diff --git a/crates/engine/src/tests/toolsettings.rs b/crates/engine/src/tests/toolsettings.rs
index 194f3ab26b..2ec7667bde 100644
--- a/crates/engine/src/tests/toolsettings.rs
+++ b/crates/engine/src/tests/toolsettings.rs
@@ -290,9 +290,11 @@ fn legacy_spiral_decay_preferences_report_the_ratio_drawn_after_reopening() {
         assert_eq!(anchors.len(), 5);
         assert!(!path.is_closed());
         let center = Point::new(200.0, 200.0);
+        // The path runs from the centre end outwards, and decay is per full wind: four quarter-turn
+        // segments give the decay.
         for pair in anchors.windows(2) {
             let ratio = pair[0].p.distance(center) / pair[1].p.distance(center);
-            assert!((100.0 * ratio - decay).abs() < 1e-10);
+            assert!((100.0 * ratio.powi(4) - decay).abs() < 1e-9);
         }
         assert_eq!(s.prefs.tool_settings["spiral"]["decay"], json!(99.99));
         s.execute("edit.undo", &json!({})).unwrap();
diff --git a/crates/tools/src/draw2/family.rs b/crates/tools/src/draw2/family.rs
index 97193ec431..2e93888d02 100644
--- a/crates/tools/src/draw2/family.rs
+++ b/crates/tools/src/draw2/family.rs
@@ -545,12 +545,13 @@ mod tests {
             let (id, params) = t.command(Point::ZERO, Point::new(100.0, 0.0), Mods::default());
             assert_eq!(id, "shape.spiral");
             assert_eq!(params["decay"], json!(expected));
-            // Inspect the actual adjacent radii, not a second copy of the tool's clamp.
+            // Inspect the actual adjacent radii, not a second copy of the tool's clamp. Decay is per full
+            // wind and a segment is a quarter turn, so four segments give the decay.
             let path = vectorcraft_geom::shapes::spiral(Point::ZERO, 100.0, params["decay"].as_f64().unwrap(), 2, true);
             let anchors = &path.subpaths[0].anchors;
             let outer = anchors[2].p.distance(Point::ZERO);
             let next = anchors[1].p.distance(Point::ZERO);
-            assert!((100.0 * next / outer - expected).abs() < 1e-10);
+            assert!((100.0 * (next / outer).powi(4) - expected).abs() < 1e-9);
         }
         for invalid in [Value::Null, json!("80"), json!(false), json!({})] {
             t.set_option("decay", &invalid);