diff --git a/lib/src/interval/size.dart b/lib/src/interval/size.dart
index 3a196b18..230366d9 100644
--- a/lib/src/interval/size.dart
+++ b/lib/src/interval/size.dart
@@ -130,12 +130,9 @@ extension type const Size._(int size) implements int {
   /// ```
   int get semitones {
     final absSimple = simple.abs();
-    final octaveShift = chromaticDivisions * (absShift ~/ octave);
-    // We exclude perfect octaves (simplified as 8) from the lookup to consider
-    // them 0 (as if they were modulo `Size.octave`).
-    final size = Size(absSimple == octave ? 1 : absSimple);
+    final octaves = (abs() - absSimple) ~/ 7;
 
-    return (_sizeToSemitones[size]! + octaveShift) * sign;
+    return (_sizeToSemitones[absSimple]! + octaves * 12) * sign;
   }
 
   /// The absolute [Size] value taking octave shift into account.
diff --git a/test/src/interval/interval_test.dart b/test/src/interval/interval_test.dart
index 0dadf292..b76782f3 100644
--- a/test/src/interval/interval_test.dart
+++ b/test/src/interval/interval_test.dart
@@ -405,7 +405,33 @@ void main() {
 
         expect(const Interval.perfect(Size(15)).semitones, 24);
         expect(const Interval.perfect(Size(22)).semitones, 36);
+        expect(
+          const Interval.imperfect(Size(23), ImperfectQuality.minor).semitones,
+          37,
+        );
+        expect(
+          const Interval.imperfect(Size(23), ImperfectQuality.major).semitones,
+          38,
+        );
+        expect(
+          const Interval.imperfect(Size(-24), ImperfectQuality.minor).semitones,
+          -39,
+        );
+        expect(
+          const Interval.imperfect(Size(24), ImperfectQuality.major).semitones,
+          40,
+        );
+        expect(const Interval.perfect(Size(25)).semitones, 41);
+        expect(const Interval.perfect(Size(-26)).semitones, -43);
         expect(const Interval.perfect(Size(29)).semitones, 48);
+        expect(
+          const Interval.imperfect(Size(30), ImperfectQuality.minor).semitones,
+          49,
+        );
+        expect(
+          const Interval.imperfect(Size(-30), ImperfectQuality.major).semitones,
+          -50,
+        );
       });
     });