How to Calculate Video File Size (and the Bitrate for a Size Limit)
The simple formula behind every video file size, worked examples, and how to reverse it to find the bitrate that fits a 10, 16, 20, or 25 MB limit.
Every compression decision gets easier once you can estimate file size in your head. The math is simple; the only trap is mixing up bits and bytes. This guide shows the formula, works through examples, and then turns it around so you can find the bitrate that fits any upload limit.
Bits vs bytes
Bitrates are measured in bits per second (kbps, Mbps). File sizes are measured in bytes (KB, MB, GB). One byte is 8 bits. So a video at 8 Mbps uses 1 megabyte every second. Forgetting to divide by 8 is the most common mistake in file size estimates.
The formula
size (MB) = total bitrate (Mbps) x duration (s) / 8
total bitrate = video bitrate + audio bitrateContainers such as MP4 add a little overhead for indexes and metadata, usually around 1 to 2%. For quick estimates you can ignore it; for hard limits, leave a margin.
Worked examples
| Video | Bitrates | Calculation | Size |
|---|---|---|---|
| 90 s phone clip, 1080p | 16 Mbps video + 0.2 Mbps audio | 16.2 x 90 / 8 | About 182 MB |
| 5 min screen recording | 2 Mbps + 0.128 Mbps | 2.128 x 300 / 8 | About 80 MB |
| 30 s compressed clip | 4 Mbps + 0.128 Mbps | 4.128 x 30 / 8 | About 15.5 MB |
Working backwards from a size limit
To fit a limit, solve the formula for bitrate:
video bitrate (Mbps) = size limit (MB) x 8 / duration (s) - audio bitrate
example: 20 MB limit, 45 s clip, 128 kbps audio
20 x 8 / 45 = 3.56 Mbps total
3.56 - 0.128 = about 3.4 Mbps for video
leave about 5% headroom: about 3.2 MbpsThe headroom covers container overhead and the fact that single-pass encoders do not hit their target exactly. Two-pass encoding, explained in CRF vs bitrate, lands much closer.
Quick reference: video bitrate that fits common limits
| Duration | 10 MB | 16 MB (WhatsApp) | 20 MB (Discord free) | 25 MB (Gmail) |
|---|---|---|---|---|
| 15 s | 5.0 Mbps | 8.0 Mbps | 10.0 Mbps | 12.5 Mbps |
| 30 s | 2.4 Mbps | 3.9 Mbps | 4.9 Mbps | 6.2 Mbps |
| 60 s | 1.2 Mbps | 1.9 Mbps | 2.4 Mbps | 3.0 Mbps |
| 2 min | 0.5 Mbps | 0.9 Mbps | 1.2 Mbps | 1.5 Mbps |
Values assume 128 kbps audio and 5% headroom. Below about 1 Mbps, 1080p footage starts to look rough; that is the point where trimming, dropping to 720p, or sending a link makes more sense. For your exact numbers, use the video size calculator, which does this math instantly.
Is the bitrate enough for good quality?
The formula tells you what fits, not whether it will look good. That depends on resolution, frame rate, motion, and codec. The typical ranges in bitrate, resolution, and file size explained are a good sanity check.
Summary
File size is total bitrate times duration, divided by 8. Turn the formula around and you can find the bitrate for any limit in seconds. Leave a little headroom, and if the answer is below about 1 Mbps for 1080p, shorten the video instead of squeezing it harder.
Quick answers
What is the formula for video file size?
Size in MB = (video bitrate + audio bitrate in Mbps) x duration in seconds / 8. Container overhead adds roughly 1 to 2%.
What bitrate fits a 25 MB limit for a 60 second video?
About 3.3 Mbps total. Subtract 128 kbps for audio and leave a little headroom, so aim for roughly 3 Mbps of video.
Try it on your own video
Upload a video, see the estimated size for each level, and download a smaller file. Free, no install.
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