WOFF is a web font, which is a compressed wrapper around a TTF or an OTF. WOFF2 is the second web font format, compressed with Brotli. fontTools sits between the two, and this page says exactly what happens to a WOFF file on the way to becoming a WOFF2 one.
What runs when a WOFF file becomes WOFF2
fontTools in our own Python, on a machine we rent and watch. Your WOFF file is uploaded once, fontTools runs once, the WOFF2 comes back, and neither file is kept. WOFF to WOFF2 is one of the 3649 pairs that engine was probed on with a real file, which is why it has a page here and why the pairs the probe could not prove do not.
What survives from the WOFF into the WOFF2
Every glyph, unchanged. This pair does not redraw anything: the outlines that leave the WOFF arrive in the WOFF2 point for point, so the letters render exactly as the designer drew them. The engine counts them on the way out and refuses the file if the count moved. Measured on this pair: the probe file came back at 258 516 bytes in 2.7 s.
Hinting, kerning pairs and OpenType features: the tables that tell a renderer how to space and substitute glyphs. This pair moves the container around them rather than through them.
Size, and this is the reason to do it. Measured on DejaVu Sans, 6253 glyphs: 759 720 bytes of WOFF become 258 392 bytes of WOFF2, a third of the original. Nothing was removed to get there; it is compression, and the browser undoes it before rendering.
Browser support you do not have to think about. Every browser in use today takes WOFF2, and it is what a modern build pipeline emits by default.
What WOFF to WOFF2 costs you
Nothing about the WOFF itself, and that is worth saying plainly rather than inventing a caveat. What this pair changes is how the font is packaged, never what it draws.
The ability to double-click it. A WOFF2 is not installable: operating systems do not accept it, and design tools do not list it. It is made to be served over HTTP and read by a browser, which is the whole point.
Nothing else, and specifically not quality. This is compression, not resampling: the WOFF2 decompresses to the same bytes the WOFF held.
Where WOFF and WOFF2 files come from
WOFF. A W3C recommendation from 2012, made for one job: shipping a font over HTTP. It wraps an existing TrueType or PostScript font and compresses it with zlib. Measured on DejaVu Sans: 759 720 bytes become 379 328.
WOFF2. A W3C recommendation from 2018 and what every browser prefers today. Same idea as WOFF, better compression: Brotli instead of zlib, plus a transform that rewrites the glyph table before compressing. Measured on DejaVu Sans: 759 720 bytes become 258 392, a third of the original.
If a model is going to read the WOFF2
Structure is what survives from the WOFF file, and structure is what a model needs. WOFF in, WOFF2 out, with the heading tree intact instead of flattened into one long paragraph.
WOFF to WOFF2, measured rather than promised
Running WOFF to WOFF2 against the real engine with a real file, fontTools wrote 258,516 bytes of WOFF2 in 2.7 seconds, machine otherwise idle. That is one WOFF file on one day and not an average, which is why the number is given together with the file that produced it.
The WOFF to WOFF2 verdict was reached by reading the font back with fontTools and checking what kind of outlines it holds, meaning the output was read back and recognised. Which check was used matters, because they do not all prove the same thing, and a status code of 200 proves nothing whatsoever about whether the WOFF2 file has anything inside it.
Other ways into WOFF2, and what they measured
Among the published routes into WOFF2, WOFF is the first largest output of the 3 measured. The witness files differ, so this ranks the probe run and not your document.
TTF to WOFF2: 258,512 bytes in 2.8 seconds.
OTF to WOFF2: 53,724 bytes in 0.5 seconds.
What we will not pretend about WOFF to WOFF2
A WOFF file over 25 MB is refused before the upload finishes rather than after it, so you do not wait for a rejection.
A WOFF to WOFF2 run that passes 50 seconds is killed, and the fontTools process is killed with it. A run left behind would sit on one of the machine's two cores until somebody noticed.
A WOFF wraps either kind of outline, so this pair works for some files and not others. Most web fonts wrap TrueType, which is why WOFF to TTF usually works and WOFF to OTF usually does not. The refusal is immediate, costs nothing, and names the target that would have worked for your file.
One WOFF file at a time, chosen in the browser. There is nothing else to set up and nothing else on offer.
WOFF to WOFF2: what people ask
What actually converts my WOFF file to WOFF2?
fontTools does it, in our own Python, on a machine we rent and watch. Not a browser trick and not somebody else service: the WOFF file is uploaded once, fontTools runs once, the WOFF2 comes back, and neither file is kept afterwards.
How long does WOFF to WOFF2 take?
On the file the probe used, fontTools took 2.7 seconds and wrote 258,516 bytes of WOFF2. That is one real measurement on one real WOFF file, not an average and not a promise about yours: a larger WOFF takes longer, and past 50 seconds the run is stopped.
What do I lose going from WOFF to WOFF2?
The one to know about first: Nothing about the WOFF itself, and that is worth saying plainly rather than inventing a caveat. What this pair changes is how the font is packaged, never what it draws.
Will the WOFF2 look different from the WOFF?
No. Not slightly, not in edge cases: the outlines are copied rather than redrawn, so every glyph has the same coordinates it had before. The engine counts the glyphs on the way out and refuses the file if the count moved, which is the one way this could go wrong silently.
How much smaller is the WOFF2?
Measured on DejaVu Sans, a 6253 glyph font: 759 720 bytes of WOFF become 258 392 bytes of WOFF2. Your font will differ, but the ratio holds: WOFF2 uses Brotli and lands near a third.
Can I install the WOFF2 on my computer?
No, and that is not a limitation of this tool. A WOFF2 is a web format: browsers fetch it, operating systems do not install it. If you want to use the font locally, convert it to TTF or OTF instead.
Is WOFF to WOFF2 free?
There is a free allowance every month, and one WOFF to WOFF2 conversion costs half a credit against it. When the allowance runs out the tool says so and stops, rather than quietly handing you a worse WOFF2.