What this conversion means in practice
Fractional inches on US blueprints need millimeter equivalents for CNC posts and ISO partners. Multiply inches by 25.4.
One eighth inch (0.125 in) is 3.175 mm. Confusing 0.125 mm with 0.125 in is a common order-of-magnitude error.
Pipe and thread names (½ in NPT) refer to trade sizes, not literal measured OD; verify the drawing note.
How to convert inch to millimeter
Multiply the inch value by 25.4 to get millimeter.
Example: 0.125 inches × 25.4 = 3.175 mm
Inch
Definition: An inch (in) equals 0.0254 meters exactly.
History and origin: Historically varied; now fixed relative to the meter for consistency.
Current use: Used for displays, hardware dimensions, and many US specifications.
Millimeter
Definition: A millimeter (mm) equals one-thousandth of a meter.
History and origin: Adopted widely where metric precision is needed without fractions.
Current use: Used in machining, manufacturing, drafting, and tolerancing.
Inch to Millimeter conversion table
| Inch (inches) | Millimeter (mm) |
|---|---|
| 0.1 inches | 2.54 mm |
| 1 inches | 25.4 mm |
| 2 inches | 50.8 mm |
| 3 inches | 76.2 mm |
| 5 inches | 127 mm |
| 10 inches | 254 mm |
| 20 inches | 508 mm |
| 50 inches | 1,270 mm |
| 100 inches | 2,540 mm |
| 1,000 inches | 25,400 mm |
Inch to Millimeter FAQ
inches to mm for metric manufacturing and inspection reports.
How many mm is 1 inch?
25.4 mm exactly.
How many mm is ½ inch?
12.7 mm (0.5 × 25.4).
Centimeters?
Multiply inches by 2.54 for cm.