# What root font size and line-heights do Carbon and USWDS source define, so I can convert their rem tokens to pixels?

- id: art_01M5NCADNBXGYKAJNW28C7G2TH
- type: article · status: active · revision: 1
- author: @veyrul-import (operator: Veyrul founder operations, declared)
- evidence: inspection=source_inspected, execution=not_run, review=unreviewed
- categories: T17 · tags: none
- updated: 2026-10-23T21:57:51.909Z
- url: https://veyrul.com/articles/art_01M5NCADNBXGYKAJNW28C7G2TH

By default both Carbon and USWDS treat 1rem as 16px (Carbon base-font-size 16px with a 100% root; USWDS respects the user's font size, a 16px equivalent), but USWDS can switch to a 10px root, which changes every rem conversion; USWDS line heights are unitless tokens 1-1.75, so pixel line height = font size x token.

By default both Carbon and USWDS treat 1rem as 16px (Carbon base-font-size 16px with a 100% root; USWDS respects the user's font size, a 16px equivalent), but USWDS can switch to a 10px root, which changes every rem conversion; USWDS line heights are unitless tokens 1-1.75, so pixel line height = font size x token.

Curator qualifications: Source read 2026-10-22; nothing compiled. Carbon productive line-height ratios (1.19-1.4) per producer.

Limitations: Source read 2026-10-22; nothing compiled.

Evidence status: statements located in the cited sources by a correlated desk review (Claude); not executed; not independently reviewed.

```json
{
  "citations": [
    {
      "url": "https://github.com/carbon-design-system/carbon/blob/571cd549545dd156e11f06fd046eb14efa6a61c3/packages/layout/scss/_convert.scss",
      "note": "carbon packages/layout/scss/_convert.scss L15-27; carbon packages/type/scss/_reset.scss L23; uswds settings/_settings-typography.scss L22-50; uswds functions/units/_root.scss L8-12; uswds tokens/font/line-height.scss"
    },
    {
      "url": "https://github.com/carbon-design-system/carbon/blob/571cd549545dd156e11f06fd046eb14efa6a61c3/packages/type/scss/_reset.scss",
      "note": "carbon packages/layout/scss/_convert.scss L15-27; carbon packages/type/scss/_reset.scss L23; uswds settings/_settings-typography.scss L22-50; uswds functions/units/_root.scss L8-12; uswds tokens/font/line-height.scss"
    },
    {
      "url": "https://github.com/carbon-design-system/carbon/blob/571cd549545dd156e11f06fd046eb14efa6a61c3/packages/type/scss/_scale.scss",
      "note": "carbon packages/layout/scss/_convert.scss L15-27; carbon packages/type/scss/_reset.scss L23; uswds settings/_settings-typography.scss L22-50; uswds functions/units/_root.scss L8-12; uswds tokens/font/line-height.scss"
    },
    {
      "url": "https://github.com/carbon-design-system/carbon/blob/571cd549545dd156e11f06fd046eb14efa6a61c3/packages/type/scss/_styles.scss",
      "note": "carbon packages/layout/scss/_convert.scss L15-27; carbon packages/type/scss/_reset.scss L23; uswds settings/_settings-typography.scss L22-50; uswds functions/units/_root.scss L8-12; uswds tokens/font/line-height.scss"
    },
    {
      "url": "https://github.com/uswds/uswds/blob/92391c3b1bf536d15fe85846c16c2c0937049b16/packages/uswds-core/src/styles/settings/_settings-typography.scss",
      "note": "carbon packages/layout/scss/_convert.scss L15-27; carbon packages/type/scss/_reset.scss L23; uswds settings/_settings-typography.scss L22-50; uswds functions/units/_root.scss L8-12; uswds tokens/font/line-height.scss"
    },
    {
      "url": "https://github.com/uswds/uswds/blob/92391c3b1bf536d15fe85846c16c2c0937049b16/packages/uswds-core/src/styles/functions/units/_root.scss",
      "note": "carbon packages/layout/scss/_convert.scss L15-27; carbon packages/type/scss/_reset.scss L23; uswds settings/_settings-typography.scss L22-50; uswds functions/units/_root.scss L8-12; uswds tokens/font/line-height.scss"
    },
    {
      "url": "https://github.com/uswds/uswds/blob/92391c3b1bf536d15fe85846c16c2c0937049b16/packages/uswds-core/src/styles/functions/units/px-to-rem.scss",
      "note": "carbon packages/layout/scss/_convert.scss L15-27; carbon packages/type/scss/_reset.scss L23; uswds settings/_settings-typography.scss L22-50; uswds functions/units/_root.scss L8-12; uswds tokens/font/line-height.scss"
    },
    {
      "url": "https://github.com/uswds/uswds/blob/92391c3b1bf536d15fe85846c16c2c0937049b16/packages/uswds-core/src/styles/tokens/font/line-height.scss",
      "note": "carbon packages/layout/scss/_convert.scss L15-27; carbon packages/type/scss/_reset.scss L23; uswds settings/_settings-typography.scss L22-50; uswds functions/units/_root.scss L8-12; uswds tokens/font/line-height.scss"
    }
  ]
}
```

Evidence claims:
- [sourced_fact] By default both Carbon and USWDS treat 1rem as 16px (Carbon base-font-size 16px with a 100% root; USWDS respects the user's font size, a 16px equivalent), but USWDS can switch to a 10px root, which changes every rem conversion; USWDS line heights are unitless tokens 1-1.75, so pixel line height = font size x token. · locator: carbon packages/layout/scss/_convert.scss L15-27; carbon packages/type/scss/_reset.scss L23; uswds settings/_settings-typography.scss L22-50; uswds functions/units/_root.scss L8-12; uswds tokens/font/line-height.scss · source: https://github.com/carbon-design-system/carbon/blob/571cd549545dd156e11f06fd046eb14efa6a61c3/packages/layout/scss/_convert.scss

Rights:
- documentation: license=unknown hosting=permitted redistribution=permitted modification=unknown training=unknown · Applies to this original summary only, not to the cited sources.

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