📐 Systems
Metric vs imperial: where the numbers come from
Most conversion tables hand you a row of decimals and move on. This one explains where those decimals came from, which of them are fixed by international agreement, and which are only habits that differ from one country to the next.
Two systems, one planet
The metric system is the official system of measurement almost everywhere. The United States is the only large economy that still uses customary units as its everyday default, and even there the metric system is lawful for every purpose and is standard in science, medicine, the military and much of manufacturing. Liberia and Myanmar, long cited as the exceptions, have both announced metrication programmes.
The result is not two clean worlds but a great deal of overlap. A British driver buys fuel in litres and talks about miles per gallon. An American engineer designs in millimetres and buys fasteners by the inch. An Irish pub pours pints while the food packaging behind the bar lists grams. Any of those people might need a converter in the same afternoon, which is why this site exists.
Imperial was not as arbitrary as it looks
The old English units were built on the human body and on things people could carry. An inch was three barleycorns laid end to end, a foot was — depending on the century — anywhere from 11 to 13 inches, and a mile came from the Roman mille passus, a thousand double paces. A yard was the distance from the nose to the outstretched thumb of a king, conveniently standardised by a bronze bar.
That is charming, and it is also why the units drifted. By the nineteenth century Britain had several incompatible gallons and at least one legal dispute about whether a bushel was heaped or struck. The Imperial Weights and Measures Act of 1824 imposed one set of standards across the empire, which is when the imperial gallon, the imperial pint and the stone as we know them were fixed.
America had already gone its own way. The US gallon had been defined in 1707 as 231 cubic inches — the volume of a cylinder 7 inches in diameter and 6 inches high — and that is still the definition today, written into law as exactly 3.785411784 litres.
The factors that are exact
In 1959 the United States, the United Kingdom, Canada, Australia, New Zealand and South Africa agreed on a single international yard and pound. Since then the customary–metric conversions have been not approximations but exact definitions: the inch is defined as exactly 25.4 millimetres, and the pound as exactly 0.45359237 kilograms. Every other length and mass factor is derived from those two numbers.
| Unit | Exactly equals |
|---|---|
| 1 inch | 25.4 mm |
| 1 foot | 0.3048 m |
| 1 yard | 0.9144 m |
| 1 mile | 1.609344 km |
| 1 nautical mile | 1.852 km (by definition since 1929) |
| 1 pound | 0.45359237 kg |
| 1 ounce | 28.349523125 g |
| 1 US gallon | 3.785411784 L |
| 1 imperial gallon | 4.54609 L |
| 1 acre | 4046.8564224 m² |
Notice how many decimal places some of those have. That is not false precision: the numbers are exact, and the trailing digits exist because the starting units are incommensurable. One kilogram really is 2.2046226218… pounds, forever.
Metric is one system with a ladder attached
The SI has seven base units — second, metre, kilogram, ampere, kelvin, mole and candela — and every other unit is built from them by multiplication. What makes the system easy is not the units themselves but the prefixes: kilo, mega, milli, micro, nano, each a power of ten. Converting 4,700 metres to kilometres is a decimal shift, not a calculation.
Two details surprise people. First, the kilogram is the only base unit whose name already contains a prefix, a historical accident nobody has been willing to fix. Second, since 2019 all seven base units are defined by fixed constants of nature — the speed of light, the Planck constant, the elementary charge — rather than by physical objects. The platinum-iridium cylinder in Paris that used to define the kilogram is now a museum piece.
Metric also separates mass from weight, which customary English does not. A kilogram is a quantity of matter; a newton is a force. In everyday speech "it weighs five kilos" is fine, but in engineering the distinction is the difference between a correct calculation and a wrong one.
Temperature is the exception
Length, mass, volume and area all convert by multiplying. Temperature does not, because Celsius and Fahrenheit disagree about where zero is as well as how big a degree is. Water freezes at 0 °C and 32 °F, and boils at 100 °C and 212 °F, so one Celsius degree is 1.8 Fahrenheit degrees and the scales are offset by 32 at the freezing point.
The formulas are C = (F − 32) × 5/9 and F = C × 9/5 + 32. The two scales meet at −40, which is the one temperature you can quote without saying which scale you mean. Kelvin has the same step size as Celsius but starts at absolute zero, so K = C + 273.15 and there are no negative Kelvin values.
The important edge case: a temperature difference converts by ratio alone. If a recipe says the oven is 20 °C hotter, that is a 36 °F difference, not 68. Confusing absolute and differential temperatures is one of the most common errors in engineering spreadsheets.
Seven conversions worth carrying in your head
These are rough approximations for when you need an answer without a calculator; the actual value is beside each rule. Use the length and weight converters when the answer has to be right.
| Conversion | Quick rule | Actual value |
|---|---|---|
| Metres to yards | Add 10 percent | 100 m ≈ 109.4 yd |
| Kilograms to pounds | Double and add 10 percent | 70 kg ≈ 154.3 lb |
| Kilometres to miles | Multiply by 0.6 | 100 km ≈ 62.1 mi |
| Litres to US quarts | Add 5 percent | 4 L ≈ 4.23 qt |
| Hectares to acres | Multiply by 2.5 and subtract 1 percent | 10 ha ≈ 24.71 acres |
| Bar to psi | Multiply by 14.5 | 2.5 bar ≈ 36.26 psi |
| Celsius to Fahrenheit | Double and add 30 | 20 °C = 68 °F |
Precision: how many digits does the answer deserve?
Conversion tools output a lot of digits because that is what floating point does, not because the measurement supports it. If you measured a table with a tape marked in millimetres and got 1830 mm, you know the length to about four significant figures; saying it is 6.003937 feet implies a precision you do not have. Round the result to the same number of significant figures as the input, and keep the extra digits only if you are going to do more arithmetic with them.
There is a second trap in rounding twice. Convert metres to feet, round to two decimals, then convert back, and you will not return to the number you started with. Convert in one step, with the full factor, and round once at the end.
Where these conversions go wrong
The expensive failures are rarely arithmetic errors; they are unit mismatches — a value in pounds treated as kilograms, a US gallon treated as an imperial one, a ton that meant 907 kg in one document and 1,016 kg in another. The ten unit mistakes guide walks through the famous cases, and the temperature, volume and area converters handle the units that cause most of them.
Frequently asked questions
Sources
- BIPM, The International System of Units (SI Brochure), 9th edition, 2019 — base units, prefixes and the 2019 redefinition.
- NIST Special Publication 811, Guide for the Use of the International System of Units (SI) — conversion factors for US customary units.
- United States Code, Title 15, Chapter 6 — legal definitions of the yard and pound.
- UK Weights and Measures Act 1985 — imperial gallon and related definitions.