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Use CSS font-size-adjust to keep a chosen font metric—usually lowercase letter height—closer to the same proportion of the declared font size when a fallback font renders. Set a ratio that matches your preferred font, then test the preferred and fallback faces at the sizes and weights your site uses. The property adjusts one metric; it does not make different typefaces look identical.
What font-size-adjust does
font-size-adjust scales each font used to render the text so that a selected metric corresponds to a target proportion of font-size. This is useful when a preferred font is unavailable or has not loaded and a fallback font has noticeably different proportions.
The value is a ratio, not pixels or a percentage. With a bare number, the browser uses the ex-height metric by default, which targets lowercase letter height relative to the font size. The property is inherited and applies to every font used to render the text—including the preferred font when it loads. Choose a value suitable for that preferred face rather than assuming only fallback fonts will be adjusted.
Set a useful value
Start with the preferred font’s relevant metric ratio, measured or documented for the font file and weight you use. This example uses an illustrative ratio only; 0.52 is not a universal recommendation.
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.article-copy {
font-family: "Preferred Sans", Arial, sans-serif;
font-size: 1rem;
font-size-adjust: 0.52;
}Because a bare number defaults to ex-height, this targets the preferred font’s lowercase-height proportion. Compare the result with the preferred font loaded and with each fallback font at your real text sizes and weights. If the preferred face becomes too large or small, revise the target rather than assuming the property affects only the fallback.
Specify a metric explicitly
CSS also allows a metric name before the ratio, for example:
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font-size-adjust: ex-height 0.52;The available metrics are:
ex-height: lowercase letter height; the default when a metric is omitted.cap-height: uppercase letter height.ch-width: the width of the “0” glyph.ic-widthandic-height: the width and height of the CJK ideograph “水”.
Use cap-height when uppercase height is the metric that matters most. The width metrics are specialized choices for designs where those glyph measures are important; the ic-* metrics are chiefly relevant to CJK fonts.
Use from-font when appropriate
The syntax also supports font-size-adjust: from-font, which uses a metric from the font rather than a numeric target. Decide whether that behavior fits your font stack, and verify the output in your target browsers; do not assume it will normalize every face to your preferred font’s proportions.
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Understand what gets preserved
The scaling relationship is u = (m / m′) s: m is the desired metric ratio, m′ is that metric ratio in the current font, s is the declared font size, and u is the adjusted size.
For example, MDN’s worked example uses a 12px preferred size, a target cap-height ratio of 0.20 and a fallback ratio of 0.15. The fallback is scaled to 16px: (0.20 / 0.15) × 12. That maintains the cap-height proportion, not identical letter shapes, line breaks, or spacing.
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MDN’s illustrative figures give Verdana an x-height aspect ratio of 0.545 and Times 0.447, and cap-height ratios of 0.73 and 0.66, respectively. These are values in MDN’s example, not universal measurements for every installed font file or version. See the MDN font-size-adjust reference, last modified September 16, 2026, for the syntax, formula, examples and compatibility summary.
Choose a metric for the design
- For ordinary body copy where lowercase readability is the concern, begin with
ex-heightand the preferred font’s x-height-to-font-size ratio. - For uppercase-heavy text, consider
cap-heightif capital height is the proportion you need to align. - For designs that depend on the width of “0” or the size of “水”, evaluate
ch-widthor the relevantic-*metric. - Keep a suitable font stack and tune
font-size,line-heightand spacing as needed. One adjusted metric cannot compensate for every difference between typefaces.
Check browser support and preserve a fallback
MDN labels font-size-adjust Baseline 2024 and says it has worked across the latest devices and browser versions since July 2024. Older browsers may not support it, and support for newer syntax may vary. Check the browser versions your project targets and retain a usable font-family stack without the property. MDN’s reference links the CSS Fonts Module Level 5, a living draft; its syntax should not be taken to mean every browser implements every part.
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If you want a separate enhancement path, test support with:
@supports (font-size-adjust: 0.5) {
.article-copy {
font-size-adjust: 0.52;
}
}Use a target number of zero or greater; zero produces text of zero height, while negative values are invalid. If an @font-face rule sets the size-adjust descriptor, that overridden metric is used in the font-size-adjust calculation. Combining the two can therefore behave differently from an adjustment based on the font’s unmodified metric.
Troubleshoot unexpected sizing
- The preferred font looks wrong too: the property applies to it as well as fallbacks. Measure or document the preferred face’s metric, then choose a matching target and check its weight and size.
- The fallback is still visually different: the property matches only the chosen metric. Adjust line height, spacing or the font stack separately; different glyph shapes and wrapping remain possible.
- The declaration has no effect: confirm support in the browser version being tested, and verify that the CSS selector applies to the text. Keep the font stack usable when the property is unsupported.
- The result changes after adding
size-adjust: the overridden metric participates in the calculation. Test the two adjustments together rather than treating them as independent. - Text disappears or becomes unusably small: check for a zero target. Negative values are invalid; use a nonzero ratio appropriate to the intended font metric.
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