How Video Compression Actually Works

"Compress the video" sounds like one action, but a video file's size comes from four independent factors, and a compressor is really adjusting some combination of them.

Resolution

The pixel dimensions of each frame. Dropping from 1080p to 720p reduces the number of pixels per frame by more than half, which is one of the most effective ways to shrink a file, but it's a visible, permanent change to the video's sharpness.

Bitrate

How much data is used to encode each second of video, usually measured in megabits per second. Bitrate and file size are directly proportional. Half the bitrate is roughly half the file size for the same duration. Lower bitrate means the codec has less budget to represent detail and motion, showing up as blockiness or blur, especially in fast-moving or high-detail scenes.

Codec

The compression algorithm itself. Newer codecs (H.265/HEVC, AV1, VP9) squeeze more visual quality out of the same bitrate than older ones (H.264), because they use more sophisticated techniques to predict and encode frames. Switching codecs without touching resolution or bitrate can still shrink a file meaningfully, but newer codecs are also slower to encode and not universally supported by every player or editor.

Frame rate

How many frames are captured per second. Dropping from 60fps to 30fps roughly halves the number of frames that need encoding, cutting file size, at the cost of motion looking noticeably less smooth, which matters more for fast action than for talking-head or static content.

Why every lossy compression changes something

These four levers all trade file size against a specific, identifiable kind of visual quality, sharpness, smoothness, detail in motion, or fine texture. There's no setting that shrinks a file "for free"; a compressor is always giving something up, and a good result is about choosing which quality loss matters least for a given video, not avoiding loss entirely.

Compress Video on this site uses one fixed quality setting rather than exposing these controls individually. It re-encodes at a lower bitrate automatically rather than letting you dial in resolution or codec by hand.