Found a bug, want a format we don't support yet, or have an idea? We'd love to hear from you.
When reporting a problem, mention your browser and the tool you used. Please don't send copyrighted media: a short sample that shows the problem is enough.
Yes. Every tool on FastSub AI is free, with no sign-up, no watermark and no usage limits.
No. Your files are read and processed inside your browser tab. The only downloads are the app itself and, for AI tools, the model files.
The AI runs on your own device, so the model has to be on it. It downloads once and your browser keeps it cached, so later runs start straight away and even work offline.
Any modern browser works. AI tools are fastest in Chrome or Edge on a desktop with WebGPU; other browsers fall back to the CPU, which is slower but works.
SRT, WebVTT, ASS/SSA, SBV, SUB (MicroDVD), SAMI, TTML/DFXP, LRC and CapCut projects in; all of those plus TXT, CSV, JSON, FCPXML, Word and PDF out.
Subtitle files up to 20 MB and videos up to several GB, limited only by your device's memory.