Guide · Published September 15, 2026 · By the site maintainer
Metric prefixes explained: kilo, milli, micro, and more
A metric prefix tells you the scale of a unit. A kilometer is not a separate kind of length from a meter; it is 1,000 meters. A millimeter is one thousandth of a meter. The prefix moves the quantity up or down by a stated power of ten while the underlying unit still measures the same physical property.
This decimal structure is why many metric conversions can be done by shifting place value instead of memorizing unrelated ratios. The official list is maintained by the International Bureau of Weights and Measures, or BIPM. Its SI prefix table runs from quetta at 1030 to quecto at 10−30. Most daily work uses a much smaller middle section, but every prefix follows the same rule.
What a prefix changes
Start with a unit such as the meter. Adding kilo means multiplying that unit by 1,000, so 1 km equals 1,000 m. Adding centi means multiplying by one hundredth, so 1 cm equals 0.01 m. Adding milli means multiplying by one thousandth, so 1 mm equals 0.001 m. The number and prefix work together as a single quantity.
The direction matters when you convert. Moving from a larger unit to a smaller unit produces more units: 2.4 meters equals 240 centimeters. Moving from a smaller unit to a larger unit produces fewer units: 2,400 millimeters equals 2.4 meters. A quick reasonableness check catches many errors. If you changed meters into millimeters and your numerical answer became smaller, the operation probably went the wrong way.
Common metric prefixes
| Prefix | Symbol | Power of ten | Factor | Example |
|---|---|---|---|---|
| giga | G | 109 | 1,000,000,000 | gigawatt (GW) |
| mega | M | 106 | 1,000,000 | megawatt (MW) |
| kilo | k | 103 | 1,000 | kilometer (km) |
| hecto | h | 102 | 100 | hectopascal (hPa) |
| deca | da | 101 | 10 | decameter (dam) |
| deci | d | 10−1 | 0.1 | deciliter (dL) |
| centi | c | 10−2 | 0.01 | centimeter (cm) |
| milli | m | 10−3 | 0.001 | milliliter (mL) |
| micro | µ | 10−6 | 0.000001 | micrometer (µm) |
| nano | n | 10−9 | 0.000000001 | nanometer (nm) |
| pico | p | 10−12 | 0.000000000001 | picosecond (ps) |
The prefixes above are not the full official range. Very large and very small prefixes support astronomy, particle science, data quantities, and other specialized work. The CGPM added ronna, quetta, ronto, and quecto to the SI prefix range in 2022. For everyday conversions, it is more useful to understand exponent differences than to memorize all twenty-four names.
Convert by comparing exponents
Write each prefix as a power of ten, subtract the destination exponent from the source exponent, and multiply by the resulting power. Kilo is 103 and milli is 10−3. The difference is 3 − (−3) = 6, so 1 kilometer equals 106, or 1,000,000, millimeters.
For 0.045 kilograms to grams, kilo contributes 103 while the gram has no added prefix and therefore uses 100. The exponent difference is 3. Multiply 0.045 by 1,000 to get 45 grams. The kilograms and pounds converter is useful when the destination is not another metric prefix and the relationship needs a non-decimal conversion factor.
For 680 micrograms to milligrams, micro is 10−6 and milli is 10−3. The difference is −3, so multiply 680 by 10−3. The result is 0.68 milligrams. Keep the unit symbol with every intermediate number; a bare “0.68” does not reveal what was converted.
Symbols are case-sensitive
Prefix symbols can change meaning when their case changes. Lowercase m means milli, while uppercase M means mega. One milliwatt is 0.001 watt, but one megawatt is 1,000,000 watts, a difference of a billion. Lowercase k is the symbol for kilo. The symbol for a unit named after a person may itself be uppercase, such as W for watt or Pa for pascal, even though the written unit name begins with a lowercase letter.
