To convert inches to millimeters, multiply by 25.4. So 1 in = 25.4 mm exactly, and 12 in = 304.8 mm. The factor is exact - one inch is defined as exactly 25.4 mm since the International Yard and Pound Agreement of 1959.
- Exact factor: 1 in = 25.4 mm (defined, not approximate).
- 1 in = 25.4 mm. 12 in = 304.8 mm (= 1 foot). 36 in = 914.4 mm (= 1 yard).
- Quick mental estimate: multiply by 25 (result is ~1.6% low).
- Essential for engineering, 3D printing, and hardware spec conversion.
How to Use This Converter
Type any inch value in the in field and the mm result appears instantly. You can also type in the mm field to convert back to inches - it works in both directions without pressing a button.
How Do You Convert Inches to Millimeters?
Multiply inches by 25.4. One inch is defined as exactly 25.4 mm - this is the foundational international definition from which all other inch-based metric conversions are derived.
$$\text{mm} = \text{in} \times 25.4$$
Worked examples: 0.5 in = 12.7 mm. 1 in = 25.4 mm. 6 in = 152.4 mm. 12 in = 304.8 mm.
Mental shortcut: multiply by 25 for a quick estimate. The error is 1.57%, which is usually acceptable for rough checks but not for precision work.
How Do You Convert Millimeters to Inches?
Divide millimeters by 25.4. For a dedicated reverse converter with reference table, see the millimeters to inches converter.
$$\text{in} = \text{mm} \div 25.4$$
Worked examples: 50 mm ÷ 25.4 = 1.9685 in. 100 mm ÷ 25.4 = 3.937 in. 254 mm ÷ 25.4 = 10 in.
Inches to Millimeters Reference Table
Common values converted using the exact factor 25.4 mm/in:
| Inches | Millimeters | Common context |
|---|---|---|
| 1/4 in | 6.35 mm | Common drill bit size |
| 1/2 in | 12.7 mm | Standard pipe size |
| 1 in | 25.4 mm | Definition anchor point |
| 1.5 in | 38.1 mm | Standard 1-1/2 inch lumber |
| 2 in | 50.8 mm | Common camera filter diameter |
| 3 in | 76.2 mm | Standard 3-inch drain pipe |
| 4 in | 101.6 mm | Standard concrete block width |
| 6 in | 152.4 mm | Half a foot |
| 8 in | 203.2 mm | Common tile size |
| 10 in | 254 mm | iPad screen (approximate) |
| 12 in | 304.8 mm | 1 foot exactly |
| 36 in | 914.4 mm | 1 yard exactly |
When Do You Need to Convert Inches to Millimeters?
Inches-to-mm is a daily conversion in technical fields where US and international standards intersect:
- Engineering drawings - converting US imperial specifications to metric for international manufacturing or ISO standards.
- 3D printing - many models are designed in inches; metric slicers require mm dimensions.
- Hardware and fasteners - US bolts are specified in fractional inches; metric equivalents are in mm.
- Woodworking - US lumber and tools use inch fractions; European machinery uses mm settings.
- Electronics - PCB trace widths, connector pitches, and enclosure dimensions often mix both systems.
Frequently Asked Questions
How many mm is 1 inch?
Exactly 25.4 mm. This is the international definition: the inch was standardized as 25.4 mm in 1959 by agreement between the US, UK, Canada, Australia, New Zealand, and South Africa. Every inch-to-mm value is a multiple of 25.4.
How do you convert inches to mm?
Multiply by 25.4. The calculation is straightforward because the factor is exact: 2 in = 50.8 mm, 3 in = 76.2 mm, 4 in = 101.6 mm. No rounding is required for any integer-inch input.
How many mm is 12 inches?
Exactly 304.8 mm. Since 12 inches = 1 foot, this means 1 ft = 304.8 mm = 30.48 cm = 0.3048 m - all the foot-based definitions chain from the same 25.4 mm anchor. For the cm conversion, see the inches to centimeters converter.
What is 2 inches in mm?
2 inches equals exactly 50.8 mm. Multiply 2 × 25.4 = 50.8. This is a common size for pipe fittings, drill bits, and camera filter threads.