How the text is added
Your caption is drawn in the browser as a transparent image the size of the GIF, in bold condensed capitals with an outline. FFmpeg then lays it over every frame and builds a new colour palette from the result, because a GIF can only hold 256 colours in each frame and white letters with a black outline need their own.
A GIF keeps its own frames and timing. A video is cut to its first seconds at 12 frames a second, which keeps the file a size chat apps and social posts accept.
Size and width
A GIF narrower than the width you pick keeps its own size; it is never scaled up.
| Width | Good for |
|---|---|
| 320 px | Reaction GIFs in chats, smallest files |
| 480 px | Most posts and messages, a good default |
| 640 px | Sharper text, noticeably bigger files |
Private by design
The GIF is processed on your device with FFmpeg running in WebAssembly and saved from there. Nothing is sent to Percify or anyone else. Files up to 250 MB work in the browser.
Platform facts checked on 17 September 2026 against each platform's own help pages.