Inches to MM Converter

How to convert inches to millimeters

Formula: millimeters = inches × 25.4

The constant is exact by definition: 1 inch = 25.4 millimeters. It is not a rounded approximation.

The inch is defined exactly in terms of the meter, and a millimeter is a thousandth of a meter, so 25.4 is exact rather than rounded. What can run long is the decimal you get back. A fraction with a power of two underneath, such as 1/64, always ends: 1/64 in = 0.396875 mm.

Worked example: 2 1/4 in to millimeters

This result is complete, not rounded.

2 1/4 in

2 1/4 in = 2.25 in
2.25 × 25.4 = 57.15 mm

Common inches to millimeters conversions

Inch fractions to millimeters chart

Every figure here is complete, not rounded: each fraction is a whole number over a power of two, and an inch is exactly 25.4 millimeters. Type any of these fractions into the calculator as it is, or use the decimal from the second column.

Fractions of an inch and mixed numbers

A fraction of an inch is a number of equal parts of one inch. 3/8 means three of the eight parts an inch is split into. A mixed number such as 2 1/4 is a whole number of inches plus a fraction, so it means 2 inches and 1/4 inch together. Type it with a space between the whole number and the fraction.

To convert a mixed number, add the parts first, then multiply: 2 1/4 is 2.25 inches, and 2 1/4 in = 57.15 mm. The calculator does the same with the text you type. In a mixed number the fraction has to be less than 1, so 2 5/4 gives no result; write 3 1/4 instead.

A decimal inch is a fraction in disguise. 0.375 is 375 thousandths, and dividing the top and the bottom by 125 gives 3/8, so both give the same answer: 0.375 in = 9.525 mm and 3/8 in = 9.525 mm. To turn a fraction into a decimal, divide the top number by the bottom number: 3 ÷ 8 = 0.375.

Reading fractions on an inch ruler

On an inch ruler or tape, each inch is divided by marks of different lengths. The longest mark between whole inches is the half. Next come the quarters, then the eighths and the sixteenths, and finer rulers add thirty-seconds and sixty-fourths. Each shorter mark halves the gap between the marks before it.

To read a measurement, find the last whole inch, then count the marks after it and write that count over the number of marks in an inch. On a ruler marked in sixteenths, the fifth mark after the inch is 5/16, and 5/16 in = 7.9375 mm. A fraction is usually written in lowest terms, so 4/16 is written as 1/4. The converter accepts either form and gives the same result: 4/16 in = 6.35 mm and 1/4 in = 6.35 mm.

Exact conversion versus real-world size

Converting inches to millimeters is arithmetic. 1/4 inch is exactly 6.35 mm, and this page gives that figure to every digit shown. Whether a physical part measures that much is a different question, and this page cannot tell.

Many sizes quoted in inches are nominal, meaning they are names for a size rather than measurements. In the US, a board sold as 2 by 4 inches usually measures about 1 1/2 by 3 1/2 inches, which is 38.1 mm by 88.9 mm. A pipe sold as 1/2 inch is named for its nominal size, and its outside diameter is larger, about 0.84 inch, or 21.336 mm.

Manufactured parts also vary within a tolerance. A drawing might ask for 6.35 mm with a tolerance of plus or minus 0.1 mm, so a part that measures 6.4 mm can still be correct. Treat the converted number as the target size, and use the drawing, the datasheet or the manufacturer for the nominal size, the tolerance and whether a dimension is a minimum or a maximum.

When you need to convert inches to millimeters

• Drill bits and hole sizes: a 1/4 inch bit is 6.35 mm and a 3/8 inch bit is 9.525 mm.
• Dimension drawings and equipment specifications that give inches, for someone ordering or machining in millimeters.
• Sheet, plate and tube thicknesses quoted in fractions of an inch.
• Woodworking and craft plans in fractions of an inch, checked against a metric tape.

Things worth knowing

• Results are decimal millimeters. The converter does not give an answer as a fraction of an inch, and it does not round to a ruler mark such as 1/16 or 1/32.

Converting back and related converters

To go the other way, use the MM to Inches Converter, which applies inches = millimeters / 25.4. For any other pair of units, open the Unit Converter, or browse every unit converter.

• Inches to CM Converter: the same inches in centimeters, with a chart of fractions and heights.
• CM to Inches Converter: for a metric length you want in inches.

Frequently asked questions