SI Prefixes Explained
Metric · Last updated: July 2025
The International System of Units (SI) defines seven base units—including the meter, kilogram, second, and ampere—and attaches prefixes to scale them up or down by powers of ten. Instead of inventing new names for every size (like inches, feet, yards, and miles), metric uses one base name plus a prefix: millimeter, centimeter, meter, kilometer. Once you learn the prefixes, converting between them is arithmetic, not memorization.
How prefixes attach to base units
A prefix combines directly with a unit symbol: k + m = km (kilometer), m + g = mg (milligram). The prefix carries the power of ten; the unit carries the physical dimension (length, mass, time, etc.). You cannot attach two prefixes to one unit—“kilomegagram” is not valid. For very large or small numbers, use scientific notation with one prefix (e.g. 3.2 × 106 m = 3.2 Mm).
Note the kilogram: the base unit for mass in SI is actually the kilogram, and the prefix attaches to the gram (10−3 kg). Historically the gram was defined as 10−3 of the kilogram prototype; the naming stuck even after the kilogram became the defined unit in 2019.
Common prefixes you will meet daily
| Prefix | Symbol | Factor | Example |
|---|---|---|---|
| tera- | T | 1012 | 1 TB ≈ 1 trillion bytes (storage marketing uses this) |
| giga- | G | 109 | 1 GHz = 1 billion cycles per second |
| mega- | M | 106 | 1 MW = 1 megawatt power plant output |
| kilo- | k | 103 | 1 km = 1,000 m; 1 kg = 1,000 g |
| centi- | c | 10−2 | 1 cm = 0.01 m (centimeter) |
| milli- | m | 10−3 | 1 mm = 0.001 m; 1 mL = 0.001 L |
| micro- | μ | 10−6 | 1 μm = 1 micrometer (print resolution, chip features) |
| nano- | n | 10−9 | 1 nm — scale of visible-light wavelengths |
Length: mm, cm, m, km
Everyday distances span many orders of magnitude. A credit card is about 0.8 mm thick (800 μm). A smartphone screen might be 15 cm diagonal. A doorway is roughly 2 m tall. A marathon is 42.195 km. Converting among these is pure decimal movement: 15 cm = 0.15 m = 150 mm. Unlike inches-to-feet (divide by 12) and feet-to-miles (divide by 5,280), metric length conversions use factors of ten only.
Construction and engineering drawings often use millimeters exclusively—even for room-sized dimensions—because integers avoid rounding noise. Surveyors and geographers use kilometers. Science may use nanometers for wavelengths or megameters for planetary scales.
Mass: mg, g, kg
Medicine doses frequently appear in milligrams: ibuprofen tablets of 200 mg, vitamin D measured in micrograms (μg). Grocery store produce is sold in kilograms outside the US. A paperclip is about 1 g. Shipping freight uses metric tonnes (1 t = 1,000 kg), not to be confused with the US short ton (2,000 lb) or UK long ton (2,240 lb).
When converting mg to g, divide by 1,000. A common error in pharmacy is misplacing that decimal—500 mg is 0.5 g, not 5 g. Prefix clarity is safety-critical in this domain.
Volume and liters
The liter (L) is an SI-accepted unit equal to one cubic decimeter. Milliliters (mL) measure small volumes: medicine syringes, recipe teaspoons (5 mL in many countries). Kiloliters (kL) appear in water-utility billing. Cubic meters (m³) are the strict SI derived unit for volume in science, but liters dominate daily life and retail labeling.
Time, electrical, and digital prefixes
Seconds use prefixes less often in casual speech, but milliseconds (ms) measure camera shutter speeds and network latency; nanoseconds (ns) appear in CPU timing. Electrical units follow the same pattern: milliamps (mA) for small currents, kilowatts (kW) for appliance power draw, megohms (MΩ) for resistance.
In computing, mega- and giga- describe storage and bandwidth. Be aware that disk manufacturers sometimes use decimal gigabytes (109 bytes) while operating systems historically used binary gibibytes (230 bytes)—a separate source of confusion unrelated to SI but often discussed alongside prefixes.
Combining prefixes: rules and pitfalls
- One prefix per unit. Write 10 km, not 10 kk m.
- No spaces for compound symbols. Correct: mm, kg, μm. Incorrect: k m, micro m.
- Do not mix prefixes in one conversion step. Convert to base unit first if needed: 2.5 km = 2,500 m = 250,000 cm.
- Centi- and deci- are less common globally than milli- and kilo-, but centimeters remain standard for body height and clothing in many countries.
- Hecto-, deka-, and deci- appear mainly in scientific contexts (hectopascal hPa for atmospheric pressure).
Why prefixes matter for conversion
If you understand that kilo- means ×1,000 and milli- means ÷1,000, you never need a lookup table to convert km to m or mg to g. Customary units lack this regularity: 12 inches per foot, 3 feet per yard, 1,760 yards per mile—each step is a new number. That is why SI dominates science and why metrication reduces training burden in industry.
When converting between metric and US customary, convert prefix to base metric first (e.g. 5 km → 5,000 m), then apply the inch or mile factor. Mixing prefixes with imperial factors in one step invites error.
References
Convert mm, km, mg, kg, and more on the tounits.com converter.