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Italiano1 mm = 0.03937 inch: the formula
To convert mm to inches, divide by 25.4: inches = mm ÷ 25.4. The factor is exact. Example: a 6 mm drill bit is 6 ÷ 25.4 = 0.236 in.
The converter on this page is already set to millimeters to inches and opens on 10 mm = 0.393701 in. The answer updates as you type, to six significant figures, and the "Formula:" line reads 1 mm = 0.0393700787402 in. The ⇄ "Swap units" button flips it to inches to mm, and the "In every unit" table adds centimeters, feet and yards.
mm to inches conversion chart
| Millimeters | Inches | Where you see it |
|---|---|---|
| 1 | 0.0394 | Smallest mark on a metric ruler |
| 6 | 0.2362 | M6 bolt, just under 1/4 in |
| 7.8 | 0.3071 | Thickness of a slim phone |
| 8 | 0.3150 | M8 bolt, close to 5/16 in |
| 10 | 0.3937 | 10 mm socket |
| 13 | 0.5118 | 13 mm wrench, a hair over 1/2 in |
| 18 | 0.7087 | European plywood sheet |
| 19 | 0.7480 | 19 mm socket, near 3/4 in |
| 25 | 0.9843 | Heavy rain in a day |
| 25.4 | 1 | One inch |
| 50 | 1.9685 | Close to 2 in |
| 100 | 3.9370 | Just under 4 in |
Where metric meets imperial
Metric bolts and SAE tools
Most cars use metric fasteners, plenty of toolboxes are still SAE. To find the closest fraction, count sixteenths: mm ÷ 25.4 × 16. A 13 mm nut gives 8.19 sixteenths, so a 1/2 in wrench (12.7 mm) is too small. A 19 mm nut gives 11.97 sixteenths, which is why a 3/4 in socket (19.05 mm) usually fits it.
Tire sizes
A 225/45 R17 tire mixes both systems: 225 mm of tread width, or 8.86 in, on a 17 in wheel. The sidewall is 45% of 225, that is 101.25 mm, so the overall diameter is 17 × 25.4 + 2 × 101.25 = 634.3 mm, or 24.97 in.
Rainfall and lumber
The UK Met Office and most forecasts outside the US give rain in millimeters: 25 mm in a day is 0.98 in. European 18 mm plywood is 0.709 in, thinner than a 3/4 in sheet.
Mental math
Multiply by 4 and move the decimal point two places left: 50 mm × 4 = 200, so about 2.00 in against an exact 1.9685. The shortcut always reads 1.6% high, fine for a rough idea, not for a drill bit.
Frequently asked questions
How many inches is 10 mm?
0.3937 in, shown as 0.393701 in by the converter. That is a little more than 3/8 in, which is 9.525 mm.
How many millimeters are in an inch?
Exactly 25.4 mm. The figure is a definition, not a rounded value.
Will a 3/8 in wrench fit a 10 mm nut?
No. A 3/8 in wrench opens to 9.525 mm, almost half a millimeter smaller than the nut. Do not confuse it with a 3/8 in drive ratchet, where the size is the square drive, not the nut.