# Inches Conversion

The inch is a unit used to measure length. Use one of the conversion calculators below to convert to another unit of measure, or read on to learn more about inches.

## Inch Conversion Calculators

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**US Customary & Imperial Units**

**SI Units**

## Inch Definition and Usage

An inch is a unit of linear length measure equal to ^{1}/_{12} of a foot or ^{1}/_{36} of a yard.
Because the international yard is legally defined to be equal to exactly 0.9144 meters, one inch is equal to 2.54 centimeters.^{[1]}

The inch is a US customary and imperial unit of length. Inches can be abbreviated as *in*, for example 1 inch can be written as 1 in.

Inches can also be denoted using the *″* symbol, otherwise know as a double-prime.
Often a double-quote (") is used instead of a double-prime for convenience.
Using the double-prime, *1 in* can be written as 1″.

The standard ruler has 12", and are a common measuring tool for measuring inches. They are also often measured using tape measures which commonly come in lengths from 6' - 35'. Other types of measuring devices include scales, calipers, measuring wheels, micrometers, yardsticks, and even lasers.

### Background and Origin

One of the earliest definitions of the inch declare the length as being equal to three barleycorns.
The inch is now the subdivision of ^{1}/_{12} of a foot.

Even so, the actual length of the inch has varied with the variance in the true length of the foot.
In 1959 the world agreed to the exact length of the national yard being equal to .9144 meters, which set the definition of the inch to being equal to 2.54 centimeters.^{[2]}

Inches are a standard unit of length that are most commonly used in the United States, Canada, and the UK.

### Why Are There 12 Inches in a Foot?

The early Romans subdivided the foot into 16 fingers and are credited with eventually dividing the foot into 12 inches.^{[3]} But why did they choose to divide the foot into 12 inches?

The Romans used the duodecimal number system, which is a base-12 number system. They preferred to use halves and twelfths for subdivision, and they used a notation system that used a dot, called an uncia, to indicate one-twelfth.

The word uncia later became the source for the word "inch" that we use today. This is the origin of the inch and the reason why there are 12 inches in a foot.

In addition to a preference for the number 12 with the number system of the period, the length of a thumb from the first joint is about an inch. The length of the thumb tip offers a convenient and semi-consistent form of measure, and subdividing the foot into these smaller units makes logistical sense for the time period.

The number 12 also more factors than other numbers such as the number 10, which makes it easier to work with mathematically. This makes it easy to handle fractions like 2/3 and 3/4, which are difficult to handle in a base-10 number system.

You'll probably also be interested in our inch fraction calculator, which converts inch values from decimal to a fraction and solves inch fractions.

## Inches Measurement Conversion Table

inches | miles | yards | feet | kilometers | meters | centimeters | millimeters | micrometers | nanometers |
---|---|---|---|---|---|---|---|---|---|

1" | 0.000015783 mi | 0.027778 yd | 0.083333' | 0.0000254 km | 0.0254 m | 2.54 cm | 25.4 mm | 25,400 µm | 25,400,000 nm |

2" | 0.000031566 mi | 0.055556 yd | 0.166667' | 0.0000508 km | 0.0508 m | 5.08 cm | 50.8 mm | 50,800 µm | 50,800,000 nm |

3" | 0.000047348 mi | 0.083333 yd | 0.25' | 0.0000762 km | 0.0762 m | 7.62 cm | 76.2 mm | 76,200 µm | 76,200,000 nm |

4" | 0.000063131 mi | 0.111111 yd | 0.333333' | 0.000102 km | 0.1016 m | 10.16 cm | 101.6 mm | 101,600 µm | 101,600,000 nm |

5" | 0.000078914 mi | 0.138889 yd | 0.416667' | 0.000127 km | 0.127 m | 12.7 cm | 127 mm | 127,000 µm | 127,000,000 nm |

6" | 0.000094697 mi | 0.166667 yd | 0.5' | 0.000152 km | 0.1524 m | 15.24 cm | 152.4 mm | 152,400 µm | 152,400,000 nm |

7" | 0.00011 mi | 0.194444 yd | 0.583333' | 0.000178 km | 0.1778 m | 17.78 cm | 177.8 mm | 177,800 µm | 177,800,000 nm |

8" | 0.000126 mi | 0.222222 yd | 0.666667' | 0.000203 km | 0.2032 m | 20.32 cm | 203.2 mm | 203,200 µm | 203,200,000 nm |

9" | 0.000142 mi | 0.25 yd | 0.75' | 0.000229 km | 0.2286 m | 22.86 cm | 228.6 mm | 228,600 µm | 228,600,000 nm |

10" | 0.000158 mi | 0.277778 yd | 0.833333' | 0.000254 km | 0.254 m | 25.4 cm | 254 mm | 254,000 µm | 254,000,000 nm |

11" | 0.000174 mi | 0.305556 yd | 0.916667' | 0.000279 km | 0.2794 m | 27.94 cm | 279.4 mm | 279,400 µm | 279,400,000 nm |

12" | 0.000189 mi | 0.333333 yd | 1' | 0.000305 km | 0.3048 m | 30.48 cm | 304.8 mm | 304,800 µm | 304,800,000 nm |

13" | 0.000205 mi | 0.361111 yd | 1.083333' | 0.00033 km | 0.3302 m | 33.02 cm | 330.2 mm | 330,200 µm | 330,200,000 nm |

14" | 0.000221 mi | 0.388889 yd | 1.166667' | 0.000356 km | 0.3556 m | 35.56 cm | 355.6 mm | 355,600 µm | 355,600,000 nm |

15" | 0.000237 mi | 0.416667 yd | 1.25' | 0.000381 km | 0.381 m | 38.1 cm | 381 mm | 381,000 µm | 381,000,000 nm |

16" | 0.000253 mi | 0.444444 yd | 1.333333' | 0.000406 km | 0.4064 m | 40.64 cm | 406.4 mm | 406,400 µm | 406,400,000 nm |

17" | 0.000268 mi | 0.472222 yd | 1.416667' | 0.000432 km | 0.4318 m | 43.18 cm | 431.8 mm | 431,800 µm | 431,800,000 nm |

18" | 0.000284 mi | 0.5 yd | 1.5' | 0.000457 km | 0.4572 m | 45.72 cm | 457.2 mm | 457,200 µm | 457,200,000 nm |

19" | 0.0003 mi | 0.527778 yd | 1.583333' | 0.000483 km | 0.4826 m | 48.26 cm | 482.6 mm | 482,600 µm | 482,600,000 nm |

20" | 0.000316 mi | 0.555556 yd | 1.666667' | 0.000508 km | 0.508 m | 50.8 cm | 508 mm | 508,000 µm | 508,000,000 nm |

## References

- National Institute of Standards and Technology, Handbook 133 - 2019 Edition, https://www.nist.gov/sites/default/files/documents/2017/04/28/12-apde-gentab-11-hb133-final.pdf
- Wikipedia, Inch, https://en.wikipedia.org/wiki/Inch
- National Institute of Standards and Technology, Handbook 44 - 2012 Edition, https://www.nist.gov/sites/default/files/documents/2017/04/28/AppB-12-hb44-final.pdf

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