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REM to Percentage Converter
Quickly convert CSS rem units to percentage.
Percentage (%): 0.00
Note: REM is relative to the HTML root font size. Percentage is relative to its parent element's font size (or other specified reference). To convert REM to Percentage, we first convert REM to PX using the root font size, then calculate what percentage that PX value is of the Reference (Parent) Font Size.
Formula: Percentage = (REM Value * Root Font Size) / Reference Font Size * 100
What is a REM to Percentage Converter?
A REM to Percentage converter calculates what percentage a rem value represents relative to a parent element's font size. Percentage-based sizing is a cornerstone of responsive web design, allowing elements to scale fluidly based on their container rather than fixed values. This tool bridges the gap between root-relative REM sizing and context-dependent percentage sizing, which is especially useful when refactoring layouts or migrating between CSS methodologies. For more on percentage values in CSS, see the MDN Web Docs on CSS percentage values.
How REM to Percentage Conversion Works
The conversion first resolves the REM value to pixels using the root font size, then calculates what percentage that pixel value represents of the parent (reference) element's font size. The formula is:
Percentage = (REM × Root Font Size ÷ Parent Font Size) × 100
When both the root and parent font sizes are 16px, 1rem = 100%. But if the parent element has a font size of 20px while the root stays at 16px, then 1rem = (16 ÷ 20) × 100 = 80% of the parent's size.
REM to Percentage Conversion Table
| REM | % (root=16px, parent=16px) | % (root=16px, parent=20px) |
|---|---|---|
| 0.5rem | 50% | 40% |
| 0.75rem | 75% | 60% |
| 1rem | 100% | 80% |
| 1.25rem | 125% | 100% |
| 1.5rem | 150% | 120% |
| 2rem | 200% | 160% |
When to Use Percentage Units
- Fluid layouts where elements should scale relative to their parent container
- Setting
font-sizeon the<html>element (e.g.,html { font-size: 62.5%; }to make 1rem = 10px) - Responsive width and height values that adapt to container size
- Creating scalable component libraries where internal sizing is proportional
- Accessibility-friendly font sizing that respects user browser preferences
FAQs
What does percentage refer to in CSS font-size?
When used for font-size, a percentage value is relative to the parent element's computed font size. So font-size: 150% on an element whose parent has a 16px font size results in 16 × 1.5 = 24px. This is similar to how EM works for font sizing.
Why use the 62.5% trick on the HTML element?
Setting html { font-size: 62.5%; } changes the root font size from 16px to 10px (62.5% of 16 = 10). This makes REM calculations much simpler: 1.6rem = 16px, 2.4rem = 24px, etc. It's a popular technique that preserves user accessibility settings while simplifying the math.
How is percentage different from EM for font sizing?
For font-size, percentage and EM are functionally identical: 150% equals 1.5em. The difference appears in other properties — for width, percentage refers to the parent's width, while EM still refers to the element's own font size. Choose based on readability and convention in your codebase.
Can percentage values exceed 100%?
Yes. A percentage above 100% means the element is larger than its reference. For font-size, 200% means double the parent's font size. For width, 150% means the element is 1.5 times wider than its containing block, which may cause overflow.
Related Tools
- PX to Percentage Converter – Convert pixels directly to percentage values
- REM to EM Converter – Convert REM to parent-relative EM units
- REM to VW Converter – Convert REM to viewport-width percentages
- REM to VH Converter – Convert REM to viewport-height percentages