Every question about video size comes down to one line of arithmetic: bitrate multiplied by time. Once you can do that sum in either direction, you can predict how big a recording will be, work out what bitrate fits a 25 MB email, and understand why a compressor dropped your resolution. The calculator below does both directions; the rest of the page explains the numbers that go into it.
The calculator
128 kbps AAC is typical; 0 for no audio.
About 93.0 MB (88.7 MiB)
(4 Mbps video + 128 kbps audio) × 180 s ÷ 8, plus a little container overhead. Rate-controlled encoders land within a few percent of this.
The calculator uses the same arithmetic as the ClipHeron compressor, including the 4% safety margin and the small container allowance, so a target you work out here matches the prediction line on the tool. The share link carries only the numbers you typed.
The formula
A video file is a stream of bits per second, so:
Size in bytes = (video bitrate + audio bitrate + container overhead) × duration in seconds ÷ 8
Bitrates are in bits per second; dividing by 8 turns bits into bytes. Container overhead (the index, timestamps and headers) is small, about 4 kbps for MP4, and only matters for very low bitrates.
Turned around to find a bitrate for a size:
Video bitrate = (target bytes × 8 × 0.96 − audio bitrate × duration − overhead × duration) ÷ duration
The 0.96 is a safety margin, because real encoders land near their target rather than exactly on it, and platforms count bytes in their own way. If the result is below the minimum sensible bitrate for your resolution (see the table), either lower the resolution or accept a soft picture.
Units trip people up more than the formula does. Mbps is megabits per second; MB is megabytes. A byte is 8 bits, so 8 Mbps sustained for one second is 1 MB. ClipHeron uses decimal units throughout (1 MB = 1,000,000 bytes), which is how platforms state their limits; the result card shows the binary figure (MiB) alongside for anyone comparing with a file manager that uses it.
Typical bitrates by resolution
These are planning figures for H.264 at 30 frames per second. “Camera” is what phones and consumer cameras record; “good” is what looks fine on the device it will be watched on; “minimum” is the floor below which the next resolution down looks better.
| Resolution | Camera | Good | Minimum | Size per minute at “good” (with 128 kbps audio) |
|---|---|---|---|---|
| 2160p (4K) | 45 to 100 Mbps | 14 Mbps | 2.5 Mbps | 106 MB |
| 1440p | 25 to 50 Mbps | 9 Mbps | 1.2 Mbps | 68 MB |
| 1080p | 8 to 20 Mbps | 4 Mbps | 0.6 Mbps | 31 MB |
| 720p | 5 to 10 Mbps | 2 Mbps | 0.3 Mbps | 16 MB |
| 480p | 2 to 5 Mbps | 1 Mbps | 0.15 Mbps | 8.5 MB |
| 360p | 1 to 2 Mbps | 0.6 Mbps | 0.08 Mbps | 5.5 MB |
The gap between “camera” and “good” is where compression lives. Cameras write high bitrates because they cannot afford to get a once-only moment wrong; a compressor working on the finished file can spend far fewer bits for the same visible result. HEVC and AV1 need roughly 30 to 50% less than these H.264 figures at the same quality, which is why phones record in HEVC, but H.264 is what plays everywhere.
Worked examples
How big is a 3-minute 1080p clip at 4 Mbps?
(4,000,000 + 128,000 + 4,000) × 180 ÷ 8 = 92,970,000 bytes, about 93 MB. The audio is 2.9 MB of that; the container about 90 KB.
What bitrate gets a 60-second 1080p clip under 10 MB?
Budget: 10,000,000 × 8 × 0.96 = 76.8 Mb. Audio at 128 kbps for 60 s: 7.68 Mb. Container: 0.24 Mb. Video: (76.8 − 7.68 − 0.24) ÷ 60 = 1.15 Mbps. That is above the 0.6 Mbps minimum for 1080p, so the resolution can stay; expect a slightly soft but perfectly watchable picture. Predicted size: (1.15 + 0.132) Mbps × 60 ÷ 8 ≈ 9.6 MB.
Why a 20-minute recording cannot reach 10 MB
Audio alone at 128 kbps for 1,200 s is 19.2 MB, nearly twice the target. With the audio removed, the whole 76.8 Mb budget over 1,200 s is 64 kbps for video, below even the 360p minimum of 80 kbps. The honest answer is that it cannot be done: trim the recording, or raise the target. A good tool says so before it starts, which is what the compressor’s “can’t reach” message is for.
Constant, variable and target bitrate
Constant bitrate (CBR) spends the same bits every second whether the scene is a still landscape or a crowd scene. It makes sizes perfectly predictable and wastes bits on easy content. Variable bitrate (VBR) lets the encoder spend more on hard scenes and less on easy ones for the same average, which looks better at the same size but can wander from the average by a few percent on unusual content. Constant quality modes (the CRF setting in desktop encoders) fix the quality instead and let the size fall where it may, which is ideal for archiving and useless for hitting a limit.
A target-size compressor uses VBR with an average set from the budget above, then checks the result. ClipHeron runs a second pass automatically if the first overshoots the target by more than 3%, scaling the bitrate by the ratio, and reports the final size honestly either way rather than quietly passing a file that is over the line.
Don’t forget the audio
At everyday video bitrates the audio is a rounding error: 128 kbps of AAC is 3% of a 4 Mbps video. At chat-app sizes it is not. In the 10 MB example above the audio took 10% of the budget; for the 20-minute recording it took all of it and more. Three options when audio is eating the budget: lower it to 96 kbps (fine for speech), remove it if the clip does not need sound, or trim the clip so there is less of everything. The extract audio tool is the other side of the same arithmetic: a 3-minute AAC track at 128 kbps is about 3 MB on its own, which is why pulling the audio out of a video gives such a small file.
Compress a video to a target size
Frequently asked questions
What bitrate should a 1080p video be?
For H.264 at 30 fps, 2 to 4 Mbps looks good on a phone or laptop screen, 5 to 8 Mbps is broadcast-like, and anything above about 12 Mbps is hard to tell from the camera original. At 60 fps add roughly half again.
How big is one minute of 1080p video?
At 4 Mbps video plus 128 kbps audio, about 31 MB. At a phone camera’s 12 Mbps, about 91 MB. At 1 Mbps for a chat app, about 8.5 MB.
What is the difference between Mbps and MB?
Mbps is megabits per second, a rate; MB is megabytes, a quantity. There are 8 bits in a byte, so 8 Mbps for one second is 1 MB. Mixing them up is the commonest reason estimates come out eight times wrong.
Why is my file bigger than the calculator says?
Three usual reasons: the encoder was set to a variable bitrate and the content was complex, so it spent more than the average; the audio bitrate was higher than you assumed; or the container index and metadata added a few hundred kilobytes. Rate-controlled encoders normally land within a few percent of the average they were asked for.
Does resolution change the file size?
Not directly. Size is bitrate times time. Resolution changes how good a given bitrate looks: the same 2 Mbps spread over 4K is far softer than over 720p. That is why target-size compressors lower the resolution when the budget is tight.
Sources
This guide explains how things work in general terms. Platform limits and codec support change; the figures were checked on the date shown above. Spotted something out of date? Email errors@clipheron.com and we will check it against the source.