diff --git a/src/tiers/__tests__/get-quantity-for-tier.test.ts b/src/tiers/__tests__/get-quantity-for-tier.test.ts
new file mode 100644
index 00000000..d62e631c
--- /dev/null
+++ b/src/tiers/__tests__/get-quantity-for-tier.test.ts
@@ -0,0 +1,68 @@
+import { describe, it, expect } from 'vitest';
+import { getQuantityForTier } from '../get-quantity-for-tier';
+
+describe('getQuantityForTier', () => {
+  it('should return the difference between max and min when quantity >= max', () => {
+    expect(getQuantityForTier({ min: 10, max: 100, quantity: 150 })).toBe(90);
+  });
+
+  it('should return the difference between quantity and min when min <= quantity < max', () => {
+    expect(getQuantityForTier({ min: 10, max: 100, quantity: 50 })).toBe(40);
+
+    // Edge case: quantity just below max
+    expect(getQuantityForTier({ min: 10, max: 100, quantity: 99.9 })).toBe(89.9);
+
+    // Edge case: quantity equal to min
+    expect(getQuantityForTier({ min: 10, max: 100, quantity: 10 })).toBe(0);
+  });
+
+  it('should throw error when min is not a number', () => {
+    expect(() => {
+      // @ts-ignore - Testing runtime validation
+      getQuantityForTier({ min: 'not-a-number', max: 100, quantity: 50 });
+    }).toThrow('Tier min quantity must be a number');
+
+    expect(() => {
+      getQuantityForTier({ min: NaN, max: 100, quantity: 50 });
+    }).toThrow('Tier min quantity must be a number');
+  });
+
+  it('should throw error when max is not a number', () => {
+    expect(() => {
+      // @ts-ignore - Testing runtime validation
+      getQuantityForTier({ min: 10, max: 'not-a-number', quantity: 50 });
+    }).toThrow('Tier max quantity must be a number');
+
+    expect(() => {
+      getQuantityForTier({ min: 10, max: NaN, quantity: 50 });
+    }).toThrow('Tier max quantity must be a number');
+  });
+
+  it('should throw error when min >= max', () => {
+    expect(() => {
+      getQuantityForTier({ min: 100, max: 100, quantity: 150 });
+    }).toThrow('Tier min quantity must be less than tier max quantity');
+
+    expect(() => {
+      getQuantityForTier({ min: 110, max: 100, quantity: 150 });
+    }).toThrow('Tier min quantity must be less than tier max quantity');
+  });
+
+  it('should throw error when quantity < min', () => {
+    expect(() => {
+      getQuantityForTier({ min: 50, max: 100, quantity: 40 });
+    }).toThrow('Normalized quantity must be greater than tier min quantity');
+
+    expect(() => {
+      getQuantityForTier({ min: 50, max: 100, quantity: 49.9 });
+    }).toThrow('Normalized quantity must be greater than tier min quantity');
+  });
+
+  it('should handle decimal values correctly', () => {
+    // All decimal values
+    expect(getQuantityForTier({ min: 10.5, max: 100.5, quantity: 50.5 })).toBe(40);
+
+    // Quantity at max boundary
+    expect(getQuantityForTier({ min: 10.5, max: 100.5, quantity: 100.5 })).toBe(90);
+  });
+});