GIF Compressor

Because a GIF has no quality setting to slide. Unlike a JPEG it is already compressed without loss, from a palette of at most 256 colours, so there is nothing to turn down — anything that makes the file smaller has to remove frames, colours or pixels. A tool showing you one "compression" dial is choosing among those three on your behalf without telling you which it took.

Not here. Each kept frame takes on the time of the frames it replaces, so the animation runs for exactly as long as it did and a loop still lines up. Plenty of tools skip this, which is why a "compressed" GIF often comes back looking twitchy — it is not the same animation, it is a shorter one played at speed.

The estimate updates as you change the settings, and the real figure is shown after it runs. They will not match exactly, and that is honest rather than sloppy: how well a GIF compresses depends on the picture, so two files with identical dimensions and frame counts can differ threefold. The estimate is reliable about the shape of the trade — halving the frames roughly halves the file — rather than about the byte.

No. It is decoded, rewritten and re-encoded in your browser — which is unusual for a compress gif online tool, where the normal arrangement is an upload, a queue behind other people's files and a watermark. There is none of that here, and nothing on our side to store or delete.

Frames, almost always. Most GIFs are recorded at far more frames per second than the eye needs for the kind of motion they usually contain, and keeping every second frame roughly halves the file with very little visible difference. Colours next — most clips look the same at 64 as at 256 unless they contain a gradient. Size last, because it is the one you always notice.

Occasionally it will, and the usual cause is that the original used one palette for the whole animation while this writes one per frame — which looks better and can cost more. If that happens, drop the colour count: at 64 or 32 the per-frame palettes are small enough that the trade goes back the other way.

Yes. Transparent pixels stay transparent through the decode and the re-encode. What is not preserved is partial transparency, because GIF has never supported it — a pixel is either fully see-through or fully opaque, in the original as much as in the result.

Up to 30 MB, which is a very large GIF indeed. The limit is memory rather than policy: a GIF decodes to full uncompressed frames, so a few megabytes of file becomes hundreds of megabytes of pixels, and that is what the browser has to hold while it works.