The size limit is the spec. Work backwards to the bitrate
A form that accepts 50 MB, a Discord upload of 10, a course platform with 200: you know the number you cannot cross. Enter it together with the running time and you get the one value the encoder actually needs.
The Video Bitrate Calculator finds the bitrate a video needs to hit a target file size, or the file size a bitrate produces for a given length. For a 10-minute video at 128 kbit/s of audio and 2,000 kbit/s of video, the file is about 160 MB.
What the usual ceilings buy you at 1080p
Every limit below is a file size, and a file size means nothing until you divide it by a running time. At the bitrate most people export 1080p with - around 5 Mbps of video plus 128 kbps of audio - a 10 MB upload is a fifteen-second clip, and a 512 MB upload is a thirteen-minute video. Same quality, different ceilings.
That is the whole trick of this calculator: the limit and the length are fixed, so the bitrate is the only thing left to choose. Once you know it, you know whether the export settings you are used to will fit or whether the video has to be shorter, smaller or compressed harder.
| Ceiling | Where | Time at 1080p |
|---|---|---|
| 10 MB | Discord, free tier | 16 seconds |
| 25 MB | most email attachments | 40 seconds |
| 50 MB | Discord Nitro, small forms | 1 min 20 s |
| 100 MB | upload forms, course platforms | 2 min 40 s |
| 512 MB | X (Twitter) | 13 minutes |
| 2 GB | WhatsApp, Telegram | 54 minutes |
Three numbers and one subtraction
The arithmetic is short. A file size in megabytes becomes bits by multiplying by 8 and by 1,048,576 - the number of bytes in a megabyte. Divide that by the length in seconds and you have the total bitrate the file can average, in bits per second. Divide by a thousand once more and you are in the kbps that encoders ask for.
Then subtract the audio, because the sound is inside the same budget. A 3.4 Mbps total with 128 kbps of audio leaves 3.27 Mbps for the picture. Subtract a further two percent for the container, the index and the metadata, and what is left is the number to type into the video bitrate field.
The result is an average, not a ceiling. A variable bitrate encoder spends more on the fast scene and less on the talking head, which is exactly what you want; a constant bitrate encoder spends the same everywhere and looks worse in both. If your encoder offers "average bitrate" or "two-pass", that is the mode this number is meant for.
- check_circleKnow the real limit, not the one you remember
- check_circleUse the exact running time, including the fade out
- check_circleDecide the audio bitrate first - it is part of the budget
- check_circlePut the number in the video bitrate field, not the total
- check_circleCheck the finished file size before uploading
- check_circleKeep the master: a re-export always loses a little
From a limit to a setting
Enter the ceiling and the length
The size the platform accepts and the real duration of the cut. Both are known before you open the encoder.
Pick the audio bitrate
128 kbps is fine for speech, 192 for music, 64 only for a voice note. It comes out of the same budget.
Read the video bitrate
That is the number for the encoder. The list underneath shows which resolutions it comfortably covers.
What changes the number without changing the size
H.265 (HEVC) reaches the same visible quality as H.264 at roughly half the bitrate, and AV1 goes further on static scenes. If the target platform accepts them, a 4 Mbps H.265 export can look like an 8 Mbps H.264 one - which doubles the length that fits in the same ceiling.
The catch is compatibility and encoding time. Older phones, some browsers and a few editors cannot open HEVC, and the export takes longer on the same machine. For a clip that will be watched inside one app, check that app's accepted codecs first; for anything that has to open anywhere, H.264 is still the safe answer.
Sixty frames per second costs bitrate for the same visual quality - usually twenty to fifty percent more than thirty, depending on the motion. A talking head gains nothing from 60 fps; gameplay, sport and anything with fast camera movement does.
If a clip does not fit and the frame rate was a habit rather than a decision, try 30 fps before you touch the resolution. Dropping from 60 to 30 often saves more than dropping 1080p to 720p, and it damages the picture less on a screen where the extra frames were barely visible.
A single-pass variable bitrate export guesses how hard each scene will be while it encodes. Two-pass encodes the whole file once to learn where the difficult parts are, then spends the budget accordingly. On a short clip the difference is small; on a long one with mixed scenes it is visible, especially in the dark scenes and the transitions.
Constant bitrate is the wrong tool here: it spends the same amount on a static title card as on a fireworks finale, so the simple parts look wasteful and the hard parts look broken. Use the average from this calculator with a variable-bitrate mode.
Questions about the number
Why is the bitrate lower than what my encoder suggests?expand_more
The result is negative. What does that mean?expand_more
Should I count the audio in the budget?expand_more
How exact is the two percent container overhead?expand_more
Does a higher bitrate always look better?expand_more
Can I use this for audio-only files?expand_more
One number, typed once
Enter the ceiling and the length, read the bitrate, export. No trial and error uploads.
fact_checkQuick facts
- File size in megabytes is (video bitrate + audio bitrate) in kbit/s, multiplied by the seconds, divided by eight and by 1024.
- Audio takes its share of the budget: a 128 kbit/s track in a 10-minute video occupies about 9.4 MB of the total.
- H.264 is the codec that made HD video practical on the web: it is decoded by every current browser and phone without a plugin.menu_bookITU-T
- Lowering the bitrate of H.264 throws away detail in fast movement first, which is why sport and confetti are the hardest scenes to compress.menu_bookITU-T
- A 128 kbit/s stereo MP3 uses about 0.9 MB of storage per minute — 128,000 bits each second, divided by eight.menu_bookFraunhofer IIS
helpQuestions people ask
How do I calculate the bitrate for a target file size?expand_more
What is a good bitrate for YouTube?expand_more
Does a higher bitrate always mean better quality?expand_more
hubWhat this page is about
- H.264 / AVCopen_in_new
The video codec standardised by ITU-T as H.264 and by ISO/IEC as MPEG-4 AVC, decodeable by every current browser.
- MP3open_in_new
A lossy audio format from MPEG-1 Audio Layer III, developed at Fraunhofer IIS, that removes sounds the ear is unlikely to notice.
library_booksOfficial sources cited on this page
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