diff --git a/apps/web/lib/api/sales/calculate-sale-earnings.ts b/apps/web/lib/api/sales/calculate-sale-earnings.ts
index b119674aceb..79a0d227a52 100644
--- a/apps/web/lib/api/sales/calculate-sale-earnings.ts
+++ b/apps/web/lib/api/sales/calculate-sale-earnings.ts
@@ -21,7 +21,7 @@ export const calculateSaleEarnings = ({
   if (reward.type === "flat") {
     return sale.quantity * amount;
   } else if (reward.type === "percentage") {
-    return sale.amount * (amount / 100);
+    return Math.round((sale.amount * amount) / 100);
   }
 
   return 0;
diff --git a/apps/web/tests/commissions/calculate-sale-earnings.test.ts b/apps/web/tests/commissions/calculate-sale-earnings.test.ts
new file mode 100644
index 00000000000..0c9c149afaa
--- /dev/null
+++ b/apps/web/tests/commissions/calculate-sale-earnings.test.ts
@@ -0,0 +1,162 @@
+import { calculateSaleEarnings } from "@/lib/api/sales/calculate-sale-earnings";
+import { describe, expect, test } from "vitest";
+
+describe("calculateSaleEarnings", () => {
+  describe("percentage – same as the old truncate path", () => {
+    test.each([
+      {
+        label: "whole cents: $10 sale at 20%",
+        saleAmount: 1000,
+        percent: 20,
+        expected: 200,
+      },
+      {
+        label: "whole cents: $10 sale at 50%",
+        saleAmount: 1000,
+        percent: 50,
+        expected: 500,
+      },
+      {
+        label: "whole cents: $19 sale at 10%",
+        saleAmount: 1900,
+        percent: 10,
+        expected: 190,
+      },
+      {
+        label: "1.4¢ truncates and rounds to the same value",
+        saleAmount: 7,
+        percent: 20,
+        expected: 1,
+      },
+      {
+        label: "0.4¢ stays 0 (below half a cent)",
+        saleAmount: 2,
+        percent: 20,
+        expected: 0,
+      },
+      {
+        label: "zero sale amount",
+        saleAmount: 0,
+        percent: 20,
+        expected: 0,
+      },
+      {
+        label: "zero percent",
+        saleAmount: 1000,
+        percent: 0,
+        expected: 0,
+      },
+    ])("$label → $expected", ({ saleAmount, percent, expected }) => {
+      expect(
+        calculateSaleEarnings({
+          reward: {
+            type: "percentage",
+            amountInCents: null,
+            amountInPercentage: percent,
+          },
+          sale: { amount: saleAmount, quantity: 1 },
+        }),
+      ).toBe(expected);
+    });
+  });
+
+  describe("percentage – half-up (new vs old truncate)", () => {
+    test.each([
+      {
+        label: "3¢ sale at 20% (ticket: 0.6¢, used to store 0)",
+        saleAmount: 3,
+        percent: 20,
+        expected: 1,
+      },
+      {
+        label: "15¢ sale at 10% (1.5¢, used to store 1)",
+        saleAmount: 15,
+        percent: 10,
+        expected: 2,
+      },
+      {
+        label: "1¢ sale at 50% (0.5¢, used to store 0)",
+        saleAmount: 1,
+        percent: 50,
+        expected: 1,
+      },
+      {
+        label: "500¢ at 2.9% (14.5¢; float 2.9/100 used to round to 14)",
+        saleAmount: 500,
+        percent: 2.9,
+        expected: 15,
+      },
+    ])("$label → $expected", ({ saleAmount, percent, expected }) => {
+      expect(
+        calculateSaleEarnings({
+          reward: {
+            type: "percentage",
+            amountInCents: null,
+            amountInPercentage: percent,
+          },
+          sale: { amount: saleAmount, quantity: 1 },
+        }),
+      ).toBe(expected);
+    });
+  });
+
+  test("percentage ignores quantity (uses sale amount only)", () => {
+    expect(
+      calculateSaleEarnings({
+        reward: {
+          type: "percentage",
+          amountInCents: null,
+          amountInPercentage: 20,
+        },
+        sale: { amount: 1000, quantity: 5 },
+      }),
+    ).toBe(200);
+  });
+
+  describe("flat", () => {
+    test.each([
+      {
+        label: "single sale",
+        amountInCents: 5000,
+        quantity: 1,
+        expected: 5000,
+      },
+      {
+        label: "quantity multiplies the flat amount",
+        amountInCents: 500,
+        quantity: 2,
+        expected: 1000,
+      },
+      {
+        label: "zero quantity",
+        amountInCents: 500,
+        quantity: 0,
+        expected: 0,
+      },
+    ])("$label → $expected", ({ amountInCents, quantity, expected }) => {
+      expect(
+        calculateSaleEarnings({
+          reward: {
+            type: "flat",
+            amountInCents,
+            amountInPercentage: null,
+          },
+          sale: { amount: 1000, quantity },
+        }),
+      ).toBe(expected);
+    });
+  });
+
+  test("returns 0 when reward type is neither flat nor percentage", () => {
+    expect(
+      calculateSaleEarnings({
+        reward: {
+          type: "unknown" as "flat",
+          amountInCents: 500,
+          amountInPercentage: 20,
+        },
+        sale: { amount: 1000, quantity: 1 },
+      }),
+    ).toBe(0);
+  });
+});