Optimize Font Awesome for only used classes

fontello is very good but IcoMoon is even more awesome.

Sass has no idea what classes you are actually using. This is something you will have to manually trim down yourself. Open up the provided .scss file and hack out anything you don't need.

Editing the font file itself to eliminate unneeded glyphs requires a 3rd party application to do so and is beyond the scope of this question.

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Fontello is an online web service that can do all of this for you. It lets you mix and match between multiple icon font collections to create the perfect font file for your project. In addition to the customized font file, it provides multiple .css files containing styles already generated for you (changing the extension to .scss will allow you to import them into your existing Sass project).