Video Size Estimator
Estimate the final file size of a video based on its bitrate and duration.
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Estimated File Size
This is an estimate for constant bitrate (CBR). Variable bitrate (VBR) files may differ. Presets follow typical H.264 values.
A video's file size is not a mystery — it is almost entirely determined by one number, the bitrate, multiplied by the duration. Bitrate is how much data the video uses per second, and it is the single lever that controls the trade-off between quality and file size. Understanding this relationship is what lets you plan a file to fit a size limit, estimate an upload before it finishes, or diagnose why a short clip is somehow gigabytes. This calculator turns bitrate and duration into an expected file size, and works backward too, so you can find the bitrate that fits a video into a target size.
How the calculator works
Enter the video's bitrate and its duration, and the calculator multiplies them to estimate the final file size. It handles the unit conversions that trip people up — bits versus bytes, and the factor of eight between them — so you do not have to. It also runs in reverse: given a target file size and a duration, it tells you the maximum bitrate that fits, which is exactly what you need when a platform imposes an upload size limit and you must re-encode to meet it.
The formula
File size (MB) = Bitrate (Mbps) × Duration (s) ÷ 8
The ÷ 8 converts bits to bytes
(8 bits = 1 byte)
Example: 10 Mbps video, 60 seconds
10 × 60 ÷ 8 = 75 MB
With audio: add audio bitrate to video bitrate
before multiplying (e.g. 10 Mbps + 0.128 Mbps)The factor of eight is where nearly every manual estimate goes wrong. Bitrate is quoted in megabits per second (Mbps), but file size is measured in megabytes (MB), and a byte is eight bits — so you must divide by eight to convert. Forget it, and your estimate is eight times too large. The other subtlety is that total bitrate includes audio: a 10 Mbps video with 128 kbps audio is really 10.128 Mbps, small but not zero. For long files even the audio track adds up.
What to know about video file size
- 1Bitrate, not resolution, is the real driver of file size. A 4K video at a low bitrate can be smaller than a 1080p video at a high one. Resolution sets the ceiling for detail, but bitrate decides how much of that detail is actually stored — which is why two videos at the same resolution can differ several-fold in size.
- 2Match bitrate to resolution, or you waste one of them. Too low a bitrate for a high resolution produces blocky, smeared footage; too high a bitrate for a low resolution is wasted data storing detail the resolution cannot hold. Each resolution has a sensible bitrate range, and staying within it is how you get efficient files that look their best.
- 3Modern codecs cut file size dramatically at the same quality. H.265 (HEVC) and AV1 achieve the same visual quality as older H.264 at roughly half the bitrate, so re-encoding with a newer codec can halve a file without visible loss. When file size is the constraint, the codec is often a bigger lever than the bitrate setting itself.
- 4Variable bitrate makes estimates approximate. Most real videos use variable bitrate, spending more data on complex, fast-moving scenes and less on static ones, so the actual size varies around the average this calculator uses. The estimate is reliable for planning, but the final file can land somewhat above or below depending on the footage's complexity.
- 5To fit a size limit, work backward from the target. When a platform caps uploads at, say, 100 MB, use the reverse calculation: divide the target size by the duration to find the maximum bitrate, then encode at or just below it. This is far more reliable than guessing a bitrate and hoping the file comes in under the limit.
Frequently asked questions
How do I calculate a video's file size?
Multiply the bitrate by the duration, then divide by eight to convert bits to bytes. A 10 Mbps video running 60 seconds is 10 × 60 ÷ 8 = 75 MB. Add the audio bitrate to the video bitrate before multiplying if you want to include the audio track. This calculator does the arithmetic and unit conversion for you, which avoids the factor-of-eight error that makes most manual estimates eight times too big.
Why is my video file so large?
Almost always because of a high bitrate, not the resolution or duration alone. Bitrate is the amount of data stored per second, so a high setting produces a large file regardless of how the video looks. Other culprits are an older codec like H.264 storing data inefficiently, or a bitrate far higher than the resolution needs. Lowering the bitrate to match the resolution, or re-encoding with a modern codec, usually shrinks the file substantially.
What is the difference between bitrate and resolution?
Resolution is the number of pixels — 1920×1080, 3840×2160 — which sets the maximum possible detail. Bitrate is how much data is used to store that picture each second, which decides how much of the potential detail is actually kept. A high resolution at a low bitrate looks soft and blocky because there is not enough data to fill the pixels well. Both matter, and matching them is what produces an efficient, good-looking file.
How do I make a video fit under a size limit?
Work backward from the limit using the reverse calculation: divide the target file size (in the right units) by the duration to get the maximum bitrate that fits, then re-encode at or just below that bitrate. For example, to fit a 5-minute video into 100 MB, you compute the bitrate ceiling and encode to it. Using a modern codec like H.265 or AV1 lets you hit that size with better quality than an older codec would allow.
Does the audio track affect the file size?
Yes, though usually less than the video. Audio has its own bitrate — commonly 128 to 320 kbps — which adds to the video bitrate for the total. On a short high-bitrate video the audio is a small fraction, but on a long video, or one with a low video bitrate, the audio track becomes a meaningful share of the total size. For an accurate estimate, add the audio bitrate to the video bitrate before multiplying by duration.