Video bitrate explained: how it sets file size and quality

Bitrate is how much data a video uses per second, usually given in megabits per second (Mbps). It decides file size directly: size equals bitrate times duration, divided by 8 to turn bits into bytes. The video file size calculator does that arithmetic in both directions, and this guide explains the formula, how bitrate relates to quality and why real files never match exactly.

What bitrate means

A bit is the smallest unit of digital data, and a byte is 8 bits. Video bitrate is quoted in bits per second: kilobits (kbps, thousands) for audio and megabits (Mbps, millions) for video. File sizes are quoted in bytes: megabytes (MB) and gigabytes (GB). The capital B for bytes and lower-case b for bits look alike but differ by a factor of 8, which is the most common source of confusion. A file's total bitrate is the video bitrate plus the audio bitrate plus a small amount of container overhead.

The file size formula

File size in megabytes is about total bitrate in Mbps times duration in seconds, divided by 8. Some worked examples:

• Ten minutes at 8 Mbps video plus 128 kbps audio: 8.128 × 600 ÷ 8 = 609.6 MB.
• One minute of 1080p at YouTube's recommended 8 Mbps, with 384 kbps stereo audio: 8.384 × 60 ÷ 8 = about 63 MB.
• One minute of phone video at about 17 Mbps with 192 kbps audio: roughly 129 MB.

Storage works the same way in reverse. A 128 GB card holds 128,000 MB, so at 50 Mbps video and 192 kbps audio it fits about 128,000 × 8 ÷ 50.192 = 20,400 seconds, or 5 hours 40 minutes. Note that drive makers use decimal units; your computer may report 609.6 MB as about 581 MB because it counts in units of 1,048,576 bytes.

Working back from a size limit

The same formula tells you what bitrate fits a limit. To fit a three-minute clip into a 25 MB email attachment: 25 × 8 ÷ 180 = 1.11 Mbps in total, and after 128 kbps of audio about 0.98 Mbps is left for video. That is too little for crisp 1080p, so it is better to lower the resolution as well, giving each remaining pixel more bits. The calculator shows the bitrate for a target size, and the video compressor applies a lower resolution and bitrate together. Trimming is the most effective step of all: the video trimmer cuts what you don't need, and half the duration is half the file.

Bitrate and quality

Uncompressed 1080p video at 30 fps, with 8-bit colour and the 4:2:0 chroma subsampling used for most delivery, would run at about 746 Mbps. At 8 Mbps the encoder is therefore discarding more than 98 per cent of the raw data, which it can do because most of each frame repeats the last one. How much bitrate a video needs depends on three things. Resolution and frame rate: more pixels per second need more bits. Content: a static talking head compresses easily, while water, confetti, foliage and film grain change constantly and need far more. Codec: newer codecs such as HEVC, VP9 and AV1 typically reach the same quality as H.264 at a noticeably lower bitrate. How video compression works: bitrate, keyframes and codecs explains where the savings come from.

Why real files differ

Few encoders hold one constant bitrate. Variable bitrate spends more bits on complex scenes and fewer on simple ones, and quality-based modes such as the constant rate factor in x264 aim for consistent quality and let the bitrate float entirely. Real files therefore often land 10 to 20 per cent away from a simple estimate, and container overhead adds a little more. Video info shows a file's actual overall bitrate, its size in bits divided by its duration, which is the best way to learn what your own camera produces.

Typical bitrates

YouTube recommends these video bitrates for standard-dynamic-range uploads at 24 to 30 fps, with higher figures at 48 to 60 fps:

• 720p: 5 Mbps (7.5 at high frame rates);
• 1080p: 8 Mbps (12);
• 1440p: 16 Mbps (24);
• 4K: 35 to 45 Mbps (53 to 68).

It recommends 384 kbps for stereo audio. Video calls use far less, often around 1 to 2 Mbps, while phones recording 4K can use 50 Mbps or more. WebM vs MP4: containers, codecs, and which one to send covers the formats these bitrates are delivered in.