diff --git a/memory-bank/activeContext.md b/memory-bank/activeContext.md
index 2d4df6b..1170181 100644
--- a/memory-bank/activeContext.md
+++ b/memory-bank/activeContext.md
@@ -7,6 +7,8 @@
 ## Recent Changes
 - Created `memory-bank/projectBrief.md`.
 - Created `memory-bank/productContext.md`.
+- Implemented unit conversion on toggle in `src/main.ts`.
+- Extracted `convertUnits` utility and added tests.
 
 ## Current Focus
 - Completing Memory Bank initialization.
@@ -18,3 +20,4 @@
 - Using **Vanilla TypeScript** to avoid framework overhead.
 - Using **Vitest** for unit testing calculation logic.
 - Target: A single-page application that can be built into a self-contained output.
+- **Unit Toggle Behavior**: Switching units now automatically converts existing input values using a factor of 25.4. MM values are rounded to 1 decimal, Inches to 2 decimals.
diff --git a/src/calculator.test.ts b/src/calculator.test.ts
index 71bb3f9..d62f0da 100644
--- a/src/calculator.test.ts
+++ b/src/calculator.test.ts
@@ -1,5 +1,22 @@
 import { describe, it, expect } from 'vitest';
-import { calculateGridfinity, DEFAULT_SPEC } from './calculator';
+import { calculateGridfinity, DEFAULT_SPEC, convertUnits } from './calculator';
+
+describe('Unit Conversion', () => {
+  it('converts inches to mm correctly', () => {
+    expect(convertUnits(1, 'in', 'mm')).toBe(25.4);
+    expect(convertUnits(2, 'in', 'mm')).toBe(50.8);
+  });
+
+  it('converts mm to inches correctly', () => {
+    expect(convertUnits(25.4, 'mm', 'in')).toBe(1);
+    expect(convertUnits(50.8, 'mm', 'in')).toBe(2);
+  });
+
+  it('returns same value if units are same', () => {
+    expect(convertUnits(42, 'mm', 'mm')).toBe(42);
+    expect(convertUnits(1, 'in', 'in')).toBe(1);
+  });
+});
 
 describe('Gridfinity Calculator', () => {
   it('calculates standard 42mm grids correctly in mm', () => {
diff --git a/src/calculator.ts b/src/calculator.ts
index f10d1b5..03d848d 100644
--- a/src/calculator.ts
+++ b/src/calculator.ts
@@ -29,7 +29,14 @@ export interface CalculatorResults {
   remainingHeight: number;
 }
 
-const INCH_TO_MM = 25.4;
+export const INCH_TO_MM = 25.4;
+
+export function convertUnits(value: number, from: 'mm' | 'in', to: 'mm' | 'in'): number {
+  if (from === to) return value;
+  if (from === 'in' && to === 'mm') return value * INCH_TO_MM;
+  if (from === 'mm' && to === 'in') return value / INCH_TO_MM;
+  return value;
+}
 
 export function calculateGridfinity(inputs: CalculatorInputs): CalculatorResults {
   const { width, length, height, unit, spec } = inputs;
diff --git a/src/main.ts b/src/main.ts
index 7a83c78..b01fede 100644
--- a/src/main.ts
+++ b/src/main.ts
@@ -1,4 +1,4 @@
-import { calculateGridfinity, GridfinitySpec } from './calculator';
+import { calculateGridfinity, GridfinitySpec, convertUnits } from './calculator';
 
 // DOM Elements
 const inputWidth = document.getElementById('input-width') as HTMLInputElement;
@@ -69,6 +69,17 @@ function updateResults() {
 });
 
 unitMmBtn.addEventListener('click', () => {
+  if (currentUnit === 'mm') return;
+
+  // Convert values
+  const width = parseFloat(inputWidth.value) || 0;
+  const length = parseFloat(inputLength.value) || 0;
+  const height = parseFloat(inputHeight.value) || 0;
+
+  if (width > 0) inputWidth.value = convertUnits(width, 'in', 'mm').toFixed(1);
+  if (length > 0) inputLength.value = convertUnits(length, 'in', 'mm').toFixed(1);
+  if (height > 0) inputHeight.value = convertUnits(height, 'in', 'mm').toFixed(1);
+
   currentUnit = 'mm';
   unitMmBtn.classList.add('bg-white', 'shadow-sm');
   unitMmBtn.classList.remove('text-slate-600');
@@ -78,6 +89,17 @@ unitMmBtn.addEventListener('click', () => {
 });
 
 unitInBtn.addEventListener('click', () => {
+  if (currentUnit === 'in') return;
+
+  // Convert values
+  const width = parseFloat(inputWidth.value) || 0;
+  const length = parseFloat(inputLength.value) || 0;
+  const height = parseFloat(inputHeight.value) || 0;
+
+  if (width > 0) inputWidth.value = convertUnits(width, 'mm', 'in').toFixed(2);
+  if (length > 0) inputLength.value = convertUnits(length, 'mm', 'in').toFixed(2);
+  if (height > 0) inputHeight.value = convertUnits(height, 'mm', 'in').toFixed(2);
+
   currentUnit = 'in';
   unitInBtn.classList.add('bg-white', 'shadow-sm');
   unitInBtn.classList.remove('text-slate-600');