Metric Prefixes: Kilo, Milli, Centi and More

What a metric prefix does

A metric prefix is a short word placed in front of a unit name to multiply it by a fixed power of ten. “Kilo” means one thousand, so a kilometre is a thousand metres and a kilogram is a thousand grams. “Milli” means one thousandth, so a millimetre is a thousandth of a metre. The prefix has its own symbol, which is placed in front of the unit symbol: km, mm, kg, mg.

The idea is powerful because it is universal. The same prefixes attach to metres, grams, litres, seconds and almost every other unit, so you learn the pattern once. There is no need for separate words for twelve inches or sixteen ounces, just a consistent set of multipliers.

The common prefixes at a glance

The prefixes you will meet most often in everyday measurement are these:

• kilo (k) = 1,000 (10³)
• hecto (h) = 100 (10²)
• deca (da) = 10 (10¹)
• deci (d) = 0.1 (10⁻¹)
• centi (c) = 0.01 (10⁻²)
• milli (m) = 0.001 (10⁻³)
• micro (µ) = 0.000001 (10⁻⁶)

Larger steps include mega (M, 10⁶), giga (G, 10⁹) and tera (T, 10¹²), and smaller ones include nano (n, 10⁻⁹) and pico (p, 10⁻¹²). In daily life, kilo, centi and milli dominate, with the others appearing in science, electronics and technology.

Converting with prefixes

Converting between prefixed units means moving the decimal point. Going from a bigger unit to a smaller one gives a bigger number, because you need more of the small units. Going the other way gives a smaller number. A useful check is to ask whether the result should be larger or smaller before you calculate.

Some examples show the pattern. 2.5 kilometres is 2.5 × 1,000 = 2,500 metres. 450 millimetres is 450 ÷ 1,000 = 0.45 metres. 3 centimetres is 30 millimetres, since there are 10 millimetres in a centimetre. 0.75 litres is 750 millilitres. And 1.2 kilograms is 1,200 grams, which is 1,200,000 milligrams, because a gram is 1,000 milligrams.

Prefixes and square or cubic units

The most frequent mistake appears with area and volume. A prefix applies to the unit it is attached to, and when that unit is squared or cubed, the prefix is squared or cubed along with it. One centimetre is 10 millimetres, but one square centimetre is 10 × 10 = 100 square millimetres, not 10.

The same rule applies on the larger side. One kilometre is 1,000 metres, so one square kilometre is 1,000 × 1,000 = 1,000,000 square metres. For volume, a cubic centimetre is a millilitre, and 1 cubic metre holds 1,000 litres. The hectare shows a prefix in a familiar unit: it equals 10,000 square metres, which is a square 100 metres on each side. If an area result seems off by a factor of 10 or 1,000, the squared prefix is the first thing to check.

The kilogram exception

One unit does not follow the pattern neatly. The kilogram is the SI base unit of mass, yet its name already contains a prefix. The rule against stacking prefixes means that multiples and fractions of the mass unit are formed by attaching prefixes to the gram instead. So a millionth of a kilogram is one milligram (1 mg), written that way and not as a “microkilogram”.

This is a small detail, but it explains why mass conversions often pass through grams in practice. A thousand kilograms is a metric tonne (t), a unit that is accepted for use with the SI even though it is not itself a prefixed form.

Case and spelling matter

Prefix symbols are case-sensitive, and the difference can be enormous. A lowercase m means milli (10⁻³), while a capital M means mega (10⁶). So 5 mm is five millimetres, but 5 Mm would be five thousand kilometres. Kilo is a lowercase k; the capital K is the symbol for the kelvin. The micro prefix uses the Greek letter µ, which is sometimes typed as “u” when the Greek letter is unavailable.

The system has also grown. In 2022 four new prefixes were added: ronna (R, 10²⁷) and quetta (Q, 10³⁰) for very large values, and ronto (r, 10⁻²⁷) and quecto (q, 10⁻³⁰) for very small ones. You are unlikely to need them for everyday conversions, but they show that the prefix system is still maintained and extended.

Practical tips for everyday use

Choose the prefix that keeps numbers readable. A road is 12 km long, not 12,000,000 millimetres, and a pill might contain 500 mg, not 0.0005 kg. For results you want to double-check, a unit converter such as this one will show the same value in every unit of a category, making it easy to compare metres, centimetres and millimetres side by side.

When you convert, write the unit with every number, move the decimal point in the right direction, and remember to square or cube the prefix factor for areas and volumes. For medical, engineering or other safety-critical work, always follow the units stated in the original source.