1 millimeter = 0.039370079 in
1 mm = 0.0393701 in · type math like (12*4)+6.5
Millimeters to inches matters in workshops, fabrication and product design, where drawings come in metric but tools, drill bits and stock material are sold in fractional inches. It is also how photographers and gun owners translate lens and caliber sizes.
Woodworkers frequently face this when a European hinge, drawer slide or router bit arrives with millimeter specifications. A 35 mm Forstner bit, the standard size for concealed cup hinges, is 1.378 inches, and there is no common fractional-inch bit that matches it exactly, so even American cabinet shops buy that bit in metric.
Bicycle components are another common case. Seatposts are measured in millimeters, such as 27.2 mm (1.071 in) and 31.6 mm (1.244 in), while older American handlebars used 1 inch (25.4 mm) and 1 1/8 inch (28.6 mm) steerer tubes.
Tyre widths are given in millimeters even in the US, so a 225 mm tyre is 8.86 inches wide.
Watch case sizes are in mm: a 42 mm watch is 1.65 inches across.
A quick check is that 25 mm is almost exactly 1 inch, so every 25 mm adds roughly an inch. A 100 mm measurement should be close to 4 inches (3.94 exactly), and a 300 mm ruler close to 12 inches (11.81 exactly). If your answer is larger than the millimeter figure, the direction is wrong. For workshop use, convert decimals to the nearest 1/16 or 1/32 to match a tape measure.
in = mm × 0.03937007874
Reverse: 1 in = 25.4 mm.
Find the nearest fractional inch drill bit to a 6 mm hole.
Answer: 6 mm = 0.2362 in; the nearest bit is 15/64 inch.
| mm | in |
|---|---|
| ¼ mm | 0.0098425197 in |
| ½ mm | 0.019685039 in |
| ¾ mm | 0.029527559 in |
| 1 mm | 0.039370079 in |
| 2 mm | 0.078740157 in |
| 3 mm | 0.11811024 in |
| 4 mm | 0.15748031 in |
| 5 mm | 0.19685039 in |
| 6 mm | 0.23622047 in |
| 7 mm | 0.27559055 in |
| 8 mm | 0.31496063 in |
| 9 mm | 0.35433071 in |
| 10 mm | 0.39370079 in |
| 11 mm | 0.43307087 in |
| 12 mm | 0.47244094 in |
| 13 mm | 0.51181102 in |
| mm | in |
|---|---|
| 14 mm | 0.5511811 in |
| 15 mm | 0.59055118 in |
| 16 mm | 0.62992126 in |
| 17 mm | 0.66929134 in |
| 18 mm | 0.70866142 in |
| 19 mm | 0.7480315 in |
| 20 mm | 0.78740157 in |
| 25 mm | 0.98425197 in |
| 30 mm | 1.1811024 in |
| 40 mm | 1.5748031 in |
| 50 mm | 1.9685039 in |
| 75 mm | 2.9527559 in |
| 100 mm | 3.9370079 in |
| 250 mm | 9.8425197 in |
| 500 mm | 19.685039 in |
| 1 000 mm | 39.370079 in |
18 mm plywood is 0.709 inches, which is why it is sold as a substitute for 3/4 inch (0.75 in) sheet.
A full-frame sensor is 36 × 24 mm, which is 1.417 × 0.945 inches.
A heavy storm dropping 50 mm of rain is 1.97 inches, about 2 inches in US forecasts.
6 mm is 0.236 in, closest to 15/64. Choosing 1/4 in (6.35 mm) leaves a loose fit for a 6 mm dowel.
US 3/4 inch plywood actually measures about 23/32 in (18.3 mm), so a metric 18 mm sheet often fits the same dado.
Dividing by 25 makes 100 mm look like 4 inches, but it is 3.937 inches. In machining that 1.6 mm error is unacceptable.
A millimeter is one thousandth of a meter. There are exactly 25.4 mm in an inch.
Origin. Part of the original 1795 metric system; milli- comes from the Latin mille, a thousand.
Used today. Millimeters are the default unit on engineering drawings worldwide, including in the US automotive and electronics industries. They also measure rainfall, sheet thickness and camera focal lengths.
One inch equals exactly 25.4 mm since the 1959 international agreement.
Origin. From the Latin uncia, a twelfth of a Roman foot.
Used today. Inches, and their fractions in sixteenths and sixty-fourths, are used for US drill bits, wrenches, plywood and pipe in American hardware stores.
10 ÷ 25.4 = 0.3937 inches, a little over 3/8 inch (0.375).
Close but not identical. A 9 mm bullet is 0.355 in in diameter, while a .38 Special bullet is about 0.357 in.
0.118 in, which is just under 1/8 inch (0.125 in).
Global supply chains, ISO standards and most CAD defaults use millimeters, so American car and electronics makers work in metric internally.