Convert inches to millimeters easily.

Imagine you're building a wooden shelf and the plans call for 18-inch spacing between brackets. But your drill bits are labeled in millimeters. Panic? Nah. Converting inches to millimeters is simpler than assembling IKEA furniture—if you know the trick. This guide breaks down everything from the basic formula to why your tape measure has both units.

Unit definitions

What is a inch (in)?

• Description: The inch is a traditional unit of length, primarily used in the U.S. and UK.
• Symbol: in
• Common uses: Measuring screen sizes, plumbing pipes, and construction materials.
• Definition: Since 1959, 1 inch has been defined as exactly 25.4 millimeters.

What is a millimeter (mm)?

• Description: A millimeter is a metric unit, representing one-thousandth of a meter.
• Symbol: mm
• Common uses: Engineering blueprints, medical devices, and precision manufacturing.
• Definition: 1 millimeter equals 0.001 meters or 0.03937 inches.

Conversion formula

The magic number here is 25.4. To convert inches to millimeters:

Millimeters = Inches × 25.4

For example, 5 inches become 5 × 25.4 = 127 mm. To go the other way, divide millimeters by 25.4.

Example calculations

1. Converting a 12-inch ruler:
   12 in × 25.4 = 304.8 mm
   That's why school rulers often show 30 cm (300 mm) on one side and 12 inches on the other.
2. Smartphone screen size:
   A 6.1-inch display measures 6.1 × 25.4 = 154.94 mm diagonally.

Conversion tables

Inches to millimeters

Millimeters to inches

Historical background

The inch has roots in ancient Rome, where an "uncia" was 1/12 of a Roman foot. Fast forward to medieval England, where King Edward II declared an inch as three barleycorns end-to-end. Meanwhile, the millimeter emerged during the French Revolution as part of the metric system, designed to simplify trade and science. The 1959 international yard agreement finally locked in the 25.4 mm per inch ratio we use so much today.

Interesting facts?

1. Global glue: 95% of the world uses millimeters, but the inch persists in U.S. manufacturing.
2. Precision matters: NASA's Mars Climate Orbiter crashed in 1999 due to a unit mix-up between pounds-force and newtons.
3. Double identity: In manufacturing, "mil" means 0.001 inches—not millimeters!
4. Nature's ruler: The inchworm caterpillar moves by stretching 1-inch segments.
5. Screen smarts: Phone screens use millimeters internally but market sizes in inches globally.

FAQ