There are many interesting “must-have” web design tool available on the internet, I tried to collect my favorite typography tools:

1. Type Scale is a great visual font size calculator, helps to set your typography correctly.

type scale

2. Colorable
helps to set the correct font and background color/contrast ratio of your site. You can generate correct font and background colors with this visual editor.

3. Golden Ratio Typography Calculator 
helps to “discover the perfect typography for your website by entering your current font size and content width.”

4. PX to EM calculator
is great for px to em, em to px conversions, and you can learn more about pix, em, % and pt.