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.