What this calculator does
Find out how much space a video will take up before you record or export it. A compressed video's size depends on two things. One is its bitrate, the amount of data (bits) used for each second of picture and sound. The other is how many seconds it lasts. Multiply them, divide by eight to turn bits into bytes, and you have the file size.
The calculator does that for the video plus any audio tracks. It shows the result in the usual megabytes and gigabytes (MB and GB). It also shows the slightly different MiB and GiB units that many computers display.
You can also work backward to plan an export (an encode). Give a target size, such as a file-sharing limit or the space left on a card. The calculator finds the video bitrate that fits after the audio takes its share.
Where it's used
Why this math matters outside the classroom:
- Memory card planning. A videographer recording at 100 Mbps finds each minute uses 750 MB, so a 64 GB card holds about 85 minutes.
- Upload limits. A teacher fits a 10-minute lesson under a 100 MB limit by encoding at about 1.33 Mbps total.
- Disc authoring. Fitting 2 hours onto a 4.7 GB disc leaves a total bitrate of about 5.2 Mbps for picture and sound.
- Security cameras. Installers multiply 4 cameras by 2 Mbps for 24 hours to size the recorder's drive at about 86 GB per day.
- Streaming data use. A viewer works out that two hours of a 5 Mbps stream uses 4.5 GB of a monthly data plan.
Grade level and standards: Grade 6; Common Core State Standards for Mathematics 6.RP.A.3.d, 6.RP.A.3.b. © Copyright 2010. National Governors Association Center for Best Practices and Council of Chief State School Officers. All rights reserved.
How to use it
- Choose File size, or Video bitrate for a target size.
- Enter the video bitrate in Mbps or kbps (or the target size in GB or MB), plus the audio bitrate per track and the number of tracks.
- Enter the length in hours, minutes and seconds. If you like, add a few percent for container overhead (the file's own bookkeeping data).
- Read the size or bitrate, the same size in computer (binary) units, and the steps.
What the inputs mean
- Video bitrate
- Export software and cameras state this in Mbps (megabits per second). It's the average rate. Variable-bitrate files swing above and below it.
- Target file size
- The size you need to stay under, in decimal units (1 GB = 1,000,000,000 bytes). That's how file-size limits and storage are usually labeled.
- Audio bitrate per track
- Common stereo values are 128 to 320 kbps (kilobits per second). Uncompressed 48 kHz, 16-bit stereo audio is 1,536 kbps.
- Audio tracks
- Each language or commentary track adds its own bitrate. Use 0 for video with no sound.
- Container overhead
- The file format (the container) adds headers and an index. That's usually a small part of the file. Leave it at 0 for a quick estimate.
Formula and method
total bitrate (kbps) = video bitrate + audio bitrate × tracks size (bytes) = total bitrate × 1,000 × seconds ÷ 8 × (1 + overhead ÷ 100) 1 MB = 1,000,000 bytes; 1 GB = 1,000,000,000 bytes; 1 GiB = 1,073,741,824 bytes video bitrate = size × 8 ÷ (1 + overhead ÷ 100) ÷ seconds ÷ 1,000 − audio bitrate
Bitrates count in thousands: 1 kbps is 1,000 bits per second, and 1 Mbps is 1,000 kbps. There are 8 bits in a byte. So a 1 Mbps stream fills 125,000 bytes every second, or 7.5 MB a minute.
File managers on some computers divide by 1,073,741,824 but still write GB. That's why a 5.53 GB file can show as 5.15. The GiB result gives that figure.
Worked example
A 90-minute recording is encoded at 8 Mbps with one 192 kbps audio track and no overhead.
- Duration: 1 × 3,600 + 30 × 60 + 0 = 5,400 seconds.
- Total bitrate: 8,000 + 192 × 1 = 8,192 kbps.
- Size: 8,192 × 1,000 × 5,400 ÷ 8 = 5,529,600,000 bytes.
- In gigabytes: 5,529,600,000 ÷ 1,000,000,000 = 5.5296 GB, which is about 5.15 GiB.
Result: The video takes about 5.53 GB. To fit the same 90 minutes in 2 GB with the same audio, the video bitrate would need to drop to about 2.77 Mbps.
Understanding the result
The estimate is exact for files with a constant bitrate. For variable-bitrate files, use the average bitrate and expect the real size to vary a little.
When you must stay under a strict size limit, aim a few percent below the bitrate shown. That leaves room for the container and for the bitrate swinging up and down.
Assumptions and limitations
- Bitrates are averages over the whole file.
- Bitrates and file sizes use decimal units; binary units are shown separately.
- Container overhead is a flat percentage of the video and audio data.
- Picture quality at a given bitrate depends on the codec (the compression method), resolution, frame rate and what's on screen. None of these are modeled.
- Subtitles, chapters and thumbnails aren't counted.
- Very short clips may have more container overhead for their size.
Frequently asked questions
How many GB is an hour of video?
It depends on the bitrate. At 5 Mbps, an hour of video alone is 5,000,000 × 3,600 ÷ 8 = 2.25 GB. At 20 Mbps, it's 9 GB. Enter your bitrate and 1 hour to see your figure.
Why does my computer show a smaller size?
Some computers count in binary units of 1,073,741,824 bytes but label them GB. The calculator's GiB result matches that display.
What's the difference between Mbps and MB/s?
Mbps is megabits per second and MB/s is megabytes per second. One byte is 8 bits, so 8 Mbps is 1 MB/s.
How do I fit a video under an upload limit?
Choose Video bitrate for a target size, and enter the limit and the length. Then export at or slightly below the bitrate shown. Lowering the audio bitrate frees a little more room for the picture.
How this calculator is checked
Method: Size = bitrate × duration ÷ 8 bits per byte; SI prefixes for bitrates and sizes, IEC binary prefixes for MiB and GiB. The calculation is covered by automated tests with known reference values and edge cases. Last updated 2026-10-18. Found a problem? Tell us.
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