Which videos work
If your browser can play it, GIFsize can convert it. MP4 (H.264) and WebM work everywhere; iPhone MOV files open in Safari and recent Chrome. Screen recordings from OBS, ShareX or the Windows Snipping Tool are ideal sources — their still backgrounds compress extremely well as GIF.
Already have a GIF that’s too heavy? That’s a different job, covered on the Compress GIF page.
The four settings that matter
| Setting | What it changes | Good starting point |
|---|---|---|
| Width | Sharpness — and the biggest effect on size | 640px for docs and stores, 480px for chat |
| Frame rate | How smooth motion looks | 12fps; 15fps for fast action |
| Colors | Smoothness of gradients | 256; 128 is enough for UI recordings |
| Dithering | Banding between colors | Pattern; Diffusion for photo-like footage |
Or skip all of it: choose a target size (for example “X (Twitter) web · 15MB”) and GIFsize tries width and frame-rate combinations until the estimate fits, keeping a safety margin.
How the estimate works
When you add a file, the whole clip is run once at low resolution while a few scenes are compressed at full size, and the two are combined. The result includes a 10% safety margin, and if a finished GIF still goes over your target it’s automatically re-made one step smaller.