Mm to Inches Converter
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Convert millimeters to inches (and back) instantly. Exact definition: 25.4 mm = 1 in — useful for 3D printing, bolts, and precision measurements. Metric drill bits are labeled in mm; US hardware stores often list inches. Swap direction to verify parts fit.
Millimeters
mm
Inches
in
1 mm = 0.03937 in
Convert Millimeters
Quick presets
Inches
1 in
25.4 mm = 1 in
- 1 mm
- 0.03937 in
- 3 mm
- 0.11811 in
- 6 mm
- 0.23622 in
- 10 mm
- 0.393701 in
- 25.4 mm
- 1 in
- 50 mm
- 1.9685 in
- 100 mm
- 3.937 in
- 210 mm
- 8.2677 in
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Millimeters vs inches
Millimeters are the everyday metric unit for small lengths — bolt diameters, 3D-printer models, phone thicknesses, and paper specs. Inches still dominate many US tool markings, pipe fittings, and DIY plans. One inch equals exactly 25.4 millimeters, so converting is exact rather than approximate.
Worked example: a part specified as 0.5 in is 0.5 × 25.4 = 12.7 mm. Three millimeters is about 0.118 in — near 1/8 inch for rough carpentry, but not exact enough for press fits or gasket shims without the precise value.
Caveat: metric fastener names (M6 = 6 mm nominal thread diameter) are a different system from US screw gauges. Entering inches into a slicer that expects millimeters will print the wrong size — convert first, then model.
Round-trip sanity check: 25.4 mm ÷ 25.4 = 1.000 in exactly — the inch is defined from the millimeter. A slicer set to inches but fed mm numbers prints 25.4× too large or small; convert before import.
Common questions
My 3D printer slicer uses mm but the part spec is in inches — what do I do?
Convert the inch dimensions to mm before modeling or scaling. Example: 0.5 in → 12.7 mm. Slicers assume mm by default; entering inches without converting will print the wrong size. Round-trip: 12.7 mm ÷ 25.4 = 0.5 in.
Are M6 bolts 6 mm in diameter?
Yes — the “M6” ISO metric designation means 6 mm nominal outer thread diameter. US #6 screws are a different system; do not mix metric mm sizing with US screw gauge numbers. Round-trip: 6 mm ÷ 25.4 ≈ 0.236 in.
How thick is 3 mm in inches?
3 mm is about 0.118 in — roughly 1/8 inch for rough work, but not exact. For gasket shims or press fits, use the precise conversion rather than fractional inch approximations. Round-trip: 0.118 in × 25.4 ≈ 3.00 mm.
Is A4 paper width easy to convert to inches?
A4 is 210 mm wide — exactly 8.27 in. US Letter is 8.5 in wide. The 0.23 in difference matters for printers, binders, and slide templates. Round-trip: 8.27 in × 25.4 = 210 mm.