Inches to Millimetres in Precision Work
One inch equals exactly 25.4 millimetres. To convert inches to millimetres, multiply by 25.4. This conversion is exact — defined by the 1959 international yard and pound agreement. Millimetres are the standard unit in engineering, CNC machining, woodworking, and 3D printing for precise dimensions. Inches persist in the US for hardware (pipe diameters: 1/2", 3/4", 1"), screen sizes (diagonal measurement in inches), and fastener sizes. When specifying tolerances in manufacturing, metric drawings use millimetres with tolerances like ±0.1 mm; imperial drawings use inches with tolerances like ±0.005".
Practical conversions: a standard sheet of paper (8.5 × 11 inches) = 215.9 × 279.4 mm. An A4 sheet is 210 × 297 mm. The common 1/8" increment used in framing = 3.175 mm. A 1/4" drill bit = 6.35 mm.
Inches to Millimetres Reference
| Inches | Millimetres | Common Item |
|---|---|---|
| 1/16 in | 1.588 mm | Thin material |
| 1/8 in | 3.175 mm | Standard framing increment |
| 1/4 in | 6.35 mm | Quarter inch bolt |
| 1/2 in | 12.7 mm | Half inch pipe |
| 1 in | 25.4 mm | One inch |
| 2 in | 50.8 mm | Two inches |
| 6 in | 152.4 mm | Six inches |
| 12 in | 304.8 mm | One foot |
