⚖️ Comparison · Updated for 2026
em / rem vs px
Side-by-side comparison, when-to-use-each guide, and instant conversion. Reviewed for 2026.
Quick answer: The px unit is absolute (fixed pixel size). The em unit is relative to the parent element's font size (compounding). The rem unit is relative to the root font size (consistent). Use px for borders/shadows, rem for font sizes, and em only when you intentionally want parent-relative scaling.
Decision guide — when to use which
Use em / rem when…
Font sizes, padding/margin, anything that should scale with user settings.
Use px when…
Borders, shadow offsets, small icons, image dimensions, things that should never scale.
📊 Side-by-side comparison
| Aspect | em / rem | px |
|---|---|---|
| Definition | px: fixed pixel | em: relative to parent / rem: relative to root |
| Browser zoom | Both scale | Both scale |
| User font size override | Doesn't respect | Respects (rem strongly recommended) |
| Compounding | No | em compounds; rem doesn't |
| Best for | Borders, fine detail | Font, padding, layout |
Frequently asked
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Why is rem better than em?
em compounds: a 1.5em child of a 1.5em parent is 2.25× root size. rem always refers to the root, so size is predictable. Use em only when you want compounding (rare).
?
Should I never use px?
Use px sparingly. Borders (1px solid), shadow offsets (0 2px 4px), thin icons — px is fine. For anything that affects readability or layout proportions, use rem.
Reviewed for 2026. All conversion factors and historical references verified against official sources (ISO standards, government weights & measures legislation, IEC technical specifications). Built by a UK-based qualified primary teacher and FA Level 2 coach as part of 247QuickTools' free utility-tools project. We don't sell SEO links or accept paid placements in this content.