CSS Unit Converter Guide
Reference for converting px, rem, em, percent, viewport, and spacing values before copying CSS into layouts.
Quick answer
Use the CSS Unit Converter to convert values such as px, rem, em, percent, and viewport-style units before using them in UI styles.
What this tool does
The converter helps compare design-token values, typography sizes, spacing, and layout measurements with clear base-size assumptions.
Step-by-step use
- Enter the source value.
- Choose the source and target units.
- Set the base size when needed.
- Review the converted value.
- Test the result in the actual layout.
Data handling and processing behavior
Processing is handled in the browser for this tool based on the current public implementation.
Examples
Typography
Convert 16px to rem when a design system uses root-relative font sizes.
Spacing
Compare px and em values before copying spacing into a component.
Design handoff
When converting spacing tokens, keep the base-size assumption in the note. For example, "24px = 1.5rem at 16px base" is easier to review than a bare rem value, and it prevents confusion if the project uses a different root font size. If the component changes font context, recheck em-based values separately before release review notes.
Assumptions and limits
- rem depends on the root font size.
- em depends on the current element context.
- Percentages depend on the property and containing box.
- Converted values still need visual review in the real layout.
Common errors
Forgetting the base size
A rem conversion is only meaningful when the root size is known.
Treating all percentages alike
Percent behavior changes by CSS property.
Next steps
- CSS Unit Converter — convert CSS units
- CSS Clamp Generator — create fluid size values
- Aspect Ratio Calculator — calculate layout ratios