The metric system works on two ideas. There’s one base unit for each kind of quantity (the meter for length, the kilogram for mass, the second for time), and prefixes scale those units by powers of ten (kilo = 1,000, centi = 1/100, milli = 1/1,000). So converting within the system means moving a decimal point: 1.5 km is 1,500 m, and 250 mL is 0.25 L. The modern version is called the International System of Units, or SI.
What are the seven SI base units? #
The SI is maintained by the International Bureau of Weights and Measures (BIPM). It has seven base units:
| Quantity | Unit | Symbol |
|---|---|---|
| Length | meter | m |
| Mass | kilogram | kg |
| Time | second | s |
| Electric current | ampere | A |
| Temperature | kelvin | K |
| Amount of substance | mole | mol |
| Luminous intensity | candela | cd |
Every other SI unit is built from these. Area is square meters, speed is meters per second, and so on. Some combinations have their own names: the newton (force), joule (energy), watt (power) and pascal (pressure) are all “derived units,” defined from the base ones.
What defines the base units? #
Since 20 May 2019, the BIPM says, “all SI units are defined in terms of constants that describe the natural world.” The meter is fixed by the speed of light (exactly 299,792,458 m/s), the second by a particular frequency of the cesium-133 atom, and the kilogram by the Planck constant. That replaced the old metal kilogram in a vault near Paris, so the definitions no longer depend on any physical object.
How do metric prefixes work? #
A prefix multiplies the unit by a power of ten. You’ll use a handful all the time:
| Prefix | Symbol | Multiplier | Example |
|---|---|---|---|
| tera | T | 1,000,000,000,000 | terabyte |
| giga | G | 1,000,000,000 | gigawatt |
| mega | M | 1,000,000 | megapixel |
| kilo | k | 1,000 | kilometer |
| hecto | h | 100 | hectopascal |
| deka | da | 10 | (rarely used) |
| deci | d | 0.1 | deciliter |
| centi | c | 0.01 | centimeter |
| milli | m | 0.001 | milliliter |
| micro | µ | 0.000001 | micrometer |
| nano | n | 0.000000001 | nanosecond |
The full list runs from quecto (10⁻³⁰) to quetta (10³⁰). According to NIST, four prefixes were added in 2022: ronna and quetta for huge numbers, ronto and quecto for tiny ones.
Capitalization matters. Big prefixes from mega up take capital letters (M, G, T), while kilo and everything smaller are lowercase. So “mm” is millimeter and “Mm” is megameter, a million times bigger.
How do you convert within the metric system? #
Count how many powers of ten apart the two prefixes are, then move the decimal point that many places.
- Bigger unit to smaller unit: move the decimal right. 2.5 km to m: kilo to base is three steps, so 2,500 m.
- Smaller unit to bigger unit: move the decimal left. 3,400 g to kg: three steps, so 3.4 kg.
- Between two prefixes: 45 cm to mm is one step right, so 450 mm. 750 mL to L is three steps left, so 0.75 L.
No 12s, 16s, 5,280s or 1,760s. That’s the whole advantage. Compare the US customary system, where 1 mile = 1,760 yards = 5,280 feet = 63,360 inches, and every step has a different number.
How do metric units connect to each other? #
The system was designed so that length, volume and mass link up:
- 1 liter = 1 cubic decimeter (a cube 10 cm on each side), and 1 mL = 1 cm³.
- 1 liter of water has a mass of about 1 kg. It was originally meant to be exactly that, and it’s still within a fraction of a percent at room temperature.
- 1 tonne = 1,000 kg, roughly the mass of a cubic meter of water.
- 1 hectare = 10,000 m², a square 100 m on each side.
The liter, tonne and hectare aren’t SI units in the strict sense, but they’re accepted for use with the SI, which is why they’re everywhere. Temperature is a partial exception to the tidy pattern. The kelvin is the base unit, and the degree Celsius is the same size of degree shifted by 273.15, as explained in the Celsius to Fahrenheit formula.
Where did the metric system come from? #
It began in France in the 1790s. The meter was originally defined as one ten-millionth of the distance from the North Pole to the equator, measured along a meridian through Paris. In 1875, seventeen nations, including the US, signed the Treaty of the Metre, which set up the BIPM. The system was modernized and renamed the SI in 1960.
Today, every US customary unit is defined in terms of SI units. The inch is exactly 2.54 cm, and the pound is exactly 0.45359237 kg. So even in the US, the metric system sits underneath every measurement. Why everyday life there still uses customary units is covered in why the US uses the imperial system.
A few writing rules worth knowing #
- Symbols don’t take a plural “s”: 5 kg, not 5 kgs.
- Symbols aren’t abbreviations, so they don’t take a period: “5 km away,” not “5 km. away.”
- Put a space between the number and the symbol: 20 °C, 5 m.
- Use the right case: kg (not Kg or KG), km (not Km), mL or ml for milliliter.
Seeing metric units side by side #
Convert Units Calculator is a quick way to see how the prefixes line up. Type one value and every unit in the category is worked out underneath: micrometers, millimeters, centimeters, meters and kilometers in Length, or bytes through petabytes in Data, with US customary equivalents on the same screen. It uses the exact definitions and works offline.
For conversions between systems, the metric to imperial conversion chart has the common factors, and how to convert units shows the method for any pair.
Frequently asked questions #
What are the 7 base units of the metric system? #
The meter (length), kilogram (mass), second (time), ampere (electric current), kelvin (temperature), mole (amount of substance) and candela (luminous intensity).
Is the kilogram or the gram the base unit? #
The kilogram. It’s the only SI base unit with a prefix built into its name, for historical reasons. Other mass units are formed by applying prefixes to the gram (milligram, microgram), not to the kilogram.
Is Celsius part of the metric system? #
Yes. The degree Celsius is an SI derived unit, the same size as the kelvin but with zero set at 273.15 K. Scientists use kelvin for absolute temperatures and Celsius for everyday ones.
Is a liter an SI unit? #
Not strictly. The liter (1 cubic decimeter) is accepted for use with the SI, along with the tonne and the hectare. The official SI unit of volume is the cubic meter.
Why is metric easier than imperial? #
Because every step is a power of ten and the prefixes are the same for every quantity. Converting millimeters to meters works exactly like converting milligrams to grams. In US customary units, each conversion has its own number: 12 inches, 3 feet, 16 ounces, 4 quarts.