Formats
Every format, in plain words.
What each format is, what it keeps and what it costs, with numbers we measured on this site. Ten pages, one for each format the converter reads or writes.
MP3 is explained below. We read it and we never write it.
Pictures
Five formats for pictures, from the one every device opens to the smallest one.
Video files
Two containers. What matters is the video and sound inside them.
Audio files
Three formats, from uncompressed to small.
Good to know
How to read these pages.
Four ideas explain most of what the format pages say.
Lossless and lossy
A lossless format stores exactly what went in, so you can convert it as often as you like. PNG, FLAC and WAV are lossless. A lossy format throws away what people are least likely to notice, which is how JPG, WebP, AVIF and M4A get small, and each lossy save throws away a little more.
The rule that follows is short: keep the original, and make lossy copies for sharing. Never convert a lossy file back to a lossless one to restore quality. It only makes it larger.
Containers and codecs
A container is a box, and a codec is how the contents are packed. MOV, MKV and MP4 are boxes for video. H.264, HEVC and AAC are codecs. Two files with the same extension can behave very differently when the codecs inside differ, which is why some MOV files convert instantly and others take time.
MP3: read, never written
Flex Convert opens MP3 files and can turn them into other formats. It does not write MP3. Browsers have no MP3 encoder, and the free encoders that exist carry licence conditions that do not suit a commercial product, so we do not include one. M4A at 192 kb/s is the usual substitute and plays almost everywhere.
If an MP3 is the only thing that will do, a desktop audio tool is the right place to make it.
Where the numbers come from
Every size and time on these pages was measured by a script that opens the real page in a real browser, drops a test file we generated, saves the result and checks it with other tools. The tables say which browser and which day. We never use someone else's photographs or music, and we do not publish a figure we did not measure.
Chrome and WebKit were tested. WebKit here is Playwright's build, which is not Safari, and Firefox was not tested.