File size is one of the most practical constraints in video production. A 20-minute video may look small during editing but become several gigabytes after export. A large file changes upload timing, cloud storage requirements, review turnaround and the amount of free space needed before rendering. Fortunately, a useful planning estimate needs only a few inputs.
1. Work in bits before converting to bytes
Bitrate is commonly expressed in megabits per second (Mbps), while files are usually reported in megabytes (MB) or gigabytes (GB). Since there are eight bits per byte, the basic decimal approximation is:
File size in GB ≈ (video Mbps + audio Mbps) × duration seconds ÷ 8,000.
If the audio is 384 kbps, convert it to 0.384 Mbps. Then add that amount to the video bitrate before multiplying by duration. This gives a planning value, not a precise output size.
2. Run a real example
Suppose you make a ten-minute tutorial, encoded at an average 8 Mbps video bitrate with 384 kbps stereo audio. The duration is 600 seconds. The result is 8.384 × 600 ÷ 8,000 = 0.629 GB (about 629 MB), ignoring container and metadata overhead. If the same project uses a 12 Mbps video stream, that estimate rises to about 0.929 GB.
The calculation does not know how much motion the shots contain, which encoder you are using, or whether you chose constant-quality, variable-bitrate or two-pass settings. Compare your rendered files against the estimate to understand how your own workflow behaves.
3. Do not mix GB with GiB
Storage manufacturers often use decimal units, where 1 GB equals 1,000,000,000 bytes. Some operating systems display a binary quantity, where 1 GiB is 1,073,741,824 bytes. A 1 GB file is approximately 0.931 GiB. The calculator shows both to prevent an apparent mismatch between the plan and a local file-properties dialog.
4. Plan for transfer overhead
At a 20 Mbps upload connection, sending a 1 GB file theoretically takes about 400 seconds before network variations and processing are considered. Actual transfer time depends on your connection, server congestion, protocol overhead, packet loss and other traffic. For the first estimate, the calculator applies an explicit, user-visible 20% overhead assumption. That is a planning margin, not a measured network guarantee.
5. Build a preflight budget
- Estimate output size before opening the final export dialog.
- Keep additional free disk space for temporary renders and retries.
- Use a supported codec and the platform's current bitrate advice.
- Measure one small real test export when possible.
- After encoding, inspect the real file size and decode quality before upload.