Understand the result
One pixel value, two root sizes
24px / 16px = 1.5rem 24px / 20px = 1.2rem
The root size is an input, not a universal constant. If the same 1.5rem is later rendered with a 20px root, it becomes 30px. The tool does not change your page root or override reader preferences.
Check the rendered layout
Inspect the computed font size on the html element in your project. Test the result with real content and browser zoom. Negative results can suit margins, but do not use negative padding or font sizes.
What this tool can and cannot tell you
REM values are relative to the computed font size of the root html element. At a 16px root, 24px is 1.5rem; at 20px it is 1.2rem. Relative units alone do not guarantee accessibility: test browser zoom and real content. Negative values suit margins, not font sizes.
Common questions
Is 16px always the root font size?
No. Read the computed html font size for the page you are changing. The converter lets you set that value explicitly.
What is the difference between rem and em?
REM uses the root font size. EM depends on the relevant element's font size and the property being evaluated; nested rules can behave differently.
Can I convert a negative value?
Yes, when the destination CSS property allows it, such as certain margins. The root size must remain positive.
Worked examples are synthetic. No customer outcome or saving is implied. Content by Orr Yoffe.