Two ways to build a measuring system
The metric system starts from a few base units and scales them by powers of ten. A kilometer is 1,000 meters, a milligram is a thousandth of a gram, and a liter is a cubic decimeter. Once you know the prefixes, you can move between sizes by shifting a decimal point.
Imperial units grew from older trade and farming habits, so their steps are uneven. There are 12 inches in a foot, 3 feet in a yard and 1,760 yards in a mile. Those numbers made sense for dividing things by hand, but they make mental conversion harder.
- kilo = 1,000
- centi = 1/100
- milli = 1/1,000
- micro = 1/1,000,000
Imperial and U.S. customary aren't the same
People say imperial when they mean American units, but the U.S. never adopted the British Imperial system of 1824. Length and mass match. Volume doesn't.
A U.S. liquid gallon is 3.785411784 liters, while an Imperial gallon is 4.54609 liters. A U.S. liquid pint is about 473 mL, and an Imperial pint is about 568 mL. So a British pint of milk holds roughly a fifth more than an American one. Recipes, fuel economy figures and old engineering notes all carry this trap.
The 1959 link that made conversions exact
Before 1959, the U.S. and British yards differed by a tiny amount. That year the inch was fixed at exactly 2.54 cm and the pound at exactly 0.45359237 kg. Since then, every foot, mile and ounce has been defined from the meter and the kilogram.
That's why 1 mile is exactly 1.609344 km and 1 foot is exactly 0.3048 m. The decimals you see in a converted value come from rounding the answer, not from fuzzy definitions. One older unit lingered: the U.S. survey foot, which NIST retired for new work from January 1, 2023.
Factors worth knowing by heart
You don't need many. With a handful of exact links, you can rebuild most everyday conversions on paper and check whether a calculator answer is sensible.
- 1 inch = 2.54 cm, so 180 cm is about 70.9 inches
- 1 mile = 1.609344 km, so 25 miles is 40.2336 km
- 1 kg = about 2.2046 lb, so 70 kg is about 154.3 lb
- 1 U.S. cup = 236.5882365 mL
- 37 deg C = 98.6 deg F
Mistakes that cause wrong conversions
The biggest one is mixing systems that share a name. Gallon, pint, ton and ounce each have more than one meaning. A troy ounce for gold is heavier than the avoirdupois ounce in your kitchen scale, and a U.S. short ton is lighter than a metric tonne.
The second is treating temperature like length. Doubling 10 deg C doesn't give you twice as much heat, and you can't convert a reading with one multiplier. Add or subtract the offset, then scale. A temperature change works differently: a 10 degree Celsius rise is an 18 degree Fahrenheit rise.
The third is false precision. If you measured a room as 12 feet, writing it as 3.6576 m claims accuracy you never had. Round to match what you measured.
When you need a quick check, our unit converter shows the factor and a reverse calculation, so you can see the path from one unit to the other.
Who uses which system
Science, medicine and most of the world's trade run on SI units. The U.S. uses customary units for most everyday things, but U.S. law has recognized the metric system since 1866, and food labels show both. The UK is mixed: road signs use miles, draught beer comes in pints, and most packaging is metric.
So if you work across borders, assume nothing. Read the unit name, find out which system it belongs to, and convert from the exact definition.
Frequently asked questions
Why does the US still use imperial units?
Strictly, the U.S. uses U.S. customary units, which are close to imperial but differ in volume. Metric has been legal since 1866, but switching everyday road signs, recipes and tools has never been mandated.
Is a metric ton the same as a ton?
No. A metric tonne is 1,000 kg. A U.S. short ton is 2,000 pounds, about 907.18 kg.
How do I convert cm to inches in my head?
Divide by 2.54, or roughly multiply by 0.4. So 30 cm is a bit under 12 inches. For an exact answer, 30 / 2.54 is 11.81 inches.
Are UK pints bigger than US pints?
Yes. An Imperial pint is about 568 mL and a U.S. liquid pint is about 473 mL, so the UK pint holds about 20% more.
What is the difference between metric and SI?
SI is the modern, formally defined version of the metric system. It fixes seven base units, such as the meter, kilogram and second, and the prefixes used with them.
References
These sources support the method or guidance used for Unit Converter. Verify time-sensitive rules at the source.
Try the calculator
Open Unit Converter, enter your scenario, and compare its supporting rows with this guide's method and checks.