Inches to Millimeters
Convert inch to millimeter instantly — type a value and read the result, with the exact formula shown.
Example
1 in = 25.4 mm, 5 in = 127 mm, 10 in = 254 mm.
How it works
Millimeter = Inch × 25.4. Every value is converted through a single meter base unit using internationally defined conversion factors, so any from/to pair stays consistent.
Good to know
Converting inches to millimeters comes up constantly whenever a project bridges the imperial and metric worlds: ordering metric drill bits or fasteners from a print marked in inches, setting up a CNC machine whose controller wants millimeters, sizing a phone screen, or matching a US plumbing fitting to a European one. The inch is an imperial/US-customary unit, while the millimeter is the metric SI sub-unit of the meter — and because mm are so much smaller and finer, engineers and machinists worldwide tend to spec tight tolerances in millimeters even in countries that still talk in inches.
The link between the two is not an approximation but a definition: since the 1959 international yard-and-pound agreement, one inch is fixed as exactly 25.4 mm. So you simply multiply inches by 25.4. A handy rule of thumb is that every inch is "25 and a bit" millimeters — roughly 25 mm — so a quick mental estimate is to multiply by 25 and then add a touch under one mm per inch.
A few values are worth memorizing because they show up everywhere:
- 1/8 in = 3.175 mm, 1/4 in = 6.35 mm, 1/2 in = 12.7 mm
- 1 in = 25.4 mm, 2 in = 50.8 mm, 12 in (1 foot) = 304.8 mm
The most common mistake is mixing up direction or rounding too early. Going the other way you divide by 25.4 (or multiply by 0.03937), not by 25 — using 25 introduces about a 1.6% error, which is enough to make a part fail to fit. The other trap is fractional inches: many people read a measurement like 0.5 in and forget it is 12.7 mm, not 5 mm. Because 25.4 is exact, keep full precision through the math and only round the final answer to the precision your job actually needs.
Frequently asked questions
How do you convert inch to millimeter?
Multiply the number of inches by 25.4 to get millimeters. For example, 1 in = 25.4 mm.
What is 1 inch in millimeters?
1 inch equals 25.4 millimeters (1 in = 25.4 mm).
How many inches are in 1 millimeter?
There are 0.03937008 inches in 1 millimeter.
Is this converter free and private?
Yes. It runs entirely in your browser, so your inputs never leave your device, there is no sign-up, and it works offline once loaded.
Are the conversions exact?
Conversions use internationally defined factors and are exact where the definitions are exact (for example, 1 inch = 2.54 cm). Displayed results are rounded for readability.
People also ask
How many mm is 1 inch exactly?
Exactly 25.4 mm. This is a defined value (1 in = 25.4 mm), not a rounded approximation, so it is precise to as many decimals as you need.
What is 1/2 inch in mm?
Half an inch is 12.7 mm (0.5 x 25.4). A quarter inch is 6.35 mm and an eighth inch is 3.175 mm.
How do I convert mm back to inches?
Divide the millimeters by 25.4, or multiply by 0.03937. For example, 100 mm = 100 / 25.4 = about 3.937 inches.
What is 6 inches in millimeters?
6 inches equals 152.4 mm (6 x 25.4). Likewise 3 in = 76.2 mm and 9 in = 228.6 mm.
How many mm are in a foot?
There are 304.8 mm in one foot, since a foot is 12 inches and 12 x 25.4 = 304.8 mm.
Why is an inch defined as 25.4 mm?
The 1959 international yard-and-pound agreement set the inch to exactly 25.4 mm to standardize imperial and metric measurements across countries. Before that, slightly different national inch definitions existed.
What is 3/4 inch in mm?
Three quarters of an inch is 19.05 mm (0.75 x 25.4). It is a common size for pipes, lumber, and hardware.
Is 25 mm the same as 1 inch?
Not quite. 25 mm is about 0.984 inch, while a true inch is 25.4 mm, so 25 mm is roughly 1.6% short of a full inch.
Related calculators