Upload .ttf files ---> recieve a .woff2.
Variable fonts are pretty cool. They give you fine-grained control over multiple font axes; most notably, font-weight. With variable fonts, you no longer have to include half a dozen different files to use the full range of weights. However, variable fonts exist as True Type Fonts (.ttf) by default, which are a whopping 250% larger than their .woff2 counterparts. That's right, converting to .woff2 gives you a file that's more than 60% smaller, and is supported by 96.35% of all browsers. It let's you do shit like this.
While they are great for working with regular .ttf files, most font conversion tools online are terrible at handling the compression of variable .ttf fonts. The only tools that seem to work are CLI-based: the fonttools and woff2 libraries. WOFFER uses the former to handle both conversion and subsetting of variable .ttf fonts to their .woff2 versions, and lets you use it through this simple GUI. Conversion usually takes between 5-10 seconds.
P.S: You can also use this app to subset any font format to the provided options, but note that they will be converted to .woff2 when you do.