diff --git a/crates/fonts/src/ttf.rs b/crates/fonts/src/ttf.rs
--- a/crates/fonts/src/ttf.rs
+++ b/crates/fonts/src/ttf.rs
@@ -718,9 +718,36 @@ impl OutlinePen for Pen {
     }
 }
 
-/// Cap height of a font in font units at 1000 units per em: the OS/2 value, else the height
-/// of its "H", else an estimate from the ascent.
+/// Cap heights the installation pins for metric-compatible substitutes, from `CADCRAFT_CAP_HEIGHT`
+/// (`LiberationSans=715.8;LiberationSans-Bold=715.8`): a substitute keeps the advance widths of
+/// the font it stands in for but not its cap height, so scaling by the substitute's own OS/2
+/// value draws text the size of the substitute, not of the requested font. Each entry names a
+/// PostScript name and its cap height in font units at 1000 per em.
+fn cap_height_overrides() -> &'static HashMap<String, f64> {
+    static OVERRIDES: OnceLock<HashMap<String, f64>> = OnceLock::new();
+    OVERRIDES.get_or_init(|| parse_cap_height_overrides(&std::env::var("CADCRAFT_CAP_HEIGHT").unwrap_or_default()))
+}
+
+/// Parse a `Name=height;…` list; entries without a name or with a non-positive or non-finite
+/// height are dropped. Names are matched normalised, like family names.
+fn parse_cap_height_overrides(spec: &str) -> HashMap<String, f64> {
+    spec.split(';')
+        .filter_map(|entry| {
+            let (name, height) = entry.split_once('=')?;
+            let height: f64 = height.trim().parse().ok()?;
+            (height.is_finite() && height > 0.0 && !name.trim().is_empty()).then(|| (norm(name.trim()), height))
+        })
+        .collect()
+}
+
+/// Cap height of a font in font units at 1000 units per em: the value `CADCRAFT_CAP_HEIGHT`
+/// pins for this font's PostScript name, else the OS/2 value, else the height of its "H",
+/// else an estimate from the ascent.
 fn cap_height(f: &FontRef) -> f64 {
+    let named = f.localized_strings(skrifa::string::StringId::POSTSCRIPT_NAME).next().map(|s| norm(&s.to_string()));
+    if let Some(height) = named.as_deref().and_then(|n| cap_height_overrides().get(n)) {
+        return *height;
+    }
     let metrics = f.metrics(Size::new(UPEM), LocationRef::default());
     let measured = || {
         let gid = f.charmap().map('H')?;
@@ -934,6 +961,33 @@ mod family_tests {
     }
 }
 
+#[cfg(test)]
+mod cap_height_override_tests {
+    use super::*;
+    use std::collections::HashMap;
+
+    #[test]
+    fn cap_height_env_parses_name_height_entries() {
+        let m = parse_cap_height_overrides("LiberationSans=715.8;LiberationSans-Bold=715.8");
+        let mut want = HashMap::new();
+        want.insert("liberationsans".to_string(), 715.8);
+        want.insert("liberationsansbold".to_string(), 715.8);
+        assert_eq!(m, want);
+        assert_eq!(parse_cap_height_overrides("LiberationSans = 715.8 ; NotoSansCJK=1466"), parse_cap_height_overrides("LiberationSans=715.8;NotoSansCJK=1466").into_iter().collect::<HashMap<_, _>>(), "spaces around names and heights are tolerated");
+        assert!(parse_cap_height_overrides("noheight;=5;Arial=0;Arial=-1;Arial=abc").is_empty(), "entries without a name or a positive finite height are dropped");
+        assert!(parse_cap_height_overrides("").is_empty());
+    }
+
+    #[test]
+    fn cap_height_override_applies_only_to_the_named_font() {
+        let m = parse_cap_height_overrides("LiberationSans=715.8");
+        assert_eq!(m.get(&norm("LiberationSans")), Some(&715.8));
+        assert_eq!(m.get(&norm("Liberation Sans")), Some(&715.8), "names match normalised");
+        assert_eq!(m.get(&norm("DejaVuSans")), None);
+        assert_eq!(m.get(&norm("LiberationSans-Bold")), None);
+    }
+}
+
 #[cfg(test)]
 mod geometry_budget_tests {
     use super::*;