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Millimetres to inches is the workshop conversion: engineering drawings, drill and tap sizes, sheet and timber thickness, watch case diameters, phone and laptop thicknesses. Everything outside the US specifies these in millimetres, and every US-facing catalogue lists them in fractional inches, so this is the bridge.
Frequently asked questions
What's the formula for mm to inches?
Divide by 25.4: 100 mm ÷ 25.4 = 3.937 inches. As a rough field check, 25 mm is about an inch and 3 mm is about an eighth.
What are the common fractional-inch equivalents?
The ones worth memorising: 25.4 mm = 1", 19.05 mm = ¾", 12.7 mm = ½", 9.525 mm = ⅜", 6.35 mm = ¼", 3.175 mm = ⅛". Hardware sold as "10 mm" is not the same as ⅜" — it's 0.394", which is why they don't interchange cleanly.