CSS Units Guide: Convert px, rem, em, and Percent Without Guessing
Learn when to use px, rem, em, and percent values, why font-size context matters, and how unit conversion supports consistent design tokens.
Browser-only utility
CSS Unit Converter, private by default
Convert interface sizes between pixels, rem, em, and percent values before updating responsive CSS or design tokens.
Convert CSS size units
Convert between px, rem, em, and percent values using your root and parent font-size context before shipping responsive interface styles.
Converted values
CSS units converted locally in your browser.
Pixels
16px
Root em
1rem
Parent em
1em
Percent of parent
100%
CSS unit conversion runs locally in your browser. Pasted values are not uploaded to tempboxs.
How to use css units safely
Why CSS units need context
Pixels, rem, em, and percentages describe size in different ways. Pixels are direct screen CSS units, rem values depend on the root font size, em values depend on the parent context, and percentages are relative to the property being measured.
Converting units before editing a layout helps teams keep typography and spacing consistent with a design system instead of guessing values by sight.
How to use the converter
Enter the source value, choose its unit, then set the root and parent font sizes used by your CSS. The converter shows equivalent px, rem, em, and percent values.
For most browser defaults, 1rem equals 16px. In nested components, 1em can be different because it follows the parent font size.
- Use px when you need a direct CSS pixel value
- Use rem for scalable typography and spacing tied to the root
- Use em for component-relative sizing
- Use percent when a size should follow its parent context
Responsive design notes
A converted value is a starting point, not a complete responsive decision. Check the component on mobile and desktop after updating CSS so text, controls, and cards still fit comfortably.
When a project has design tokens, convert values into the closest existing token when possible. That keeps the interface consistent and easier to maintain.
Why people use this tool
CSS values are converted locally in your browser. tempboxs does not receive pasted numbers or settings.
Convert common CSS units
Translate px, rem, em, and percent values using the root and parent font-size context.
Match design tokens
Review sizes before updating spacing, typography, buttons, cards, and responsive interface rules.
Copy conversion notes
Copy a concise summary into pull requests, design QA notes, tickets, or documentation.
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FAQ
No. Unit conversion runs locally in your browser and does not send pasted values or settings to tempboxs.
Many browsers use 16px as the default root font size, but users and sites can change it. Use your project's actual root size when converting.
rem is based on the root font size. em is based on the current parent or element context, so nested components can produce different pixel values.
No. Use the unit that fits the design need. Typography and scalable spacing often work well with rem, while borders and precise effects may stay in px.