diff --git a/src/lib/types/interfaces.ts b/src/lib/types/interfaces.ts
index 853e748..b54cb57 100644
--- a/src/lib/types/interfaces.ts
+++ b/src/lib/types/interfaces.ts
@@ -115,3 +115,20 @@ export interface Client {
 	/** ISO date string of the client's last order (null if no orders) */
 	lastOrderAt: string | null;
 }
+
+/**
+ * Represents aggregated financial totals for an entire order
+ */
+export interface OrderTotals {
+	/** Sum of all item costs */
+	totalCost: number;
+
+	/** Sum of all item revenues */
+	totalRevenue: number;
+
+	/** Sum of all item margins (totalRevenue - totalCost) */
+	totalMargin: number;
+
+	/** Total profit percentage ((totalMargin / totalRevenue) × 100) */
+	totalMarginPercent: number;
+}
diff --git a/src/lib/utils/calculations.spec.ts b/src/lib/utils/calculations.spec.ts
new file mode 100644
index 0000000..1155899
--- /dev/null
+++ b/src/lib/utils/calculations.spec.ts
@@ -0,0 +1,293 @@
+import { describe, it, expect } from 'vitest';
+import {
+	calculateItemCost,
+	calculateItemRevenue,
+	calculateMargin,
+	calculateProfitPercentage,
+	calculateOrderTotals
+} from './calculations';
+import type { OrderItem } from '$lib/types/interfaces';
+
+describe('Calculation Functions', () => {
+	describe('calculateItemCost', () => {
+		it('should calculate item cost by multiplying quantity and price per unit', () => {
+			const result = calculateItemCost(5, 10);
+			expect(result).toBe(50);
+		});
+
+		it('should handle decimal quantities', () => {
+			const result = calculateItemCost(2.5, 20);
+			expect(result).toBe(50);
+		});
+
+		it('should handle decimal prices', () => {
+			const result = calculateItemCost(10, 10.5);
+			expect(result).toBe(105);
+		});
+
+		it('should return 0 when quantity is 0', () => {
+			const result = calculateItemCost(0, 100);
+			expect(result).toBe(0);
+		});
+
+		it('should handle zero price', () => {
+			const result = calculateItemCost(5, 0);
+			expect(result).toBe(0);
+		});
+
+		it('should return correct result for large numbers', () => {
+			const result = calculateItemCost(1000, 999.99);
+			expect(result).toBe(999990);
+		});
+	});
+
+	describe('calculateItemRevenue', () => {
+		it('should calculate item revenue by multiplying quantity and sale price', () => {
+			const result = calculateItemRevenue(5, 15);
+			expect(result).toBe(75);
+		});
+
+		it('should handle decimal quantities', () => {
+			const result = calculateItemRevenue(2.5, 20);
+			expect(result).toBe(50);
+		});
+
+		it('should handle decimal prices', () => {
+			const result = calculateItemRevenue(10, 12.5);
+			expect(result).toBe(125);
+		});
+
+		it('should return 0 when quantity is 0', () => {
+			const result = calculateItemRevenue(0, 100);
+			expect(result).toBe(0);
+		});
+
+		it('should handle zero price', () => {
+			const result = calculateItemRevenue(5, 0);
+			expect(result).toBe(0);
+		});
+
+		it('should return correct result for large numbers', () => {
+			const result = calculateItemRevenue(1000, 1999.99);
+			expect(result).toBe(1999990);
+		});
+	});
+
+	describe('calculateMargin', () => {
+		it('should calculate margin as revenue minus cost', () => {
+			const result = calculateMargin(50, 75);
+			expect(result).toBe(25);
+		});
+
+		it('should handle decimal values', () => {
+			const result = calculateMargin(50.5, 75.75);
+			expect(result).toBeCloseTo(25.25, 5);
+		});
+
+		it('should return 0 when cost equals revenue', () => {
+			const result = calculateMargin(100, 100);
+			expect(result).toBe(0);
+		});
+
+		it('should return negative margin when cost exceeds revenue', () => {
+			const result = calculateMargin(100, 50);
+			expect(result).toBe(-50);
+		});
+
+		it('should handle zero values', () => {
+			const result = calculateMargin(0, 100);
+			expect(result).toBe(100);
+		});
+
