Bitrate, resolution and file size: how they fit together

File size is bitrate multiplied by time

Bitrate is the amount of data used for each second of video, measured in megabits per second (Mbps). To get the file size in megabytes, multiply the bitrate by the length in seconds and divide by 8, because there are 8 bits in a byte.

Size in MB = bitrate in Mbps × seconds ÷ 8

A one minute video at 8 Mbps is 8 × 60 ÷ 8 = 60 MB. Nothing else enters into it. Resolution, frame rate and codec do not change the size of a file directly. They change how good the video looks at a given bitrate.

Sound adds a little on top. Typical phone audio is 128 to 256 kbps, which is 1 to 2 MB per minute.

Resolution: how many pixels share the data

A fixed bitrate has to describe every pixel in the frame. More pixels means less data for each one.

This is why lowering the resolution is often the right move when you need a small file. At 1 Mbps a 1080p video breaks into visible blocks, because two million pixels are sharing very little data. The same 1 Mbps spread over a 720p frame looks clean. A smaller sharp picture beats a larger blocky one, especially on a phone screen.

Frame rate

A 60 frames per second video has twice as many pictures to describe as a 30 frames per second one. It does not need twice the bitrate, because neighbouring frames are more alike, but it does need noticeably more. If you are short of data, 30 frames per second is the better use of it for most footage.

What is in the picture matters

Video compression works by describing what changed since the last frame. Footage where little changes is cheap. Footage where everything changes is expensive.

• Cheap: a person talking to a camera on a tripod, a screen recording, a slideshow.
• Expensive: handheld walking shots, water, leaves in wind, confetti, crowds, rain.
• Surprisingly expensive: dark scenes. Camera noise in low light changes on every frame, and the encoder has to spend data on it.

Two videos at the same bitrate and resolution can look very different for this reason. If a compressed video looks rough, the footage may simply need more data than you gave it.

Codec

The codec is the method used to compress the video. H.264 is the one practically every device can play. HEVC, also called H.265, reaches similar quality at a noticeably lower bitrate, and many phones record with it to save space, but some older devices and apps cannot open it. When you are sending a video to other people, H.264 is the safe choice.

Working back from a size limit

Turn the formula around and a size limit gives you a bitrate.

Bitrate in Mbps = size in MB × 8 ÷ seconds

Say you need a 90 second video under 16 MB. 16 × 8 ÷ 90 = 1.42 Mbps for everything. Take away about 0.13 Mbps for the sound and 1.3 Mbps is left for the picture. That is too little for 1080p, and comfortable for 720p. So the answer is 720p at about 1.3 Mbps.

The video compressor does this sum for you when you choose Fit under a file size. It works out the bitrate from your target and the length of the video, then picks the largest resolution that still looks reasonable at that bitrate.

The short version

• Size depends on bitrate and length only.
• Shorter videos can afford more quality at the same size, so trim first.
• When the bitrate has to be low, reduce the resolution to match.
• Use 30 frames per second unless smooth motion is the point.
• Busy, shaky or dark footage needs more data than calm, bright footage.