Convert a whole CSS block from px to rem at any root font size. Runs in your browser, nothing uploaded, no length limit and no run limit.
The job
You have a stylesheet written in pixels and a reason to move it to rem, usually an accessibility review. Paste the block, set the root font size, and every px value in it is divided by that number and rewritten as rem.
It all happens in the browser. No upload, which matters when the stylesheet is a client's, no signup, no length cap, and no limit on how many times you run it.
The workflow
Three steps, and the third one is the difference between a converted stylesheet and a broken one.
Before you paste it back
The conversion is a text replacement with no knowledge of CSS, so it converts values it should have left alone. These are the four to check, and all four are quick.
The one setting
The Root font size field is the divisor. At the default of 16, which matches the browser default, 16px is 1rem, 24px is 1.5rem and 32px is 2rem. If your project declares a different value on html, put that number in instead and the whole conversion shifts with it.
Zero and negative numbers are rejected rather than producing nonsense: the tool returns Enter a valid root font size and leaves your CSS alone. A base of 10 is accepted if your project works that way, and turns 18px into 1.8rem.
Predicting the output
Useful to know before you paste a large file, because it tells you where to look afterwards.
The reason
Pixels are absolute and rem units are relative, and only one of those respects a reader's own settings. Someone who has raised their browser default font size, often for low vision or for cognitive reasons, sees rem-based type and spacing scale with that choice, while pixel values simply ignore it. That is why rem sits behind accessibility guidance on text resizing and readability.
It is also why the review list matters. Typography and spacing are what you want scaling. A one pixel divider was never meant to scale, and converting it is a regression dressed up as a fix.
Where it runs
Most px to rem converters are hosted. Pasting a client's production stylesheet into somebody else's web form is a decision worth making deliberately, and plenty of agency contracts have something to say about it.
This one is a local transformation. The JavaScript that does the conversion is downloaded with the page and runs on your machine, so nothing is transmitted during a run. Load the page, disconnect, and it still converts. That is also why there is no run limit here: limits exist to protect a server, and no server is involved.
Where it fits
Developers doing an accessibility pass on an older codebase, where the finding was that text does not resize and the remedy is a unit change across a stylesheet. This turns an afternoon of find-and-replace into a paste and a review.
Designers converting a pixel-based mockup into the units a design system actually uses, and anyone maintaining a component library who needs one file moved without disturbing the rest. In every case the value is the same: the arithmetic is trivial, doing it a hundred times by hand is where the mistakes come from.
Divide each pixel value by the root font size. Paste the block here and every px value is converted in one pass.
1rem at the default base of 16. 24px is 1.5rem and 32px is 2rem.
Yes, with no signup, no length cap and no run limit.
No. The conversion runs on your own device, so a client file never leaves your machine.
Yes. There is no length limit, because nothing is being sent anywhere.
Yes, it is the one setting on the page. Everything is divided by whatever you enter.
Zero and negative values are rejected with the message Enter a valid root font size, and your CSS is left unchanged.
Yes, to 0.0625rem. There is no skip option, so restore hairline borders during the review.
Yes. A filename containing a px value inside url() gets rewritten, so search the output for url( afterwards.
Yes, to 0rem. It works, though plain 0 is tidier.
There is no rounding step. Bases that are not powers of two produce long exact values.
No. There is no rounding setting, so trim long values by hand.
Yes. Both -8px and .5px are converted correctly.
Yes, and the output is written lowercase as rem regardless.
The number must be directly followed by the unit, with no space between them.
No. Every match is converted, which is why the review step exists.
No. It is a text replacement, so it cannot tell a property value from a filename or a comment.
No. Paste in, copy out.
Rem is relative to the root font size. Em is relative to the parent's font size, so nested elements compound.
No. Typography and spacing gain the most. Anything that must stay exactly one pixel should stay in px.
Use the rem to px converter, which multiplies by the root font size instead.