+		it('should handle large numbers', () => {
+			const result = calculateMargin(5000, 7500);
+			expect(result).toBe(2500);
+		});
+	});
+
+	describe('calculateProfitPercentage', () => {
+		it('should calculate profit percentage as (margin / revenue) * 100', () => {
+			const result = calculateProfitPercentage(25, 75);
+			expect(result).toBeCloseTo(33.33, 2);
+		});
+
+		it('should return 0 when margin is 0', () => {
+			const result = calculateProfitPercentage(0, 100);
+			expect(result).toBe(0);
+		});
+
+		it('should return 100 when margin equals revenue', () => {
+			const result = calculateProfitPercentage(100, 100);
+			expect(result).toBe(100);
+		});
+
+		it('should return negative percentage when margin is negative', () => {
+			const result = calculateProfitPercentage(-50, 100);
+			expect(result).toBe(-50);
+		});
+
+		it('should handle decimal values', () => {
+			const result = calculateProfitPercentage(33.33, 100);
+			expect(result).toBeCloseTo(33.33, 2);
+		});
+
+		it('should handle high profit scenarios', () => {
+			const result = calculateProfitPercentage(200, 250);
+			expect(result).toBe(80);
+		});
+
+		it('should return Infinity when revenue is 0 and margin is positive', () => {
+			const result = calculateProfitPercentage(100, 0);
+			expect(result).toBe(Infinity);
+		});
+
+		it('should return NaN when both margin and revenue are 0', () => {
+			const result = calculateProfitPercentage(0, 0);
+			expect(Number.isNaN(result)).toBe(true);
+		});
+	});
+
+	describe('calculateOrderTotals', () => {
+		it('should calculate totals for single item order', () => {
+			const items: OrderItem[] = [
+				{
+					productId: 'prod-001',
+					quantity: 5,
+					itemCost: 50,
+					itemRevenue: 75,
+					itemMargin: 25,
+					itemMarginPercent: 33.33
+				}
+			];
+			const totals = calculateOrderTotals(items);
+			expect(totals.totalCost).toBe(50);
+			expect(totals.totalRevenue).toBe(75);
+			expect(totals.totalMargin).toBe(25);
+			expect(totals.totalMarginPercent).toBeCloseTo(33.33, 2);
+		});
+
+		it('should calculate totals for multiple items', () => {
+			const items: OrderItem[] = [
+				{
+					productId: 'prod-001',
+					quantity: 2,
+					itemCost: 20,
+					itemRevenue: 30,
+					itemMargin: 10,
+					itemMarginPercent: 33.33
+				},
+				{
+					productId: 'prod-002',
+					quantity: 3,
+					itemCost: 30,
+					itemRevenue: 45,
+					itemMargin: 15,
+					itemMarginPercent: 33.33
+				}
+			];
+			const totals = calculateOrderTotals(items);
+			expect(totals.totalCost).toBe(50);
+			expect(totals.totalRevenue).toBe(75);
+			expect(totals.totalMargin).toBe(25);
+			expect(totals.totalMarginPercent).toBeCloseTo(33.33, 2);
+		});
+
+		it('should handle empty order', () => {
+			const items: OrderItem[] = [];
+			const totals = calculateOrderTotals(items);
+			expect(totals.totalCost).toBe(0);
+			expect(totals.totalRevenue).toBe(0);
+			expect(totals.totalMargin).toBe(0);
+			expect(Number.isNaN(totals.totalMarginPercent)).toBe(true);
+		});
+
+		it('should handle mixed profit margins', () => {
+			const items: OrderItem[] = [
+				{
+					productId: 'prod-001',
+					quantity: 10,
+					itemCost: 100,
+					itemRevenue: 150,
+					itemMargin: 50,
+					itemMarginPercent: 33.33
+				},
+				{
+					productId: 'prod-002',
+					quantity: 20,
+					itemCost: 200,
+					itemRevenue: 300,
+					itemMargin: 100,
+					itemMarginPercent: 33.33
+				}
+			];
+			const totals = calculateOrderTotals(items);
+			expect(totals.totalCost).toBe(300);
+			expect(totals.totalRevenue).toBe(450);
+			expect(totals.totalMargin).toBe(150);
+			expect(totals.totalMarginPercent).toBeCloseTo(33.33, 2);
+		});
+
+		it('should handle orders with zero margin items', () => {
+			const items: OrderItem[] = [
+				{
+					productId: 'prod-001',
+					quantity: 5,
