Length converter
Convert between length units.
Length Guide
Metric vs Imperial Systems
The metric system (meters, centimeters, kilometers) is used in most countries worldwide, including Poland. The imperial system (feet, inches, miles) dominates in the USA, Liberia, and Myanmar. The UK uses a mix of both - distances in miles, but alcohol in milliliters. 1 inch = 2.54 cm, 1 foot = 30.48 cm, 1 mile = 1.609 km.
When to Use Which System
Always use metric in science and medicine. In construction in Poland use mm and cm. In USA real estate and human height is given in feet and inches. Maritime and air navigation uses nautical miles (1852 m). When shopping online from the USA, check dimensions in inches.
Most Common Conversions
Height: 5 feet = 152 cm, 6 feet = 183 cm. Shoe size: 41 EU ≈ 7.5 UK ≈ 8 US. Laptop screen 15.6 inches is 39.6 cm diagonal. Standard shipping container 40 feet = 12.2 m. Marathon is 42.195 km = 26.2 miles.
There were two different feet, and they differed by 2 parts per million
Since 1959 an international foot has been exactly 0.3048 metres. But American surveying kept an older definition, 1200/3937 metres, for another sixty years. The two differ by two millionths — invisible in a room, and a foot of error across a hundred miles of survey line.
How it works
- Converts between metric and imperial length units using the exact 1959 definitions.
- Treats the inch as exactly 25.4 mm, which makes every other imperial length exact too.
- Keeps precision across large distances, where rounded conversion factors accumulate visibly.
1 inch = 25.4 mm exactly, by the 1959 agreement 1 foot = 12 in = 0.3048 m exactly 1 mile = 5280 ft = 1609.344 m exactly the retired US survey foot was 1200/3937 m = 0.3048006096 m difference: 2 ppm
Worked example
Following the two definitions out to distances where the gap stops being theoretical.
- over one mile the two feet differ by 3.2 mm
- over 100 miles the gap reaches 322 mm — just over one foot
- over 1000 miles it is 10.6 feet
- in a room, across a plot, on a building site: entirely undetectable
The same measurement expressed in the two feet drifts about a foot per hundred miles. That is why the difference mattered for state plane coordinate systems and not for anything you would measure with a tape.
Reading the result
- The US survey foot was formally retired at the end of 2022, and the international foot is now the single definition. Legacy survey data still carries the old one, so archived coordinates and new ones can disagree slightly.
- Because the inch is defined as exactly 25.4 mm, every imperial length is exactly convertible. There is no rounding error to worry about — only the question of which foot a historical figure used.
- A nautical mile is not an imperial mile. It is 1852 m exactly, defined from the Earth's circumference rather than from feet, which is why it survives in navigation and aviation alongside the metric system.
- The often-quoted 'a metre is about 39 inches' is 39.37, and that 0.37 compounds. Over a kilometre the shortcut is out by nearly a foot.
Common questions
- Does the survey foot still matter?
- Only for historical data. New work uses the international foot exclusively since the end of 2022, but archived surveys, deeds and state plane coordinates may still be expressed in the old definition, and mixing the two silently introduces a 2 ppm error.
- Are imperial conversions exact or approximate?
- Exact. The 1959 agreement defined the inch as precisely 25.4 mm, so feet, yards and miles all convert without rounding. Only pre-1959 measurements carry genuine definitional uncertainty.