PX REM EM Converter
Convert CSS px, rem and em values with configurable base font size.
Overview
The PX REM EM Converter switches CSS lengths between pixels, rems and ems with a configurable base font size. Type a value in any unit and see all three at once, so you can copy whichever your stylesheet prefers.
It is built for front-end developers building responsive layouts, designers translating mockups to code and accessibility-minded engineers using relative units to respect user font preferences. Mixing units is fine — knowing which is which avoids subtle layout bugs.
How it works
Browsers compute 1rem as the root element's font size, typically 16 px by default. So 1rem = 16 px and 12 px = 0.75 rem at default settings. em is the same idea but relative to the parent element's font size; the converter assumes the same base for simplicity.
The formulas are rem = px / base, px = rem * base and the same with em since this tool treats em as equivalent to rem at the chosen base. Change the base (e.g., to 10 px for the popular 62.5% trick) and all conversions rescale.
Examples
16 px at base 16 → 1 rem
24 px at base 16 → 1.5 rem
2 rem at base 10 → 20 px
0.875 em at base 16 → 14 px
FAQ
Why use rem over px?
rem respects the user's preferred font size, which matters for accessibility. Users who bump up default text get a proportionally larger layout for free.
What's the 62.5% trick?
Set the root font size to 62.5% (10 px). Then 1 rem = 10 px, making conversions trivial mental arithmetic. The trade-off is breaking the user's true default size.
Is em the same as rem?
Conceptually yes (both are font-relative), but em is relative to the parent's font size while rem is always relative to the root. In nested contexts em compounds; rem does not.
Does pt or pc matter for the web?
Print units (pt, pc) exist in CSS but are rarely used. 1pt = 4/3 px. Most modern stylesheets stick to px and rem.
Why doesn't my value round-trip exactly?
Floating-point rounding can leave a 0.9999 instead of 1. The converter displays sensibly rounded values rather than full double-precision noise.