Most of the arithmetic a developer does in a day is conversion rather than calculation: decimal to hex, px to rem, bytes to something readable, a ratio into the height that matches a width. These tools cover the conversions that are quicker to look up than to derive.
Alongside them sit the small computations that come up often enough to be worth a dedicated box: percentage change, a tip split, a primality check with the factors, or spelling a figure out in words for a contract or a cheque.
Frequently asked questions
How do I convert px to rem?
The CSS Unit Converter does px ↔ rem ↔ em ↔ pt ↔ %, against whatever root font size you set. The default is 16px, which is what browsers use unless a stylesheet says otherwise.
Can it convert between number bases?
Yes. The Base Converter shows binary, octal, decimal and hexadecimal at once, updating every field as you type in any of them.
What aspect ratio do I need for a given width?
Enter the ratio and one dimension in the Aspect Ratio tool and it solves the other, which is the usual question when resizing video frames or image containers.