Tool
Video size calculator
Two questions, one formula: size = bitrate × duration. Work out what fits under a cap, or how big a video will turn out.
A video's file size is its total bitrate multiplied by its length, divided by 8. To fit a size limit, turn it around: the limit in megabytes × 8 ÷ the length in seconds gives the most bitrate the video can use, in megabits per second. A 60-second clip under a 20 MB limit can average about 2.6 Mbps, sound included, after the 3% this calculator sets aside for the container. Nothing you type here leaves your browser.
How it works
A video's size is its bitrate — how much data it uses each second — multiplied by how many seconds it runs. Bitrate is measured in bits, and file size in bytes, and there are 8 bits in a byte:
size in bytes = (video bitrate + audio bitrate) × seconds ÷ 8
bitrate that fits = size in bytes × 8 ÷ seconds − audio bitrateThis calculator uses the same units as Ninja and as most platforms' help pages: 1 MB = 1,000,000 bytes. A few systems count 1,048,576 bytes to the megabyte; the difference is under 5%, and the decimal figure is the safer one. It also sets 3% aside for the container's own headers and indexes.
The resolution suggestion
More pixels need more bitrate to look equally clean. The suggestion is the largest resolution that gets at least 0.045 bits per pixel per frame — a rule of thumb for H.264 that looks clean at phone size. HEVC and AV1 look good with less, so treat the suggestion as conservative: if the video will be watched on a phone, the next resolution up is often fine.
Worked example
A 90-second clip at a 16 MB target: 16,000,000 × 8 × 0.97 ÷ 90 ≈ 1.38 Mbps in total. Take away 128 kbps for audio and about 1.25 Mbps is left for the picture — just enough for 720p at 30 fps by the rule of thumb above. That's exactly the combination Ninja's WhatsApp preset aims for, well under the 64 MB and 100 MB defaults in WhatsApp's Help Center. More on how to send a long video on WhatsApp.
Common video size limits
The limit you're aiming at decides the bitrate. These are the figures our guides track, each checked against the platform's own help pages; follow the link for the date and source.
| Where it's going | Limit | Guide |
|---|---|---|
| WhatsApp video (default) | 100 MB at 720p on faster connections, 64 MB at 480p on slower ones; up to 2 GB sent as a document | |
| Discord, free account | 20 MB | Discord |
| Gmail and Yahoo Mail attachments | 25 MB | |
| Outlook attachments | 10 to 25 MB, depending on the account and app |
The same arithmetic covers other limits: fit a video under Discord's 20 MB limit, email a large video without it bouncing, or check the Instagram and TikTok video size limits.
Frequently asked questions
How do I calculate video file size?
Multiply the total bitrate (video plus audio) by the duration, then divide by 8 to turn bits into bytes. A 2 Mbps video that runs for 60 seconds is 2,000,000 × 60 ÷ 8 = 15,000,000 bytes, or 15 MB — plus a little for the container.
What bitrate fits a video under a size limit?
Divide the limit in bits by the duration in seconds, then subtract the audio bitrate. For 16 MB and 60 seconds: 16,000,000 × 8 ÷ 60 ≈ 2.13 Mbps in total, so about 2 Mbps for the picture with 128 kbps audio.
How do I make a video a specific file size?
Re-encode it at the bitrate that fits, which the calculator above works out from the limit and the length. A video compressor with a target-size mode, such as Ninja on iPhone, does the arithmetic for you: you type the size and it picks the bitrate.
Why is my file slightly bigger than the calculation?
Containers like MP4 add indexes and headers, and encoders overshoot a target slightly. This calculator sets 3% aside for that, which is why its answers are a touch conservative.