TTF is a TrueType font, the kind almost every system ships with. WOFF is a web font, which is a compressed wrapper around a TTF or an OTF. fontTools sits between the two, and this page says exactly what happens to a TTF file on the way to becoming a WOFF one.
What runs when a TTF file becomes WOFF
fontTools in our own Python, on a machine we rent and watch. Your TTF file is uploaded once, fontTools runs once, the WOFF comes back, and neither file is kept. TTF to WOFF 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 TTF into the WOFF
Every glyph, unchanged. This pair does not redraw anything: the outlines that leave the TTF arrive in the WOFF 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 379 328 bytes in 0.1 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 TTF become 379 328 bytes of WOFF, half the original. Nothing was removed to get there; it is compression, and the browser undoes it before rendering.
What TTF to WOFF costs you
Nothing about the TTF 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 WOFF 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 WOFF decompresses to the same bytes the TTF held.
Where TTF and WOFF files come from
TTF. Everything that makes fonts writes it, and every operating system installs it. Its outlines are quadratic curves, which is what distinguishes it from OTF and what decides which conversions are possible at all.
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.
If a model is going to read the WOFF
Structure is what survives from the TTF file, and structure is what a model needs. TTF in, WOFF out, with the heading tree intact instead of flattened into one long paragraph.
TTF to WOFF, measured rather than promised
Running TTF to WOFF against the real engine with a real file, fontTools wrote 379,328 bytes of WOFF in 0.1 seconds, machine otherwise idle. That is one TTF file on one day and not an average, which is why the number is given together with the file that produced it.
The TTF to WOFF 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 WOFF file has anything inside it.
Other ways into WOFF, and what they measured
Among the published routes into WOFF, TTF is the first largest output of the 3 measured. The witness files differ, so this ranks the probe run and not your document.
WOFF2 to WOFF: 377,532 bytes in 0.8 seconds.
OTF to WOFF: 60,904 bytes in 0.2 seconds.
What we will not pretend about TTF to WOFF
A TTF file over 25 MB is refused before the upload finishes rather than after it, so you do not wait for a rejection.
A TTF to WOFF 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.
This family will not change the kind of outlines a font carries, and that rules out TTF to OTF. A TrueType font draws with quadratic curves, a PostScript one with cubic curves, and turning one into the other means approximating every curve in the file. It can be done, it moves points, and it is not what happens here. Asking for it returns 415 with the name of the target that would have worked.
One TTF file at a time, chosen in the browser. There is nothing else to set up and nothing else on offer.
TTF to WOFF: what people ask
What actually converts my TTF file to WOFF?
fontTools does it, in our own Python, on a machine we rent and watch. Not a browser trick and not somebody else service: the TTF file is uploaded once, fontTools runs once, the WOFF comes back, and neither file is kept afterwards.
How long does TTF to WOFF take?
On the file the probe used, fontTools took 0.1 seconds and wrote 379,328 bytes of WOFF. That is one real measurement on one real TTF file, not an average and not a promise about yours: a larger TTF takes longer, and past 50 seconds the run is stopped.
What do I lose going from TTF to WOFF?
The one to know about first: Nothing about the TTF 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 WOFF look different from the TTF?
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 WOFF?
Measured on DejaVu Sans, a 6253 glyph font: 759 720 bytes of TTF become 379 328 bytes of WOFF. Your font will differ, but the ratio holds: WOFF uses zlib and lands near a half.
Can I install the WOFF on my computer?
No, and that is not a limitation of this tool. A WOFF is a web format: browsers fetch it, operating systems do not install it. If you want to use the font locally, the TTF you started from is the file for that.
Is TTF to WOFF free?
There is a free allowance every month, and one TTF to WOFF 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 WOFF.