How to convert a length
- Type the length you want to convert.
- Pick the unit you are converting from.
- Pick the unit you want.
- Press Convert or watch the live result.
- Read the other units in the list below.
What this length converter gives you
- Metric units from millimeters to kilometers.
- Imperial units from inches to miles.
- Nautical miles for boats and planes.
- The conversion factor both ways.
- Every other unit listed next to your answer.
- Exact international factors with no rough rounding.
The definitions are exact
This surprises many people. Since 1959 an inch equals exactly 25.4 millimeters. It is not close. It is exact. The international yard and pound agreement defined imperial units in metric terms. So every conversion between the two systems is exact.
That explains the odd looking numbers. A foot is exactly 0.3048 meters. A mile is exactly 1609.344 meters. They look random because an agreement set them. No natural law produced them.
Common length conversions
| From | To | Exact factor |
|---|---|---|
| 1 inch | Centimeters | 2.54 |
| 1 foot | Meters | 0.3048 |
| 1 yard | Meters | 0.9144 |
| 1 mile | Kilometers | 1.609344 |
| 1 nautical mile | Meters | 1852 |
In the other direction a kilometer is about 0.621 miles. That explains why 100 km/h is roughly 62 mph.
How the meter got its definition
The first meter was one ten millionth of the distance from the equator to the North Pole. Surveyors measured it along a line through Paris. Their survey was slightly off. So scientists replaced it with a metal bar. Later they used a wavelength of krypton light.
Since 1983 the meter equals the distance light travels in a vacuum in 1/299792458 of a second. That fixes the speed of light exactly. Any well equipped lab can now reproduce the meter. A bar in a vault never allowed that.
Nautical miles are a different kind of unit
A nautical mile is exactly 1852 meters. It runs about 15 percent longer than a regular mile. It started as one minute of latitude. That makes navigation math simple. A knot is one nautical mile per hour. Ships and planes use it for speed.
The link to latitude is the key. One minute of arc along a line of longitude equals one nautical mile. So a distance on a chart turns straight into a change in position. Aviation and shipping never switched to kilometers for that reason.
The old US survey foot
The United States once kept a second foot for land surveys. It equaled 1200/3937 meters and not exactly 0.3048. The gap is about two parts per million. Across a room that means nothing. Across 15 kilometers it adds up to about 3 centimeters.
That small gap caused real mismatches in state map grids. The government retired the survey foot at the end of 2022. Older survey records may still use it.
A quick length converter example
You want a TV for a wall space of 140 centimeters. Enter 140 and pick centimeters. Choose inches as the target. The tool shows about 55.1 inches. Remember that TV sizes measure the diagonal and not the width. Check the product width before you buy.
Handy length benchmarks
- A US letter page is 8.5 by 11 inches or about 21.6 by 27.9 centimeters.
- A standard door in the US stands about 80 inches tall.
- A marathon runs 26.2 miles or about 42.2 kilometers.
- A football field runs 100 yards between goal lines.
Benchmarks help you sense whether a result looks right. Say a room converts to 400 meters wide. You know at once that something went wrong. Check the unit you picked and try again.
Tips for home projects
Measure twice and convert once. Write every measurement with its unit. A note that says 30 means nothing a week later. Buy materials in the unit the store uses. Most US hardware stores sell lumber in feet and inches. Tile and flooring from Europe often lists metric sizes. Convert before you order so the pieces fit the first time.
Who uses a length converter?
DIY fans buy lumber and tile. Runners convert race distances. Online shoppers check sizes. Pilots and sailors work in nautical miles. Students solve science homework.
Your numbers stay private
The tool runs in your browser and sends nothing. Want pounds and kilograms? Use the weight converter. Need volume too? Try the unit converter. Want to size a print? The pixels to inches converter helps.