# Length converter

> Convert miles to km, feet to metres, inches to cm and 16 other length units with exact factors. Accepts feet and inches such as 5 ft 11 in.

Interactive version: https://www.calcopenly.com/conversion/length-converter
Subject: Unit converters

Each length unit is defined as a fixed number of metres; a conversion multiplies by the “from” factor and divides by the “to” factor. The imperial units have exact metric values because the 1959 international yard and pound agreement fixed the yard at 0.9144 m, which makes the inch 25.4 mm, the foot 0.3048 m and the mile 1,609.344 m.

The default converts 1 mile: 5,280 ft × 0.3048 m = 1,609.344 m, so 1 mi = 1.609344 km with no rounding. A height typed as “5 ft 11 in” is read as one value and gives 180.34 cm.

Results that never terminate, such as 1 nautical mile = 6,076.115… ft, are labelled as repeating and shown to 12 significant figures. Astronomical units follow the IAU: 1 au = 149,597,870,700 m, and a light-year is the distance light travels in 365.25 days.

## Inputs

- **Value**: A number in the “from” unit, or with its own unit: 5 ft 11 in, 3 mi
- **From** (options: metre (m), kilometre (km), centimetre (cm), millimetre (mm), micrometre (µm), nanometre (nm), ångström (Å), inch (in), foot (ft), yard (yd), mile (mi), nautical mile (nmi), thou (mil), hand, fathom (ftm), chain (ch), furlong (fur), astronomical unit (au), light-year (ly), parsec (pc), point (typographic) (pt), pica (pc (type)))
- **To** (options: metre (m), kilometre (km), centimetre (cm), millimetre (mm), micrometre (µm), nanometre (nm), ångström (Å), inch (in), foot (ft), yard (yd), mile (mi), nautical mile (nmi), thou (mil), hand, fathom (ftm), chain (ch), furlong (fur), astronomical unit (au), light-year (ly), parsec (pc), point (typographic) (pt), pica (pc (type)))

## Results

- Converted value — main result
- Conversion
- Conversion factor
- Exactness

## Formula

$$
\begin{gathered} x_{\text{to}} = x_{\text{from}} \times \dfrac{f_{\text{from}}}{f_{\text{to}}} \\ f = \text{one unit expressed in } \text{m} \end{gathered}
$$

## Worked examples

### 1 mile to kilometres

- Value: 1 mi
- From: mile (mi)
- To: kilometre (km)
- **Converted value: 1.609344 km**
- **Conversion factor: 1.609344**
- **Conversion: 1 mi = 1.609344 km**
- **Exactness: Exact**
- Checked against: 1 mi = 5280 ft × 0.3048 m = 1609.344 m (NIST HB 44 App. C, international foot of 1959); Python 3.8 fractions from the legal definitions: 25146/15625 km

### 5 ft 11 in to centimetres

- Value: 5 ft 11 in
- From: foot (ft)
- To: centimetre (cm)
- **Converted value: 180.34 cm**
- **Exactness: Exact**
- Checked against: 71 in × 2.54 cm/in (1 in = 0.0254 m exactly); Python 3.8 fractions from the legal definitions: 9017/50

### 1 nautical mile to feet

- Value: 1 nmi
- From: nautical mile (nmi)
- To: foot (ft)
- **Converted value: 6,076.11548556 ft**
- **Exactness: Exact definitions, repeating decimal**
- Checked against: 1852 m ÷ 0.3048 m = 2315000/381 ft, a non-terminating decimal (Python 3.8 fractions from the legal definitions)

### Light-year to astronomical units (very large exact factors)

- Value: 1 ly
- From: light-year (ly)
- To: astronomical unit (au)
- **Converted value: 63,241.0770843 au**
- Checked against: 1 ly = 299792458 m/s × 365.25 d × 86400 s; 1 au = 149597870700 m (IAU 2012 B2); ratio 431996825232/6830953 (Python 3.8 fractions from the legal definitions)

## Questions

### How many kilometres are in a mile?

Exactly 1.609344 km. The 1959 international yard and pound agreement set the yard at 0.9144 m, and a mile is 1,760 yards, so 1,760 × 0.9144 m = 1,609.344 m. In the other direction 1 km is 0.621371 miles, which makes a 10 km race 6.21 miles.

### How many centimetres are in an inch?

Exactly 2.54 cm. The inch has been defined as 25.4 mm since the 1959 international yard and pound agreement (effective 1 July 1959 in the United States, according to NIST), so inch-to-centimetre results are exact, not rounded. A 12-inch ruler is 30.48 cm and a 55-inch screen diagonal is 139.7 cm.

### How do I convert feet and inches to centimetres?

Multiply the feet by 30.48 and the inches by 2.54, then add. For 5 ft 11 in, 5 × 30.48 = 152.4 cm and 11 × 2.54 = 27.94 cm, which gives 180.34 cm. Typing “5 ft 11 in” into the value box does the same sum, and the answer is exact because both factors are defined values.

### Is a nautical mile longer than a mile?

Yes. A nautical mile is exactly 1,852 m, which is 1.1508 statute miles or 6,076.1 ft. It was chosen to approximate one minute of latitude, so sea and air navigation pair it with the knot, one nautical mile per hour. The statute mile used on roads is 1,609.344 m.

### What happened to the US survey foot?

NIST declared it obsolete on 1 January 2023 and replaced it with the international foot of exactly 0.3048 m. The survey foot, 1200/3937 m, is 2 parts per million longer, which adds up to 3.2 mm over a mile. This converter uses the international foot for every foot, yard and mile.

### How accurate is the length converter?

Accuracy depends on your inputs and the method's assumptions. Decimal arithmetic uses 50 significant digits, but estimates, numerical methods and source data can be less precise; the displayed rounding does not remove those limits. It is checked against 4 worked examples whose answers come from independent sources; for example, “1 mile to kilometres” is checked against 1 mi = 5280 ft × 0.3048 m = 1609.344 m (NIST HB 44 App. C, international foot of 1959); Python 3.8 fractions from the legal definitions: 25146/15625 km.

### Where does the method come from?

NIST SP 811 (2008), Guide for the Use of the SI, Appendix B — conversion factors; NIST Handbook 44, Appendix C — General tables of units of measurement; IAU 2012 Resolution B2 — the astronomical unit.

## Sources

- [NIST SP 811 (2008), Guide for the Use of the SI, Appendix B — conversion factors](https://www.nist.gov/pml/special-publication-811)
- [NIST Handbook 44, Appendix C — General tables of units of measurement](https://www.nist.gov/pml/owm/nist-handbook-44-current-edition)
- [IAU 2012 Resolution B2 — the astronomical unit](https://www.iau.org/static/resolutions/IAU2012_English.pdf)