How Many mm in an Inch
An mm-in-an-inch page converts lengths between imperial inches and metric millimeters. Since the 1959 international agreement, the yard is defined using the metric system, setting the standard inch as exactly 25.4 millimeters.
Convert inches to millimeters and learn about international length standards. Read formulas and view fraction reference tables.
Quick Answer
Convert inches to millimeters instantly using the exact 25.4 mm definition.
How Many Millimeters (mm) in an Inch?
There are exactly 25.4 millimeters in an inch.
This definition has been the worldwide standard for industrial and general length units since 1959.
Formula: 1 Inch = Exactly 25.4 Millimeters (mm)
Common Inch Fractions to mm
| Fraction (in) | Decimal (in) | Millimeters (mm) |
|---|---|---|
| 1/16 in | 0.0625 in | 1.5875 mm |
| 1/8 in | 0.125 in | 3.175 mm |
| 1/4 in | 0.25 in | 6.35 mm |
| 1/2 in | 0.50 in | 12.70 mm |
| 3/4 in | 0.75 in | 19.05 mm |
| 1 in | 1.00 in | 25.40 mm |
| 2 in | 2.00 in | 50.80 mm |
Examples
1 Inch
25.4 mm
0.5 Inches
12.7 mm
2 Inches
50.8 mm
How it works
The relationship between the imperial inch and the metric millimeter is defined internationally as:
1 Inch (in) = Exactly 25.4 Millimeters (mm)
Formula: mm = Inches × 25.4
Converting Inches to Millimeters
To convert inches to mm, multiply the number of inches by 25.4:
Inches × 25.4 = Millimeters
Example: 3 inches × 25.4 = 76.2 mm
To convert millimeters back to inches, divide the total millimeters by 25.4:
Millimeters ÷ 25.4 = Inches
Example: 100 mm ÷ 25.4 ≈ 3.937 inches
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