Local Calculation · Real-Time · Private
PX ⇄ REM Converter
Bidirectional pixel-to-REM conversion with adjustable base font size, live formula display, and a full reference table. Conversion is calculated in your browser, so your entered values are not sent to a conversion server.
px-rem-calculator v3.0
e.g. 16 ÷ 16 = 1rem
e.g. 1 × 16 = 16px
Your conversion history will appear here
Type a value → get the exact CSS-ready unit
Real-time conversion with the formula visible underneath, so you always know what's happening mathematically.
Enter a value
Type in either the Pixels or REM field — the opposite updates live as you type.
Set your base
Keep 16px for most projects, or match your CSS root font size exactly.
Read the result
The converted value and the exact formula appear under the input field.
Copy the unit
Click the copy icon to copy the formatted CSS unit straight to your clipboard.
Precision, privacy, and predictability
What this tool does with your numbers — and what it deliberately does not.
Local conversion
Every conversion is calculated in your browser using JavaScript. The converter does not send your entered values to a conversion server.
Predictable rounding
REM values are displayed to 4 decimal places and pixel values to 2 decimal places. This keeps the results readable while preserving useful precision for CSS work.
Private local history
Up to 10 recent conversions are stored in this browser's localStorage. They remain on this device until you clear them or site data is removed.
Where PX-to-REM math shows up in real CSS
Practical scenarios where this converter saves time in daily development work.
| Use case | Best for | Why it helps |
|---|---|---|
| Typography | Headings, body, captions | Converts fixed design-mockup px into scalable rem that respects user font preferences. |
| Spacing system | Margins, padding, gap | Turns an 8px grid into clean rem values (0.5rem, 1rem, 1.5rem) for a consistent scale. |
| Components | Buttons, cards, inputs | Calculates rem dimensions that grow with the root font, keeping UI proportional. |
| Accessibility | Scalable sizing | Shows how a PX value maps to REM relative to the selected root font size. |
What it does, and what it does not
Clear boundaries so you know when to use this tool and when to reach for another.
What it does
- Bidirectional real-time conversion between px and rem.
- Adjustable base font size from 1 to 100 px.
- One-click copy of any converted value.
- Quick buttons for common typography sizes.
- Reference table for 14 / 16 / 18 px bases.
- Full dark mode support.
- Runs in your browser without an account or conversion server.
What it does not
- Convert to or from EM, %, vw/vh, or ch — those are different units.
- Replace design-tool plugins (Figma, Sketch) for live style updates.
- Handle media-query breakpoints that use non-px units.
- Store history across devices — data stays in one browser only.
Common pixel → REM equivalents
Quick lookup across three common base values. Usage labels are guidelines, not strict rules.
| Pixels | REM @16px | REM @14px | REM @18px | Typical usage | Scale |
|---|
Common input issues and fixes
Quick answers to the most frequent conversion problems.
Result shows "Invalid"
The input contains letters or symbols. Only digits, one decimal point, and an optional leading minus sign are accepted. Remove any spaces or unit suffixes like "px" or "rem" — the converter adds those for you.
Can I convert negative values?
Yes. The converter performs the mathematical conversion, but whether a negative CSS length is valid depends on the specific CSS property where you use it.
Values don't match my design tool
Check whether your design uses pixel values and confirm the root font size used by your CSS. Figma and Sketch commonly work with pixel-based design values, while your CSS root font size determines the REM conversion.
History disappeared
History lives in this browser's localStorage. Private windows, cleared site data, or a different device will show an empty list — that is intentional.
Frequently asked questions
Helpful answers about PX-to-REM conversion and CSS units.
Why use REM instead of PX?
REM is relative to the root font size, so it can scale with changes to that root size. This can make typography and spacing easier to adapt to user and project settings. PX is an absolute CSS length unit and does not scale from the root font size in the same way.
What is the base font size?
It is the font-size of the root element. Most browsers default to 16px, so 1rem = 16px. If your project sets html { font-size: 18px }, then 1rem = 18px — adjust the converter's base size to match your CSS.
Should I use REM for everything?
REM is useful for typography and spacing that should scale with the root font size. PX can still be appropriate for pixel-specific details such as thin borders or fixed visual assets. For responsive breakpoints, choose the unit that fits your CSS strategy rather than assuming one unit is always best.
What is the difference between REM and EM?
REM is relative to the root font size, so it is consistent everywhere. EM is relative to the parent element's font size, which can compound unpredictably in nested elements. REM gives you more predictable scaling.
Is my input sent to a server?
The conversion is calculated locally in your browser. This tool does not send your entered PX, REM, or base-size values to a conversion server.
How accurate is the conversion?
The conversion uses standard mathematical formulas: pixels ÷ root font size = REM, and REM × root font size = pixels. The displayed result is rounded to 4 decimal places for REM and 2 decimal places for pixels.