Component sizing

Component heights and icon sizes are curated in UDTS, not formula-derived. The rungs are picked so each height pairs cleanly with a font from the type scale, an icon from the icon ladder, and tap-target requirements — and so each rung distinguishes visually from its neighbors at every density.

When to use

• Picking a height for a button, input, chip, badge, dropdown, or any other rectangular interactive control.
• Proposing a control-height ladder for a new density mode.
• Auditing one-off heights (h-7, h-9, h-11) drifting across a codebase.
• Code review: flag heights outside the curated ladder; propose the nearest curated rung with rationale.

When NOT to use

• One-off marketing CTAs designed for a specific page (e.g. hero buttons at 72 px). The ladder doesn't apply to single-surface designs.
• Non-rectangular controls (circular avatars, range thumbs) where the size is governed by the icon or asset, not a height token.

The curated ladders

UDTS ships three default density modes. Each is a 5-rung ladder labeled xs / sm / md / lg / xl — interactive rungs start at sm per the WCAG 2.5.8 floor (see spacing-system).

Balanced (UDTS default, 4/8 grid)

Dense (2/4 grid)

Spacious (4/16 grid)

The curated icon-size ladder

UDTS's canonical icon sizes: 12, 16, 24, 32, 40, 48. Picked because:

• These are the sizes at which Material Icons, Lucide, Heroicons, and Phosphor families render cleanly without hairline glitches.
• They pair with the font-sizes from type-scale naturally (icon ≥ font-size at every rung; never undersized).
• They subdivide into 4 px steps at the small end (where it matters for inline alignment) and 8 px steps at the large end (where it doesn't).

Don't formula-derive icon sizes. A height-to-icon ratio (e.g. icon = height / 2) produces sizes like 14, 18, 22, 28 — every one of which renders worse than the curated neighbor.

Why curated, not formula-derived

A formula (height) → font-size → icon-size produces mathematically clean values that render poorly:

• Heights at every 4 px (24, 28, 32, 36, 40, ...) create visually indistinct rungs — 28 and 32 are hard to tell apart in a stack.
• Font sizes outside the type-scale (e.g. 13, 15, 17) clash with the rest of the typography system.
• Icon sizes outside the curated set glitch at common zoom levels.

The curated 5-rung ladders give visual distinguishability at each rung, predictable font and icon pairing, and tap-target safety. The trade-off is less flexibility — but design systems benefit from less flexibility at this layer.

The WCAG 2.5.8 floor

Interactive control heights start at 24 CSS px per WCAG 2.5.8 AA Target Size (added in WCAG 2.2). In practice:

• Balanced and dense: xs (24 / 24 px) is non-interactive only. Interactive rungs start at sm (32 / 28 px). The 24 px rung exists for badges, read-only chips, density tags.
• Spacious: xs lifts to 32 px, so the entire ladder is interactive-safe.

Fails of this rule are one of the most common WCAG 2.2 audit misses.

Picking a rung

Default heuristic:

• Default button / input: md. Use the same rung for buttons and their associated inputs so they align horizontally in a form.
• Compact density toggle (dense table cell): sm.
• Primary CTA / hero button: lg or xl depending on prominence.
• Density tag, badge, breadcrumb separator (non-interactive): xs.

Sibling controls in the same UI surface should share a rung — mixing sm and md buttons next to each other reads as a typo, not a hierarchy.

Verification

After picking a rung:

1. Density match: the rung is from the density mode declared in the current foundation theme. Mixing balanced and spacious ladders in the same surface is a bug.
2. Interactive floor: the height is ≥ 24 CSS px for interactive controls. xs rungs in balanced and dense are reserved for non-interactive.
3. Font pairing: the inner font-size matches the rung's pairing (not a one-off).
4. Icon pairing: the leading or trailing icon size matches the rung's pairing.
5. Sibling consistency: sibling controls in the same surface share a rung.

Cross-references

• REQUIRED BACKGROUND: spacing-system — the unit primitives (minor + major) drive which density mode applies, and the WCAG 2.5.8 floor lives there.
• For the font sizes paired with each rung: type-scale — the per-rung font is taken from the canonical scale, not free-picked.
• For line-heights inside each rung: line-height-grid — interactive controls use lh-ui (1.20), not lh-prose.

Sources

• WCAG 2.5.8 — Target Size (AA, added in 2.2).
• Material Design 3 — Component sizing guidance.
• Apple Human Interface Guidelines — touch targets (44 pt mobile; relaxed to 24 CSS px desktop per WCAG).
• The UDTS / token.design spacing-and-sizing foundations spec.