How video file size is worked out
A video’s size comes from two things: its bitrate (how much data each second uses) and its length. The formula is simple:
Size in MB = (video bitrate + audio bitrate in kbps) × seconds ÷ 8 ÷ 1000
For example, two minutes of 1080p at 6 Mbps with 128 kbps sound is (6000 + 128) × 120 ÷ 8 ÷ 1000 = about 92 MB. Resolution and frame rate don’t appear in the formula directly; they decide how much bitrate you need for the picture to look good.
Typical bitrates
| Video | Typical bitrate | Size per minute |
|---|---|---|
| 480p, shared online | 1.5 Mbps | about 12 MB |
| 720p, shared online | 3 Mbps | about 23 MB |
| 1080p, shared online | 6 Mbps | about 46 MB |
| 1080p, straight from a phone | 12–20 Mbps | about 90–150 MB |
| 4K, straight from a phone | 40–60 Mbps | about 300–450 MB |
Phone and camera recordings use generous bitrates because they’re made for editing. For sharing, a third of that usually looks the same on a phone or laptop, which is why compressing works so well.
Worked examples
- A 30-second clip for Discord (20 MB): the bitrate calculator gives about 5 Mbps, enough for sharp 1080p.
- A 2-minute video for WhatsApp (16 MB): about 0.9 Mbps, so 480p is the best choice.
- A 5-minute screen recording for email (18 MB): about 0.4 Mbps. Screen recordings compress very well, so text often stays readable; otherwise share a link.
- A 10-minute 1080p phone video: roughly 1 to 1.5 GB as recorded, or about 460 MB at a 6 Mbps sharing bitrate.
From the calculator to a smaller file
When a video doesn’t fit, press “Compress to fit” next to the limit, or the button under the bitrate result. The compressor opens with that size already set, works out the exact bitrate for your video, and saves an MP4 that’s guaranteed to be under the limit. If the video is too long for any sensible quality, trim it first.
MB, MiB and why sizes look different
This calculator uses MB as 1,000,000 bytes, like Discord, Gmail and most websites. Windows shows sizes in MiB (1,048,576 bytes) but labels them MB, so the same file looks about 5% smaller in File Explorer.
Questions
How do I calculate video file size?
Multiply the total bitrate (video plus audio, in kbps) by the length in seconds, then divide by 8,000 to get MB.
What bitrate do I need for 1080p?
About 5–8 Mbps looks good for sharing; phones record at 12–20 Mbps.
How long a video fits in 25 MB?
About 30 seconds at a 6 Mbps 1080p sharing bitrate, only around 12 seconds straight from a phone camera, or several minutes at 480p.
Why is my video bigger than the calculator says?
Phones vary their bitrate with the scene, and some record at much higher rates than typical. Read the length and size from your file to see its real bitrate.
Does the file I choose get uploaded?
No. Only its length is read, on your device.
GuideHow to Reduce Video File Size Without Losing Quality
CalculatorWatching a long video? Try the video speed calculator