diff --git a/utils/format-percentage.test.ts b/utils/format-percentage.test.ts
new file mode 100644
index 0000000..9e8e52d
--- /dev/null
+++ b/utils/format-percentage.test.ts
@@ -0,0 +1,60 @@
+/* eslint-disable @typescript-eslint/no-unused-vars, @typescript-eslint/no-var-requires */
+import { formatPercentage } from "./format-percentage";
+
+describe("formatPercentage", () => {
+  describe("null values", () => {
+    it("should return '0%' for null", () => {
+      expect(formatPercentage(null)).toBe("0%");
+    });
+  });
+
+  describe("NaN values", () => {
+    it("should return '0%' for NaN", () => {
+      expect(formatPercentage(NaN)).toBe("0%");
+    });
+  });
+
+  describe("positive integers", () => {
+    it("should format positive integers correctly", () => {
+      expect(formatPercentage(0)).toBe("0%");
+      expect(formatPercentage(1)).toBe("1%");
+      expect(formatPercentage(50)).toBe("50%");
+      expect(formatPercentage(100)).toBe("100%");
+      expect(formatPercentage(999)).toBe("999%");
+    });
+  });
+
+  describe("positive decimals", () => {
+    it("should floor positive decimal values", () => {
+      expect(formatPercentage(0.1)).toBe("0%");
+      expect(formatPercentage(0.9)).toBe("0%");
+      expect(formatPercentage(1.1)).toBe("1%");
+      expect(formatPercentage(1.9)).toBe("1%");
+      expect(formatPercentage(50.5)).toBe("50%");
+      expect(formatPercentage(99.99)).toBe("99%");
+    });
+  });
+
+  describe("negative numbers", () => {
+    it("should handle negative integers", () => {
+      expect(formatPercentage(-1)).toBe("-1%");
+      expect(formatPercentage(-50)).toBe("-50%");
+      expect(formatPercentage(-100)).toBe("-100%");
+    });
+
+    it("should floor negative decimal values", () => {
+      expect(formatPercentage(-0.1)).toBe("-1%");
+      expect(formatPercentage(-0.9)).toBe("-1%");
+      expect(formatPercentage(-1.1)).toBe("-2%");
+      expect(formatPercentage(-1.9)).toBe("-2%");
+      expect(formatPercentage(-50.5)).toBe("-51%");
+    });
+  });
+
+  describe("edge cases", () => {
+    it("should handle very large numbers", () => {
+      expect(formatPercentage(1000000)).toBe("1000000%");
+      expect(formatPercentage(999999.99)).toBe("999999%");
+    });
+  });
+});