Measurement › Metric prefixes

Metric prefixes

One letter glued to a unit can multiply it by a million, a billion, or shrink it to a billionth. The whole metric system in 14 prefixes.

Metric prefix tower

Hover any prefix to see how it scales your base unit.

Pmpeta1 petabyte ≈ all books ever written10¹⁵1,000,000,000,000,000 m

Tmtera1 TB hard drive10¹²1,000,000,000,000 m

Gmgiga1 GHz = 1 billion cycles/sec10⁹1,000,000,000 m

Mmmega1 MW power station10⁶1,000,000 m

kmkilo1 km ≈ 12 min walk10³1,000 m

hmhecto10²100 m

damdeca10¹10 m

—m (base)base unitmetre, gram, litre10⁰1 m

dmdeci10^-10.1 m

cmcenti1 cm = fingernail width10^-20.01 m

mmmilli1 mm = thickness of a coin10^-30.001 m

μmmicro1 μm = human hair / 10010^-60.000 001 m

nmnano1 nm = DNA strand width10^-90.000 000 001 m

pmpico10^-120.000 000 000 001 m

Every step up multiplies by 10 (or by 1000 between the major prefixes kilo / mega / giga). The metric system is the only measurement system where converting is just moving the decimal point.

The big idea

Pick a base unit (metre, gram, litre, second, watt …) and stick a prefix on the front. The prefix tells you which power of 10 to multiply by. 1 km = 10³ m = 1000 m. 1 ms = 10⁻³ s = 0.001 s. Same logic for every base unit there is — that's why the metric system feels so clean.

The big six you'll meet every day

Prefix	Symbol	Factor	Example
kilo	`k`	×1,000	1 km ≈ 12 min walk
mega	`M`	×1,000,000	1 MW power station
giga	`G`	×1,000,000,000	1 GHz processor
milli	`m`	÷1,000	1 mL eye-drop
micro	`μ`	÷1,000,000	Bacteria are a few μm long
nano	`n`	÷1,000,000,000	1 nm ≈ atom-scale

Capital matters

m means milli (one thousandth) but M means mega (one million). A 5 mg pill is very different from a 5 Mg (=5000 kg = 5 tonne!) anvil. The same case-sensitivity catches out a lot of pharmacy interns.

The two layouts

The prefix table has two arrangements you'll see:

Step-by-10 for everyday measures: deca, hecto, kilo on the way up; deci, centi, milli on the way down. Each step multiplies by 10.
Step-by-1000 for scientific scales: kilo, mega, giga, tera; milli, micro, nano, pico. Each step multiplies by 1000.

That's why nobody uses "hectogram" but everyone uses "kilogram" — past the centi/deci range, the metric world skips by thousands.

Sanity checks

1 km = 1000 m. 1 m = 1000 mm. So 1 km = 1,000,000 mm.
1 GB = 1000 MB = 1,000,000 kB (in marketing-talk). In computer-science-talk it's slightly different (powers of 1024).
1 Hz = 1 cycle / second, so 1 GHz = 1 billion cycles per second. A modern CPU does several of those per second.

Try it

Convert 2500 mg to g.
Convert 0.4 km to cm.
A drug dose is 5 μg per kg of body weight. How many milligrams for a 70 kg adult?
A 4 TB hard drive — how many GB?

Answers: 1) 2500 / 1000 = 2.5 g. 2) 0.4 × 1000 = 400 m → ×100 = 40,000 cm. 3) 5 × 70 = 350 μg = 0.35 mg. 4) 4 × 1000 = 4000 GB.