+					itemCost: 50,
+					itemRevenue: 50,
+					itemMargin: 0,
+					itemMarginPercent: 0
+				}
+			];
+			const totals = calculateOrderTotals(items);
+			expect(totals.totalCost).toBe(50);
+			expect(totals.totalRevenue).toBe(50);
+			expect(totals.totalMargin).toBe(0);
+			expect(totals.totalMarginPercent).toBe(0);
+		});
+
+		it('should handle orders with decimal costs and revenues', () => {
+			const items: OrderItem[] = [
+				{
+					productId: 'prod-001',
+					quantity: 2.5,
+					itemCost: 25.5,
+					itemRevenue: 38.25,
+					itemMargin: 12.75,
+					itemMarginPercent: 33.33
+				}
+			];
+			const totals = calculateOrderTotals(items);
+			expect(totals.totalCost).toBeCloseTo(25.5, 2);
+			expect(totals.totalRevenue).toBeCloseTo(38.25, 2);
+			expect(totals.totalMargin).toBeCloseTo(12.75, 2);
+			expect(totals.totalMarginPercent).toBeCloseTo(33.33, 2);
+		});
+
+		it('should return correct totalMarginPercent for order with different item margins', () => {
+			const items: OrderItem[] = [
+				{
+					productId: 'prod-001',
+					quantity: 10,
+					itemCost: 100,
+					itemRevenue: 200,
+					itemMargin: 100,
+					itemMarginPercent: 50
+				},
+				{
+					productId: 'prod-002',
+					quantity: 10,
+					itemCost: 100,
+					itemRevenue: 150,
+					itemMargin: 50,
+					itemMarginPercent: 33.33
+				}
+			];
+			const totals = calculateOrderTotals(items);
+			expect(totals.totalCost).toBe(200);
+			expect(totals.totalRevenue).toBe(350);
+			expect(totals.totalMargin).toBe(150);
+			// Total margin percent = (150 / 350) * 100 = 42.857...%
+			expect(totals.totalMarginPercent).toBeCloseTo(42.86, 2);
+		});
+	});
+});
diff --git a/src/lib/utils/calculations.ts b/src/lib/utils/calculations.ts
new file mode 100644
index 0000000..2f68018
--- /dev/null
+++ b/src/lib/utils/calculations.ts
@@ -0,0 +1,60 @@
+import type { OrderItem, OrderTotals } from '$lib/types/interfaces';
+
+/**
+ * Calculates the total cost (purchase price) for an item
+ * @param quantity - Number of units
+ * @param pricePerUnit - Purchase price per unit
+ * @returns Total cost (quantity × pricePerUnit)
+ */
+export function calculateItemCost(quantity: number, pricePerUnit: number): number {
+	return quantity * pricePerUnit;
+}
+
+/**
+ * Calculates the total revenue (selling price) for an item
+ * @param quantity - Number of units
+ * @param salePrice - Sale price per unit
+ * @returns Total revenue (quantity × salePrice)
+ */
+export function calculateItemRevenue(quantity: number, salePrice: number): number {
+	return quantity * salePrice;
+}
+
+/**
+ * Calculates the profit margin
+ * @param cost - Total cost amount
+ * @param revenue - Total revenue amount
+ * @returns Margin amount (revenue - cost)
+ */
+export function calculateMargin(cost: number, revenue: number): number {
+	return revenue - cost;
+}
+
+/**
+ * Calculates the profit percentage
+ * @param margin - Profit margin amount
+ * @param revenue - Total revenue amount
+ * @returns Profit percentage ((margin / revenue) × 100)
+ */
+export function calculateProfitPercentage(margin: number, revenue: number): number {
+	return (margin / revenue) * 100;
+}
+
+/**
+ * Calculates aggregated totals for an entire order
+ * @param items - Array of OrderItem objects
+ * @returns OrderTotals with aggregated cost, revenue, margin, and margin percentage
+ */
+export function calculateOrderTotals(items: OrderItem[]): OrderTotals {
+	const totalCost = items.reduce((sum, item) => sum + item.itemCost, 0);
+	const totalRevenue = items.reduce((sum, item) => sum + item.itemRevenue, 0);
+	const totalMargin = calculateMargin(totalCost, totalRevenue);
+	const totalMarginPercent = calculateProfitPercentage(totalMargin, totalRevenue);
+
+	return {
+		totalCost,
+		totalRevenue,
+		totalMargin,
+		totalMarginPercent
+	};
+}