5% reserve is an editable planning allowance, not a promise about every container or encoder. Round the encoder setting down and inspect the actual export. If audio alone uses the available payload budget, the tool asks for a larger file budget, shorter duration or lower audio bitrate.

Payload budget = 50 MB × 0.95 = 47.5 MB
Total bitrate = 47.5 × 8 ÷ 60 ≈ 6.3333 Mbps
Video budget = 6.3333 − 0.128 ≈ 6.2053 Mbps

Choose a rate that survives a real sample

A file budget does not establish visual quality. Export a short representative section containing motion, faces, fine text or other detail that matters to your clip. Compare it with the source at the size your audience will watch. If the quality is unacceptable, shorten the edit, reconsider the dimensions or choose another delivery route instead of treating the number as an ideal preset.

Use average bitrates for estimates when your encoder provides them. Variable bitrate and quality-based encoding can produce different sizes for different scenes. FFmpeg’s documentation distinguishes bitrate controls and encoding behavior; the calculator itself does not run an encoder.

Verify the final file before scheduling

After the full export, inspect its exact size, duration, sound and captions. Check the destination’s current requirements for format, dimensions and duration as well as file size. Passing one limit does not establish that every other requirement is met.

Save the export settings with the final media so you can reproduce the result. The calculator is useful before a long render or upload; the finished file is the evidence you use for the publishing decision.