Compress 4K Video

Why 1080p shrinks a 4K file so much

A 3840 × 2160 frame has four times the pixels of 1920 × 1080. The target this page sets is worked out per pixel, so at 1080p it is about a quarter of what the same clip would get at 4K. For 1080p at 30 fps in MP4 that is about 4 Mbps of video.

Frames above 30 fps are counted at half weight when the target is worked out, so a 60 fps clip gets 1.5 times the 30 fps figure. At the same target size, setting Frame rate to 30 fps under Advanced spreads the bits over half as many frames.

Screen size lists Original with its resolution, then 1440p, 1080p, 720p and the lower steps. The Target size buttons (10%, 30%, 50%, 70%) each show the size they give. If a size needs a lower resolution than 1080p, the panel picks it and frames that button in dashed amber.

Large files and HEVC sources

A long 4K clip can run to several GB. That works here: the file is read from your disk and the output is written to disk as it is produced, so memory use does not grow with the file. The screen is kept awake while compressing, and the progress window shows elapsed and remaining time.

An iPhone set to High Efficiency records HEVC (H.265) (Apple Support). Your browser can only read HEVC if the device has a hardware HEVC decoder; Chrome has no software HEVC decoder. On some Macs, Chrome stops decoding certain H.264 files partway through. The page then restarts with software decoding, replaces frames that still cannot be decoded with the previous frame, and states how many were replaced.

Camera audio with more than 2 channels is mixed down to stereo if it is re-encoded. Only the first audio track is kept.

Keeping 4K

If you plan to edit or crop the footage later, keep a 4K copy. Tap Original under Screen size and pick a target size; the file gets smaller and the resolution stays. The original file is never modified. If only part of the clip matters, cut it first with Trim Video ↗.

Frequently asked questions