How big is a video file?
A video file’s size depends on its bitrate, the amount of data used per second, and its duration. Resolution and frame rate matter only through the bitrate they require.
- Total bitrate (Mbps) = video bitrate + audio bitrate
- File size (MB) = total bitrate × duration in seconds ÷ 8
- File size (GB) = MB ÷ 1,000
The division by 8 converts bits to bytes. Bitrates are given in megabits per second (Mbps), while file sizes are in megabytes (MB).
Worked example
A 90-minute video recorded at 8 Mbps with 160 kbps audio:
- Total bitrate: 8 + 0.16 = 8.16 Mbps.
- Per minute: 8.16 × 60 ÷ 8 = 61.2 MB.
- Whole file: 8.16 × 5,400 ÷ 8 = 5,508 MB, or 5.51 GB.
- A 128 GB memory card holds about 34.9 hours at this bitrate.
Typical bitrates
| Video | Typical bitrate | Size per hour |
|---|---|---|
| 720p streaming | 3 to 5 Mbps | 1.4 to 2.3 GB |
| 1080p streaming | 5 to 8 Mbps | 2.3 to 3.6 GB |
| 1080p phone or camera | 12 to 20 Mbps | 5.4 to 9 GB |
| 4K streaming | 15 to 25 Mbps | 6.8 to 11 GB |
| 4K phone or camera | 50 to 100 Mbps | 22.5 to 45 GB |
| 4K professional (ProRes 422 HQ) | about 700 Mbps | about 315 GB |
Codecs matter: HEVC (H.265) and AV1 reach similar quality at roughly half the bitrate of H.264.
GB or GiB?
Storage makers count 1 GB as 1,000,000,000 bytes. Windows uses 1,073,741,824 bytes (a gibibyte, GiB) but labels it GB, so a 5.51 GB file shows up as 5.13 GB, and a 128 GB card shows about 119 GB of space. macOS uses decimal GB like the manufacturers.
Constant and variable bitrate
With constant bitrate (CBR), every second uses the same amount of data, and the calculation is exact. Most cameras and encoders use variable bitrate (VBR), which spends more data on complex, fast-moving scenes and less on still ones. The average bitrate is then lower than the maximum, and the real file is smaller than the estimate.
Planning storage and uploads
Use the size per hour to plan how many memory cards you need for an event, how long a drive will last for security camera footage, or how much data a video upload will use. On a mobile plan, streaming an hour of 1080p video can use 3 GB or more, so download over Wi-Fi when you can.