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Inches to millimetres, for when an imperial spec has to be built or bought in metric: ordering metric fasteners against an inch drawing, setting up a 3D print from a US model, matching pipe and tube sizes, or converting a fractional inch into something a digital caliper will agree with.
Frequently asked questions
What's the formula for inches to mm?
Multiply by 25.4: 2 inches × 25.4 = 50.8 mm. This is exact — since the 1959 international yard and pound agreement, the inch is *defined* as 25.4 mm rather than measured against it.
How do I convert a fractional inch like 3/8?
Divide the fraction out first, then multiply: 3 ÷ 8 = 0.375, × 25.4 = 9.525 mm. Type the decimal into the value box. The common ones: ¼" = 6.35 mm, ½" = 12.7 mm, ¾" = 19.05 mm.