diff --git a/crates/doc/src/expr.rs b/crates/doc/src/expr.rs
index 9c8ca556d4..935698e9f4 100644
--- a/crates/doc/src/expr.rs
+++ b/crates/doc/src/expr.rs
@@ -589,9 +589,11 @@ pub fn is_literal(s: &str) -> bool {
     }
 }
 
-/// Is the expression a bare number without a unit, like `20` or `-1.5`?
+/// Is the expression a bare number without a unit, like `20` or `-1.5`? A leading `=` only marks
+/// a formula (#4681), so `=20` is the same bare number.
 pub fn is_bare_number(s: &str) -> bool {
-    is_literal(s) && s.trim().trim_start_matches(['-', '+']).trim().parse::<f64>().is_ok()
+    let number = s.trim().strip_prefix('=').unwrap_or(s);
+    is_literal(s) && number.trim().trim_start_matches(['-', '+']).trim().parse::<f64>().is_ok()
 }
 
 /// Is `s` shaped like a parameter name: a letter or `_`, then letters, digits and `_`? The shape
@@ -1449,6 +1451,8 @@ mod tests {
         assert!(is_literal("20") && is_literal("-1.5 in") && is_literal("3 deg"));
         assert!(!is_literal("d1") && !is_literal("2 * 3") && !is_literal("2 width"));
         assert!(is_bare_number(" -1.5 ") && is_bare_number("+2") && !is_bare_number("20 mm") && !is_bare_number("(2)"));
+        // A leading `=` only marks a formula (#4681): `=5` is the bare number `5`, not `==5`.
+        assert!(is_bare_number("=5") && is_bare_number(" = -1.5") && !is_bare_number("=5 mm") && !is_bare_number("==5") && !is_bare_number("=d3"));
         assert_eq!((length_unit_or_mm(" in "), length_unit_or_mm("deg"), length_unit_or_mm("")), ("in", "mm", "mm"));
     }
 
diff --git a/crates/doc/src/params.rs b/crates/doc/src/params.rs
index 87f601dcbe..78e6b2c869 100644
--- a/crates/doc/src/params.rs
+++ b/crates/doc/src/params.rs
@@ -1150,6 +1150,19 @@ mod design_length_tests {
         assert_eq!(written("", 5.0), w("mm", "5 mm"));
     }
 
+    /// A bare number typed as a formula (`=5`) means the design's units like `5` does.
+    #[test]
+    fn a_formula_that_is_a_bare_number_gets_the_design_unit() {
+        let mut doc = Document::new("Units");
+        doc.units = "in".into();
+        let length = crate::expr::Kind::Length;
+        assert_eq!(doc.with_design_unit("5", length), "5 in");
+        assert_eq!(doc.with_design_unit("=5", length), "=5 in");
+        assert_eq!(doc.with_design_unit("=5 mm", length), "=5 mm");
+        let inches = doc.eval(&doc.with_design_unit("=5", length), length).unwrap();
+        assert!((inches - 127.0).abs() < 1e-9, "{inches}");
+    }
+
     #[test]
     fn the_written_value_reads_back_within_a_micrometre() {
         for units in ["mm", "cm", "m", "in", "ft"] {