How to Calculate Video File Size From Bitrate (With Examples)

By HSP · Published October 21, 2026 · 6 min read · Productivity & Skills

"Why is this 40-second video 90 MB?" is one of the most common questions people ask when a clip will not attach to an email or upload fast enough. The answer is almost never the resolution alone. A video's size depends on one number more than any other: its bitrate, the amount of data used for every second of picture and sound. Once you know that number, the file size is simple arithmetic, and you can predict it before you export, instead of guessing and trying again.

The video file size formula

Bitrate is measured in bits per second, usually megabits per second (Mbps) for video and kilobits per second (kbps) for audio. There are 8 bits in a byte, so:

File size (MB) = (video bitrate + audio bitrate in Mbps) × duration in seconds ÷ 8

Two quick notes before the examples. First, 128 kbps of audio is 0.128 Mbps, so remember to convert. Second, this gives the size in megabytes of 1,000,000 bytes. Some computers report sizes in "binary" megabytes (1,048,576 bytes), so the number you see on screen can be about 5% smaller than your result. The real file is also a little bigger than the formula, because the container (MP4, MOV) adds a small amount of extra data such as timing and index information.

Worked examples

The video bitrates below are the reference figures YouTube publishes for standard-frame-rate (24 to 30 fps) uploads, with 128 kbps audio added:

Notice how the 4K minute is five times bigger than the 1080p minute, even though both last 60 seconds. Length and bitrate multiply, so doubling either one doubles the file.

The mental shortcut: bitrate × 7.5

A minute has 60 seconds, and 60 ÷ 8 = 7.5. So for any video, MB per minute ≈ total Mbps × 7.5. At 8 Mbps that is 60 MB a minute; at 2 Mbps it is 15 MB a minute; at 20 Mbps it is 150 MB a minute. It is easy to do in your head when you are deciding whether a clip will fit somewhere.

Work backwards: find the bitrate for a size limit

This is where the formula really helps. Rearranged, it tells you the highest bitrate that fits a limit:

Total bitrate (Mbps) = target size (MB) × 8 ÷ duration (seconds)

Say you want to email a 2-minute clip from a personal Gmail account, where the attachment limit is 25 MB:

1. 25 × 8 ÷ 120 = 1.67 Mbps in total.
2. Keep about 5% spare for the container: 1.67 × 0.95 ≈ 1.58 Mbps.
3. Take away the audio: 1.58 − 0.128 ≈ 1.45 Mbps for the picture.

About 1.45 Mbps is a sensible bitrate for 480p, but it is tight for 1080p, where the picture would look blocky. So for this clip, either lower the resolution, or make it shorter. Cutting it to 1 minute doubles the bitrate you can afford to about 3 Mbps. Our video compressor lets you choose a smaller size and resolution, and the video trimmer cuts the parts you do not need first, all in your browser without uploading the file.

Find the bitrate of a video you already have

The formula also works the other way round: bitrate (Mbps) = size (MB) × 8 ÷ seconds. A 150 MB clip that lasts one minute has an average bitrate of 150 × 8 ÷ 60 = 20 Mbps. That tells you instantly that compressing it to around 5 Mbps would cut it to roughly a quarter of the size. On Windows, the Details tab of a file's Properties also shows the data rate and total bitrate; on a Mac, QuickTime Player's Movie Inspector shows the data rate.

What changes the bitrate you need

• Resolution: more pixels need more data to look sharp. That is why recommended bitrates climb steeply from 480p to 4K.
• Frame rate: 60 fps holds twice as many pictures per second as 30 fps, so YouTube suggests about 50% more bitrate for high frame rates (12 Mbps instead of 8 Mbps at 1080p).
• Motion and detail: sport, confetti, water and shaky handheld footage are harder to compress than a talking head against a plain wall. Two clips at the same bitrate can look very different.
• Codec: newer codecs such as HEVC (H.265) and AV1 usually reach the same quality at a lower bitrate than H.264, but not every device or app plays them.
• Audio: usually small. Even 384 kbps stereo is under 3 MB a minute. If the sound does not matter, removing the audio saves a little extra.

Shape matters too

For Reels, Shorts and TikTok, a vertical 9:16 frame such as 1080 × 1920 has exactly as many pixels as a 1920 × 1080 landscape video, so it needs about the same bitrate. If you are converting a landscape clip for vertical apps, our resize video for Reels tool fits it into a 9:16 frame, and the aspect ratio calculator works out matching sizes. For still pictures, the same "fewer bytes for the same look" idea is covered in our guide to reducing image size without losing quality.

VSH take: decide the size first, then export

Most people export at whatever the default is, find the file is too big, and then compress it again, which costs quality twice. We think it is far better to start from where the video is going: an email limit, a chat app, a slow connection or a phone that is almost full. Do the 30-second sum, choose the bitrate, and export once. Planning a trip video to share with family? Grab cheap fares on our flight deals hub, then use the shortcut above so the highlights reel actually sends. Students making a project video can check deadlines on our exams hub and plan revision on the study pages too.

Bitrates in this guide are reference figures for planning; real file sizes vary with the encoder, codec, content and settings. Platform limits change, so check the current limit in the app or service you are using. ViralScopeHub is not affiliated with YouTube, Google or any app mentioned here.