- How many millimeters are in an inch?
- Exactly 25.4. It isn't an approximation: since 1959 the inch has been defined as 25.4 mm, so every inches-to-millimeters conversion is a clean multiply. 2 in is 50.8 mm, 10 in is 254 mm, and a foot (12 in) is 304.8 mm.
- How do I convert a fraction of an inch to mm?
- Divide the top number by the bottom number to get decimal inches, then multiply by 25.4. 3/8 in is 3 ÷ 8 = 0.375 in, and 0.375 × 25.4 = 9.525 mm. For a mixed size like 1 3/8 in, add the whole inch first: 1.375 × 25.4 = 34.925 mm. Or type 1 3/8 into the box above and skip the arithmetic.
- What's a quick way to estimate inches to mm in my head?
- Multiply by 25, then add 2% more. 8 in → 200 → 204 mm (exact: 203.2 mm). Multiplying by 25 alone comes in about 1.6% low, which is fine for a rough idea and wrong for anything that has to fit: on a 36 in cabinet that's 14 mm short.
- Is a 5/8 in socket the same as a 16 mm socket?
- Close, but not the same. 5/8 in is 15.875 mm, so a 16 mm socket is 0.125 mm bigger. On a loose nut you won't notice. On a tight or rusted nut that small gap is enough to round the corners off, so use the matching size. The same trap shows up at 3/4 in (19.05 mm) against 19 mm, and at 1/2 in (12.7 mm) against 13 mm.
- How precise should the millimeter answer be?
- It depends on the job. Woodworking and home projects rarely need better than half a millimeter, so 9.5 mm for 3/8 in is plenty. Machining and 3D printing care about hundredths, which is why this converter keeps three decimals (1/64 in shows as 0.397 mm). The exact value is always 25.4 × your inches; Copy formula gives you the full calculation if you need to show your work.
- Why do tape measures use 1/16 in marks instead of decimals?
- Because halving is easy to see and easy to mark. Each step splits the one before it — 1/2, 1/4, 1/8, 1/16 — so you can find a mark without reading tiny numbers. The cost is that fractions don't add up cleanly in your head, and none of them land on a round millimeter. 1/16 in is 1.5875 mm, which is why metric plans and imperial tools never line up exactly.
- Does 1 3/8 mean 1.38 inches?
- No, and it's a common slip. 1 3/8 in means one inch plus three eighths, which is 1.375 in (34.925 mm). Reading it as 1.38 in is only 0.127 mm off, but reading 1 3/4 as 1.34 instead of 1.75 is more than 10 mm off. Type the fraction as written and let the converter do the division.
- Is what I type saved or sent anywhere?
- No. The conversion runs in your browser. Nothing is sent to a server or stored, and closing the tab clears it.
