There is no physical inch. Since 1959 the inch has been defined as exactly 0.0254 metres, which makes every imperial length a derived metric unit. The countries that resisted the metric system most successfully still measure everything with it, one layer down.
What the Units Actually Are
The metre was originally one ten-millionth of the distance from the equator to the North Pole, measured along a meridian through Paris. That definition was replaced by a platinum bar, then by a wavelength of krypton-86 light, and since 1983 by the distance light travels in a vacuum in 1/299,792,458 of a second.
That last definition is worth pausing on: it fixes the speed of light as exact and derives the metre from it. The speed of light is no longer measured, it is defined, and the metre floats on top of it.
The International Yard and Pound Agreement of 1959 then pinned the imperial system to the metric one. The yard became exactly 0.9144 metres, which makes the foot exactly 0.3048 metres and the inch exactly 25.4 millimetres. Before that agreement, the American and British inches differed by a fraction of a micron, enough to matter for precision engineering and not for anything else.
Exact Conversions Worth Knowing
Every figure in the middle column is exact by definition, not rounded.
| Unit | Exactly | Useful approximation |
|---|---|---|
| 1 inch | 25.4 mm | Roughly 2.5 cm |
| 1 foot | 0.3048 m | About 30 cm |
| 1 yard | 0.9144 m | Slightly under a metre |
| 1 mile | 1,609.344 m | About 1.6 km |
| 1 nautical mile | 1,852 m | One minute of latitude |
| 1 metre | 1.0936133 yd | About 3 feet 3 inches |
| 1 kilometre | 0.62137119 mi | Roughly 5/8 of a mile |
| 1 centimetre | 0.393700787 in | A bit under 0.4 inches |
The unit mismatch that lost a Mars orbiter
In 1999 NASA lost the Mars Climate Orbiter, a 125 million dollar spacecraft, because one team supplied thruster impulse figures in pound-force seconds while the receiving software expected newton-seconds. The trajectory was wrong by enough to put the craft into the atmosphere. Nobody made an arithmetic error; the units were simply never checked at the interface.
The Foot That Was Slightly Different Until 2023
For sixty years the United States maintained two feet. The international foot of exactly 0.3048 metres, and the US survey foot of 1200/3937 metres, which is longer by about two parts per million.
Two parts per million sounds negligible and is, over the length of a room. Over the width of a state it is not. Across 100 kilometres the two definitions differ by about 20 centimetres, which is enough to move a property boundary or misplace a benchmark in a geodetic survey.
The survey foot survived because changing it would have invalidated a century of land records. NIST and NOAA finally retired it at the end of 2022, and since 1 January 2023 the international foot is the only legal foot in the United States. Older survey data still carries the previous definition, so anyone working with historical coordinates needs to know which foot a dataset was recorded in.
Where Conversions Go Wrong
The arithmetic is trivial. The mistakes come from everything around it.
Rounding too early
Convert, round to two decimals, then convert again and the error compounds. Carry full precision through every intermediate step and round only when you display the final number.
Treating approximations as exact
One inch is 2.54 cm exactly, but one mile is 1.609344 km, not 1.6. Using the shortcut over a marathon distance puts you out by more than 300 metres.
Confusing area and volume scaling
A square foot is not 0.3048 square metres, it is 0.3048², about 0.0929. A cubic foot is 0.3048³. Converting an area with a linear factor is one of the most common spreadsheet errors.
Mixing up nautical and statute miles
They differ by about 15 percent. A nautical mile is one minute of latitude, which is why it persists in aviation and shipping while everything else uses the statute mile.
Assuming a decimal foot
Construction measurements are usually feet and inches, not decimal feet. 6.5 feet is 6 feet 6 inches, but 6 feet 5 inches is 6.417 feet, and reading one as the other is a five-centimetre error.
Convert between units of length
Nanometres to nautical miles, feet and inches included, updating as you type.
Who Uses Which System
Three countries have not formally adopted the metric system as their primary standard: the United States, Liberia and Myanmar. That statistic gets repeated more often than it gets qualified.
In practice the United States runs on both. Science, medicine, the military and most manufacturing are metric. Nutrition labels are metric. Car engines are specified in litres. What remains imperial is largely consumer-facing: road signs, weather, body measurements, construction lumber and cooking.
The United Kingdom is similarly mixed, and formally so. Road distances are in miles, draught beer is in pints, and milk can be sold in pints, while nearly everything else is metric by law. Someone will give their height in feet and inches and their weight in stones, then buy 500 grams of cheese.
This is why conversion tools stay useful. The question is rarely which system is better; it is that any given document, spec sheet or recipe may use either, and often both.
Frequently Asked Questions
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