To convert millimeters to inches, divide by 25.4. So 25.4 mm = 1 in exactly, and 100 mm = 3.937 in. 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).
- 25.4 mm = 1 in exactly. 50.8 mm = 2 in exactly.
- Quick mental check: divide mm by 25 for an estimate (gives result ~1.6% high).
- Used heavily in engineering, 3D printing, woodworking, and hardware specifications.
How to Use This Converter
Type any millimeter value in the mm field and the inches result appears instantly. You can also type in the in field to convert back to millimeters - it works in both directions without pressing a button.
How Do You Convert Millimeters to Inches?
Divide millimeters by 25.4. One inch is defined as exactly 25.4 mm under the International Yard and Pound Agreement of 1959. This definition is exact with no rounding.
$$\text{in} = \text{mm} \div 25.4$$
Worked examples: 10 mm ÷ 25.4 = 0.3937 in. 50 mm ÷ 25.4 = 1.9685 in. 100 mm ÷ 25.4 = 3.937 in.
Mental shortcut: divide by 25 for a quick estimate - the result is about 1.6% higher than the true value. For precision work always use 25.4.
How Do You Convert Inches to Millimeters?
Multiply inches by 25.4. This is the same exact definition used in reverse. For a dedicated reverse converter, see the inches to millimeters converter.
$$\text{mm} = \text{in} \times 25.4$$
Worked examples: 1 in = 25.4 mm. 6 in = 152.4 mm. 12 in = 304.8 mm (= 1 foot).
Millimeters to Inches Reference Table
Common values converted using the exact factor 1 ÷ 25.4 = 0.039370...
| Millimeters | Inches (decimal) | Common context |
|---|---|---|
| 1 mm | 0.03937 in | Thickness of a credit card (~0.76 mm) |
| 5 mm | 0.19685 in | Standard pencil diameter |
| 10 mm | 0.39370 in | About 3/8 in; common bolt size |
| 12.7 mm | 0.5 in | Exactly 1/2 inch |
| 25 mm | 0.98425 in | Just under 1 inch |
| 25.4 mm | 1 in | Exactly 1 inch (definition) |
| 50 mm | 1.9685 in | 2-inch lens filter (~52 mm) |
| 100 mm | 3.9370 in | Standard 4-inch pipe (~101.6 mm) |
| 150 mm | 5.9055 in | 6-inch ruler length |
| 200 mm | 7.8740 in | Standard A4 width (210 mm) |
| 254 mm | 10 in | Exactly 10 inches |
| 300 mm | 11.811 in | 30 cm ruler |
When Do You Need to Convert Millimeters to Inches?
The mm-to-inch conversion is common wherever metric and imperial specifications appear side by side. Key use cases:
- Engineering and manufacturing - technical drawings and CAD files are often specified in mm in Europe, in inches in the US.
- 3D printing - slicer software and filament specs use both units depending on the manufacturer's market.
- Woodworking and DIY - European timber and hardware is sized in mm; US hardware stores use inches.
- Camera and optics - filter diameters, sensor sizes, and lens mounts are specified in mm; screen sizes in inches.
- Plumbing and fasteners - pipe diameters, drill bits, and bolt sizes mix both systems.
Frequently Asked Questions
How many inches is 1 mm?
1 mm equals 0.0393701 inches. In fractional terms that is approximately 5/127 of an inch - too small to be useful as a fraction in everyday work. For most practical purposes, note that 25.4 mm = 1 inch exactly.
How do you convert mm to inches?
Divide by 25.4. The factor is exact: the international inch is defined as exactly 25.4 mm. So 50 mm ÷ 25.4 = 1.9685 in, and 100 mm ÷ 25.4 = 3.9370 in.
How many mm is 1 inch?
Exactly 25.4 mm. This is the foundational definition - all other mm/in conversions derive from it. Use the inches to mm converter to check any inch value.
What is 10 mm in inches?
10 mm equals 0.393701 inches - just under 13/32 of an inch, or about 3/8 in for practical rounding. This is a common size for metric bolts and drill bits that corresponds closely to the imperial 3/8-inch standard.