The Inch: Definition, Equivalents and History
An inch is exactly 25.4 millimeters. It is a twelfth of a foot and a thirty-sixth of a yard, and it is still the everyday unit of length for building, woodworking and screens in the United States.
Definition
Since 1959, when the United States, the United Kingdom and other Commonwealth countries agreed on a common yard and pound, the inch has been defined as exactly 2.54 cm. Before that, the American and British inches differed in the sixth digit. The symbol is in, or a double prime (″); a foot is marked with a single prime (′), so 5′ 6″ is five feet six inches.
One inch equals
| Unit | 1 inch equals |
|---|---|
| Feet | 1/12 ft (0.083333…) |
| Yards | 1/36 yd (0.027777…) |
| Millimeters | 25.4 mm |
| Centimeters | 2.54 cm |
| Meters | 0.0254 m |
| Thousandths of an inch (thou, mil) | 1,000 thou |
| Square inch | 6.4516 cm² |
| Cubic inch | 16.387064 cm³ (1/231 of a US gallon) |
Inch fractions
Tape measures and rulers divide the inch into halves, quarters, eighths and sixteenths (fine rules go to 1/32 or 1/64). To read a fraction as a decimal, divide the top number by the bottom one; to convert to millimeters, multiply the decimal by 25.4. The inch fraction calculator does this and the arithmetic, and the feet and inches calculator adds and subtracts tape readings.
| Fraction | Decimal | Millimeters |
|---|---|---|
| 1/16″ | 0.0625″ | 1.5875 mm |
| 1/8″ | 0.125″ | 3.175 mm |
| 1/4″ | 0.25″ | 6.35 mm |
| 3/8″ | 0.375″ | 9.525 mm |
| 1/2″ | 0.5″ | 12.7 mm |
| 5/8″ | 0.625″ | 15.875 mm |
| 3/4″ | 0.75″ | 19.05 mm |
| 7/8″ | 0.875″ | 22.225 mm |
Where the inch came from
The name comes from the Latin uncia, a twelfth, which is also the root of “ounce”. Early definitions tied it to natural measures such as the width of a thumb or three barleycorns laid end to end. Twelve inches made a foot because twelve divides evenly by 2, 3, 4 and 6, which suits carpentry and trade.
Related inches
- The US survey inch was 1/39.37 of a meter (about 2.540005 cm); it was retired with the US survey foot at the end of 2022.
- A square inch is the area of a square one inch on a side (6.4516 cm²); a cubic inch is the volume of a cube one inch on a side (16.387064 cm³).
- Other historical inches (Scottish, French, Roman) differed by a few percent and are no longer in use.