NIST's guide to writing SI units explains capitalization, spacing, plural forms, and prefix symbols. Unit symbols do not take plural endings: 5 kilometers is written 5 km, not 5 kms. A space separates the number from most unit symbols, as in 25 kg or 18 cm.
Area and volume require extra care
A prefix attached to a squared or cubed unit is also squared or cubed. One centimeter equals 0.01 meter, but one square centimeter equals (0.01 m)2, or 0.0001 square meter. There are 10,000 square centimeters in one square meter, not 100.
The effect is larger for volume. One centimeter equals 0.01 meter, so one cubic centimeter equals (0.01 m)3, or 0.000001 cubic meter. That is why moving three prefix steps in length does not mean the same factor for area or volume. Use the square meters and square feet converter for area across systems, and write all squared units explicitly.
| Quantity | Relationship | Factor |
|---|---|---|
| Length | 1 cm = 0.01 m | 10−2 |
| Area | 1 cm2 = 0.0001 m2 | 10−4 |
| Volume | 1 cm3 = 0.000001 m3 | 10−6 |
Why the kilogram looks unusual
The kilogram is the SI base unit of mass even though its name already contains the prefix kilo. Smaller and larger mass units are formed from gram: milligram, gram, and megagram. Prefixes are not stacked, so a millionth of a kilogram is normally expressed as a milligram rather than a “microkilogram.” The BIPM list of SI base units identifies kilogram as the mass base unit alongside meter for length, second for time, and kelvin for thermodynamic temperature.
This naming history does not alter the arithmetic. One kilogram is exactly 1,000 grams, one gram is 1,000 milligrams, and one milligram is 1,000 micrograms. Draw a short unit chain when several prefixes appear in one problem instead of trying to move a decimal point from memory.
Do not combine prefixes
SI uses one prefix at a time. A thousand kilometers is expressed as one megameter, not one “kilokilometer.” Combining prefixes makes the exponent unclear and is outside standard SI notation. Convert the full factor to a single prefix that keeps the numerical value readable.
A readable value usually falls into a range suited to the context. A machinist may prefer 0.25 mm to 250 µm because the drawing and tools use millimeters. A microscopy report may prefer 250 µm because that scale matches the rest of the observations. Both quantities are equal; the useful representation depends on the work.
Decimal prefixes and digital storage
SI prefixes are decimal. Kilo means 1,000 and mega means 1,000,000. Binary computing quantities sometimes use powers of 1,024 instead. Those binary multiples have separate names and symbols, such as kibibyte (KiB) and mebibyte (MiB). Do not silently substitute 1,024 for the SI meaning of kilo. Read the specification for the device, file system, or software display so you know which convention is being used.
Worked checks for everyday quantities
- 3.6 km to m: kilo is 103, so 3.6 × 1,000 = 3,600 m.
- 42 mm to cm: 42 mm is 0.042 m, while each centimeter is 0.01 m, so the result is 4.2 cm.
- 750 mL to L: milli is 10−3, so 750 × 0.001 = 0.75 L.
- 2.5 MW to W: mega is 106, so the result is 2,500,000 W.
- 8,000 µm to mm: 1,000 µm equals 1 mm, so the result is 8 mm.
For a mixed metric and U.S. customary measurement, prefix movement is only one part of the calculation. First normalize the metric value, then apply the system-to-system factor. The centimeters and inches converter, meters and feet converter, and milliliters and cups converter show the relevant formulas. The metric conversion chart keeps common factors in one reference.
A reliable prefix checklist
- Write the source value and source unit together.
- Identify the source and destination prefix exponents.
- Account for a square or cube before calculating.
- Apply the power-of-ten factor in the direction that makes sense.
- Round only as far as the original measurement justifies.
- Write the destination unit beside the final number.
Prefixes make the metric system compact because the scale is carried in a small symbol. Once you read that symbol as a power of ten, kilometer, milligram, and micrometer stop being separate facts to memorize. They become variations of one decimal rule.