How EM/REM to Pixels Converter Works

CSS length units fall into two broad groups: absolute units like px, and relative units like em and rem. Relative units scale from a reference font size, which means the same number can render differently depending on the context. That flexibility is exactly why teams use em/rem for typography and spacing—it helps the UI adapt to user font settings, zoom levels, and design-system scaling.

For rem, the reference is the root element’s font size (usually the computed font-size of the html element). For em, the reference is the current element’s computed font size, which often comes from the parent context. This tool keeps the math explicit: you provide the effective base size in pixels and the value you want to convert, and it outputs a copy-ready CSS value along with a clear formula so you can sanity-check the result.

Step-by-step conversion

• 1. Choose the direction: convert em/rem → px or px → em/rem.
• 2. Select the unit you’re working with (em or rem).
• 3. Enter the value you want to convert (supports decimals like 1.125 or 0.875).
• 4. Set the base font size in pixels (commonly 16, but your design system may differ).
• 5. Pick a precision level to control rounding, then generate your result.
• 6. Copy the output or download it as a text file for quick reuse in your stylesheet or design notes.

Formulas and practical examples

The underlying formulas are straightforward. To convert em/rem to pixels, multiply by the base: px = value × base. To convert pixels to em/rem, divide by the base: unit = value ÷ base. If your base is 16px, then 1rem equals 16px, 1.5rem equals 24px, and 0.75rem equals 12px. If your root size is 10px, then 2.4rem equals 24px. The tool is designed so you can plug in whichever base is true for the context you are modeling.

Remember that em can compound. For example, if a container sets font-size: 1.25rem and the root is 16px, the container’s computed size is 20px. Inside that container, a child with padding: 1em uses 20px as its base. In that scenario, the effective base for em conversions is 20px, not 16px. When you enter a base font size in this converter, you’re choosing the base for the exact context you care about.

Key Features

Two-way conversion (em/rem ↔ px)

Some workflows start with relative units, while others begin with pixel values from a mockup, component spec, or an existing codebase. This converter supports both directions so you can standardize units without rethinking your process every time a requirement changes. Convert a spacing token like 1.25rem into pixels for a design review, or convert a measured 18px font size into rem for a design system token.

When you’re refactoring, two-way conversion is especially useful. It lets you migrate a stylesheet gradually: you can replace pixel values with equivalent rem values in one component, confirm that the computed pixels remain consistent in the browser, and move on to the next component with confidence.

Custom base font size that matches your project

Not every site uses a 16px root size. Many teams set html { font-size: 62.5%; } to make 1rem equal to 10px. Others use clamp-based fluid typography where the root size changes across breakpoints, or they adjust the base size to improve readability for specific audiences. By letting you choose the base, this tool remains valid across all of those approaches.

For rem conversions, your base is the computed root font size in the state you care about (desktop, mobile, or a specific breakpoint). For em conversions, your base is the computed font size of the context element (often the parent). If you’re unsure, inspect the computed font-size in your browser’s devtools and use that number as the base.

Precision control and readable output

Design tokens typically use consistent increments—quarter steps, eighth steps, or a limited number of decimals. When you convert between units, you can end up with results like 1.0625rem or 13.333333px. That may be mathematically correct but visually noisy. The precision setting lets you round to a sensible number of decimals, producing stable values that are easier to maintain.

A good rule of thumb is to set precision based on your system’s granularity. If your spacing scale is in 0.25rem increments, rounding to 3 decimals is usually enough. If you’re creating a token list for typography, 2–3 decimals keeps values compact while remaining accurate for most use cases. For edge cases, increase precision temporarily, then normalize to the nearest token.

Copy-ready CSS values and one-click export

The result is formatted as a CSS value (for example, 20px or 1.25rem). That means you can paste it directly into properties like font-size, padding, gap, border-radius, or even into a design token JSON file. Copy with one click to avoid transcription errors, or download a plain-text file when you’re building a conversion table for a migration.

Export is also handy for documentation: you can keep a small file of “common conversions” alongside component specs, or attach it to a pull request when you’re introducing a new spacing scale.

Context-aware guidance for em versus rem

rem is anchored to the root font size, which makes it consistent across the page and predictable for global spacing and typography. em follows the local context, which can be powerful inside components that should scale when their font size changes (buttons, pills, alerts, badges, and menus).

This tool encourages clarity by asking you to set the effective base size explicitly. That makes it easier to reason about compounding: you can run the same conversion with different bases to see how a nested context changes the pixel outcome, and then decide whether rem or em better matches the behavior you want.

Use Cases

• Responsive spacing tokens: Convert a pixel-based spacing scale into rem so padding and margins scale with user font settings and the overall type scale.
• Typography systems: Translate type sizes from design tools into rem so headings and body text remain proportional when the root size changes across devices.
• Component libraries: Use em for components that should scale with their own font size (buttons, badges, input groups), then verify the pixel equivalent for alignment and layout.
• Bug triage and QA: When devtools shows computed pixels, convert back to em/rem to locate the original token or CSS rule faster.
• Documentation and handoff: Provide both px and rem values in guidelines so designers and developers can communicate without unit confusion.
• Accessibility checks: Evaluate how layouts respond when users increase default font size by converting fixed pixels into relative units and spotting brittle measurements.
• Legacy migration planning: Estimate the impact of switching from px to rem by converting a set of common values and comparing before/after in a staging build.

Unit conversion is rarely a one-time task. You might convert values during initial implementation, revisit them during refactors, and validate them again while debugging responsive bugs. A dedicated converter keeps that workflow lightweight: you can quickly test alternative bases, compare em and rem behavior, and produce consistent, shareable values that align with your team’s tokens and naming conventions.

Optimization Tips

Decide where you want scaling to happen

If you want global scaling with user preferences, favor rem for font sizes and most spacing. Users who adjust their browser’s default font size benefit immediately, because rem-based layouts respond naturally. If you want a component to scale with its own font size (for example, an icon and padding that should grow when the button text grows), em can be a better fit because it follows the local context.

Many teams adopt a hybrid strategy: rem for global spacing and typography, em for component internals, and px for fine details like borders. The “right” mix depends on your design system and your accessibility goals.

Round with intention, then normalize to tokens

Overly precise decimals make code harder to scan and can complicate diff reviews. Set your precision to match your token granularity and then standardize on a token set. For example, if your spacing scale is based on 4px steps, you might convert to rem and then snap to values like 0.25rem, 0.5rem, 0.75rem, and 1rem (assuming a 16px base). If you’re using a 10px base, you might prefer 0.2rem, 0.4rem, 0.6rem, and so on.

When values don’t land neatly on a token, consider whether the design can be adjusted slightly. Small changes are often invisible to users but make your CSS significantly more maintainable.

Validate the base font size from computed styles

When you’re unsure what base to use, check the computed font-size for the relevant element in your browser devtools. Use that pixel value as the base for rem conversions (root element) or em conversions (context element). If your base changes by media query, validate at the breakpoint you care about. If your project uses fluid typography, test a couple of viewport sizes and note how the computed base shifts so your conversions remain accurate.

As a final check, paste the converted value back into a sandbox component and confirm that the computed pixel value matches your expectation. That quick loop prevents surprises caused by compounding or overrides.

FAQ