How to estimate video file size from bitrate and duration
Estimated file size in decimal MB equals total average bitrate in Mbps multiplied by duration in seconds, divided by eight. Include video and audio bitrates. A 10-minute video at 8 Mbps video plus 192 kbps audio is approximately 614.4 MB before container overhead.
Begin with total average bitrate
Bitrate measures how much encoded data is used each second. Add the average video rate and all audio rates before calculating a complete file. If video is 8 Mbps and audio is 192 kbps, convert audio to 0.192 Mbps. The total is 8.192 Mbps.
Then use MB = total Mbps × seconds ÷ 8. For ten minutes, 8.192 × 600 ÷ 8 gives 614.4 MB. This example is arithmetic for stated inputs, not a recommended quality setting. Real files also contain a container and may include additional tracks or metadata.
Resolution does not determine a unique size
A label such as 1080p describes image dimensions, not the number of bytes in a complete video. Two 1080p files can have very different durations, frame rates, codecs, and encoding settings. Even the source material matters: a quiet presentation and a scene with rapid motion present different compression challenges.
Use the average bitrate or a measured sample when predicting a file size. If all you know is “4K,” a precise answer would be invented. An editor's export options may change size and quality together, but a resolution choice by itself does not establish a universal bitrate.
Distinguish average and peak values
A constant-bitrate calculation assumes a steady target rate. Variable-bitrate encoding can allocate more data to difficult scenes and less to simple ones. File size depends on the average over the full duration, while a peak or maximum bitrate describes a different constraint.
For example, a video with a 12 Mbps maximum does not necessarily average 12 Mbps. If it averages 6 Mbps for five minutes, the video stream contributes about 225 MB, before audio and other data. Using the maximum throughout would double that estimate. Check what an encoder's setting actually represents before entering it.
Work backward from a file-size ceiling
If an upload accepts a decimal 100 MB file and your clip lasts 120 seconds, the total average bitrate budget is 100 × 8 ÷ 120, or approximately 6.67 Mbps. Reserving 0.192 Mbps for audio leaves about 6.47 Mbps for video before allowing for container overhead.
A hard upload limit calls for additional margin and a final byte-size check. Do not target the last available byte using an approximate calculator. Confirm whether the service means MB or MiB, whether it limits duration separately, and whether it re-encodes the file after upload. Those rules are independent of the size formula.
Test the result and plan the transfer
Export a short representative sample before encoding a long project. Include motion, detail, and text that matter to the finished video, then inspect quality at the intended viewing size. A smaller file is only useful if the result remains acceptable for its purpose. Keep the original before experimenting with a new export.
Once exported, the actual file's byte count is more reliable than any estimate. Enter that size into the download-time tool, using your measured upload rate when sending it to a service. The video-size calculator helps explore settings; it does not encode, compress, inspect, or upload your footage.
Sources & definitions
- MDN: Web video codec guide
- MDN: Video processing concepts
- Microsoft: Exporting and saving a video in Clipchamp
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