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name: sizing-units description: rem, px or relative units for each kind of value. Use when defining spacing and sizing tokens, control heights or media queries, or when a layout breaks under enlarged text. metadata: priority: 5 pathPatterns: - "design-system/" - "ui/" - "/tokens*" - "/.css" - "**/tailwind.config." - "components/" - "src/components/" promptSignals: phrases: - "rem" - "px" - "em" - "spacing scale" - "sizing tokens" - "root font size" - "text zoom" - "which unit" retrieval: aliases: - px vs rem - css units - spacing units - sizing tokens - root font size - text scaling intents: - decide between px and rem for spacing - define a spacing scale - fix a layout that breaks when text is enlarged - choose units for media queries - set control heights that survive text scaling examples: - should spacing be in rem or px - our buttons clip when the user enlarges text - what unit for border radius and shadows - should media queries use px or em

Sizing Units

The Question the Unit Answers

A unit is not a formatting preference. It is a declaration of what happens to a value when the user enlarges text.

• rem — this value belongs to the text and must grow with it.
• px — this value belongs to the screen and must stay put.
• %, fr, ch, clamp(), min() — this value belongs to the available space.

Pick the unit by answering that question, not by picking a house style and applying it everywhere. Both absolutist positions — all-rem and all-px — produce defects, and they are different defects.

Zoom Is Not the Font-Size Setting

The common argument for px everywhere is that browsers already have full-page zoom, so the root font size can be ignored. They are separate controls serving separate people.

• Full-page zoom magnifies everything, layout included. It buys larger text at the cost of horizontal scrolling and fewer items per screen.
• The browser's default font size enlarges text and leaves the layout alone. This is what a user with low vision sets once, years ago, and never touches again. They will not switch to zoom because your product ignored it.

A product that hard-codes every dimension in px answers the first user and silently refuses the second. WCAG 2.2 SC 1.4.4 (Resize Text) requires text to reach 200% without loss of content or function, and SC 1.4.10 (Reflow) requires the page to survive it in a single column. See [[wcag-accessibility]].

Use rem For

Typography. Every font size and line height. See [[modular-scale-typography]].

Padding around text. A button's inner padding in px means large text presses against its edges. In rem the breathing room scales with the words it surrounds.

Control heights. A 40px control holding 24px text is broken. Write the height token in rem and it stays a control-shaped control at every text size. See [[component-family-consistency]].

Measure. max-width on a text column: 65ch or a rem value, never px.

Gaps between text-bearing blocks. Stack spacing in a form or an article is part of the reading rhythm.

Use px For

Hairlines and borders. 1px is one device pixel by intent. In rem it becomes 1.25px at a larger root size and renders as a blurry smear.

Shadow offsets and blur. Elevation is a screen-space effect. See [[elevation-and-depth]].

Small icons inside controls where the icon is a fixed glyph rather than text.

Sub-pixel corrections — the -1px that makes two borders overlap instead of doubling.

Use Relative Units For

Layout widths and gaps. This is where the "lock it in px so it stays stable" instinct goes wrong: a px-locked container is not stable, it is brittle. The text inside it still grows, and a fixed box is exactly what makes growing text clip.

/* Brittle — the box cannot absorb anything */
.card   { width: 320px; height: 180px; }
.sidebar { width: 280px; }

/* Resilient */
.card    { max-width: 20rem; min-height: 11.25rem; }
.sidebar { width: clamp(16rem, 22%, 22rem); }

min-height, never height, on anything containing text. A fixed height cannot grow; a minimum height holds the shape at rest and yields when it must.

Media Queries

Use em in media queries. Media-query rem and em both resolve against the browser's default font size, ignoring any html { font-size } you set — so em is the honest spelling, and both respect the user's preference where a px breakpoint does not. A user with a large default font size gets the layout their effective text size warrants, not the one their device width claims.

@media (min-width: 48em) { /* ~768px at default settings */ }

Do Not Reset the Root

html { font-size: 62.5%; } /* Do not do this */

The 62.5% trick exists to make rem arithmetic easy — 1.6rem for 16px. It buys you mental arithmetic and pays for it by overriding the user's stated preference by default. Keep the root at the browser default and let the tooling do the division.

A Measured Container Must Not Depend on What It Sizes

When script sizes an element from its container, the container's size must be definite and independent of that element on every breakpoint. If the container shrink-wraps the child, the measurement reads the child's own collapsed size and the element stays collapsed, silently, on the breakpoints where the chain loses its definite size. Give the chain a definite size down to the measured box and observe the box itself, not window events.

Review Checklist

• Is every font size, line height, and text-adjacent padding in rem?
• Are borders, hairlines, and shadow offsets in px?
• Does any element containing text use height rather than min-height?
• Are container widths relative (%, fr, clamp(), max-width) rather than fixed px?
• Are media-query breakpoints in em?
• Is html { font-size } left at the browser default?
• At a 200% browser font-size setting, does every control still contain its label, with no clipping and no horizontal page scroll?
• Is every script-measured container sized by layout rather than by the child it sizes?