diff --git a/apps/api/src/combat/combat-damage.spec.ts b/apps/api/src/combat/combat-damage.spec.ts
new file mode 100644
index 0000000..a5a2c4e
--- /dev/null
+++ b/apps/api/src/combat/combat-damage.spec.ts
@@ -0,0 +1,17 @@
+import { calculateDamage } from './combat-damage';
+
+describe('calculateDamage', () => {
+  it('applies the established armor mitigation formula', () => {
+    // raw = 12 + 15 = 27; 27 * 60 / (60 + 20) = 20.25 -> rounds to 20
+    expect(calculateDamage({ attack: 12, weaponDamage: 15 }, 20)).toBe(20);
+  });
+
+  it('never returns less than 1 damage, even against extreme armor', () => {
+    expect(calculateDamage({ attack: 1, weaponDamage: 0 }, 100_000)).toBe(1);
+  });
+
+  it('treats an attacker with no weaponDamage as having attack alone as its raw damage', () => {
+    // raw = 9; 9 * 60 / (60 + 5) = 8.307... -> rounds to 8
+    expect(calculateDamage({ attack: 9 }, 5)).toBe(8);
+  });
+});
diff --git a/apps/api/src/combat/combat-damage.ts b/apps/api/src/combat/combat-damage.ts
new file mode 100644
index 0000000..a8d8810
--- /dev/null
+++ b/apps/api/src/combat/combat-damage.ts
@@ -0,0 +1,13 @@
+export interface DamageAttacker {
+  attack: number;
+  weaponDamage?: number;
+}
+
+const ARMOR_MITIGATION_CONSTANT = 60;
+
+export function calculateDamage(attacker: DamageAttacker, targetArmor: number): number {
+  const rawDamage = attacker.attack + (attacker.weaponDamage ?? 0);
+  const mitigatedDamage =
+    (rawDamage * ARMOR_MITIGATION_CONSTANT) / (ARMOR_MITIGATION_CONSTANT + targetArmor);
+  return Math.max(1, Math.round(mitigatedDamage));
+}