Video File Size Calculator
Estimate a video file size from bitrate and duration, or find the bitrate needed to hit a target size.
About the Video File Size Calculator
The Video File Size Calculator answers two questions that come up constantly when you export, upload, or compress video: how big will this file be, and what bitrate do I need to hit a size limit? Pick a mode from the dropdown, enter your duration and bitrate (or a target size), and the answer updates the moment you stop typing — no rendering, no waiting, no guesswork. It is built for editors, streamers, YouTubers, students, and anyone bumping up against an upload cap, an email attachment limit, or a storage budget.
Everything runs entirely in your browser. Nothing you type is uploaded, there's no sign-up, and once the page has loaded the calculator keeps working even without an internet connection. Empty or invalid entries show a short, friendly prompt instead of a confusing "NaN", and impossible cases like a zero duration are caught and explained rather than crashing the result. Sizes are reported in decimal units (1 MB = 1,000,000 bytes) to match what upload dialogs, cloud drives, and hosting platforms usually mean by "MB" and "GB".
How to use
- Choose a mode at the top: "Estimate file size" if you know the bitrate, or "Find required bitrate" if you have a size limit to hit.
- Enter the duration in the hours, minutes, and seconds boxes. Any combination works — for example 90 minutes can go in as 1 hour 30 minutes or as 90 minutes.
- Add your numbers. In file-size mode, type the video bitrate and audio bitrate in kbps. In bitrate mode, type your target size in MB.
- Read the highlighted result box. File-size mode shows the estimate in MB and GB; bitrate mode shows the total bitrate you need in kbps and Mbps.
- Switch modes anytime — your duration stays put, so you can flip between "how big" and "what bitrate" without re-typing.
How the math works
File size is just bitrate multiplied by time. The tool adds your video and audio bitrates, converts kilobits per second into bytes per second (multiply by 1000, then divide by 8), and multiplies by the total number of seconds. To go the other way, it takes your target size in bytes, multiplies by 8 to get bits, divides by the duration to get bits per second, and converts back to kbps. Because real encoders use variable bitrate and every container (MP4, MKV, MOV) adds a little overhead for headers and metadata, treat the output as a close estimate rather than an exact byte count.
Frequently asked questions
How do I estimate a video's file size?
Use "Estimate file size" mode. Enter the clip length and both the video and audio bitrates in kbps. The tool computes size as (video kbps + audio kbps) × 1000 ÷ 8 × total seconds, then shows the answer in MB and GB.
What bitrate do I need to fit a size limit?
Switch to "Find required bitrate" mode, enter your target size in MB and the duration, and you'll get the total bitrate in kbps and Mbps. Set your encoder a little below that number to leave room for container overhead and audio.
Why are your MB and GB smaller than what my operating system shows?
This calculator uses decimal units, where 1 MB = 1,000,000 bytes and 1 GB = 1,000,000,000 bytes — the convention most upload limits and hosting sites use. Some file managers use binary units (1 MiB = 1,048,576 bytes), so their numbers can look slightly different for the same file.
Why is my real file a different size than the estimate?
Encoders rarely hold an exact constant bitrate, and containers add headers, indexes, and metadata that the raw bitrate math doesn't include. The estimate gets you very close, but expect small differences in practice.
Is my data private, and does this work offline?
Yes on both counts. Every calculation happens locally in your browser — no numbers, files, or settings are uploaded or tracked. After the page loads once, the calculator keeps working with no internet connection at all.