The International System of Units (SI) is the framework behind almost every metric unit you will meet, from a millimetre on a ruler to a gigawatt in a power-station report. Once you know the seven base units and the prefixes that scale them, dozens of separate-looking units fall into a single pattern.

The Seven Base Units

Every SI quantity is either a base unit, a power of base units, or a named combination of them. The seven bases are:

• Metre (m) — length. The reference for kilometre, centimetre, millimetre, micrometre and so on.
• Kilogram (kg) — mass. The base for the gram (1 g = 10⁻³ kg) and tonne (1 t = 10³ kg). This is the only base unit whose name already contains a prefix, for historical reasons.
• Second (s) — time. Minutes, hours and days are accepted for use with SI but are not themselves SI units.
• Ampere (A) — electric current. Underpins coulomb (charge), volt (potential difference) and ohm (resistance).
• Kelvin (K) — thermodynamic temperature. Same degree size as Celsius but starts at absolute zero — see the temperature scales guide.
• Mole (mol) — amount of substance. Used in chemistry to count particles in bulk.
• Candela (cd) — luminous intensity. The basis for the lumen and the lux used in lighting calculations.

Named Derived Units You Already Use

A great many familiar units are combinations of the seven bases. The SI gives the most useful combinations their own names, which is why the equations of physics and engineering remain readable.

• Newton (N) — force. 1 N = 1 kg·m/s².
• Pascal (Pa) — pressure. 1 Pa = 1 N/m².
• Joule (J) — energy. 1 J = 1 N·m.
• Watt (W) — power. 1 W = 1 J/s.
• Coulomb (C) — electric charge. 1 C = 1 A·s.
• Volt (V) — potential difference. 1 V = 1 W/A.
• Ohm (Ω) — resistance. 1 Ω = 1 V/A.
• Hertz (Hz) — frequency. 1 Hz = 1 cycle per second.
• Lumen (lm) and lux (lx) — luminous flux and illuminance.

The Decimal Prefixes

Prefixes scale a unit by an exact power of ten. The current set of SI prefixes runs from quecto (10⁻³⁰) to quetta (10³⁰), with the most common ones in everyday use shown in bold.

• quetta (Q) — 10³⁰
• ronna (R) — 10²⁷
• yotta (Y) — 10²⁴
• zetta (Z) — 10²¹
• exa (E) — 10¹⁸
• peta (P) — 10¹⁵
• tera (T) — 10¹² (terabyte, terawatt-hour)
• giga (G) — 10⁹ (gigabyte, gigahertz, gigawatt)
• mega (M) — 10⁶ (megabyte, megahertz, megawatt)
• kilo (k) — 10³ (kilometre, kilogram, kilowatt)
• hecto (h) — 10² (hectolitre, hectare)
• deca (da) — 10¹
• deci (d) — 10⁻¹ (decimetre, decibel)
• centi (c) — 10⁻² (centimetre)
• milli (m) — 10⁻³ (millimetre, millilitre, milligram)
• micro (µ) — 10⁻⁶ (micrometre, microsecond)
• nano (n) — 10⁻⁹ (nanometre, nanosecond)
• pico (p) — 10⁻¹²
• femto (f) — 10⁻¹⁵
• atto (a) — 10⁻¹⁸
• zepto (z) — 10⁻²¹
• yocto (y) — 10⁻²⁴
• ronto (r) — 10⁻²⁷
• quecto (q) — 10⁻³⁰

The very largest and very smallest prefixes were added in 2022 to accommodate fields like data storage and atomic physics, where 10²¹ and 10⁻²¹ had become routine.

Rules That Trip People Up

• Symbols are case-sensitive. "M" means mega; "m" means milli. A megametre and a millimetre differ by a factor of 10⁹.
• Kelvin has no degree symbol. Write 300 K, not 300 °K.
• Prefixes do not stack. Use micro, not "millimilli". 10⁻⁶ m is a micrometre, not a "milli-millimetre".
• "Kilo" is k, not K. Capital K is reserved for kelvin. 1 km, 1 kW, 1 kg.
• Spaces matter. Standard SI style separates the value and the unit with a non-breaking space: 10 m, not 10m.
• Decimal vs binary prefixes. In computing, "kilo" historically meant 1024, not 1000. The IEC introduced separate binary prefixes (kibi, mebi, gibi) — see the data storage units guide.

Worked Example: Reading a Datasheet

Suppose a component datasheet reads "Output: 4.7 µF, 25 V, ESR 50 mΩ, ripple 200 mA at 100 kHz". Translating:

• 4.7 µF = 4.7 × 10⁻⁶ farad — capacitance.
• 25 V = 25 volts — maximum DC working voltage.
• 50 mΩ = 50 × 10⁻³ ohm = 0.05 Ω — equivalent series resistance.
• 200 mA = 0.2 A — RMS ripple current.
• 100 kHz = 10⁵ Hz — measurement frequency.

None of these values lives in a separate "datasheet system". They are all SI base units or named derived units with prefixes.

A Short Practical Checklist

• Identify the unit symbol — is it a base unit (m, kg, s) or a derived one (N, J, W)?
• Identify the prefix — case matters (M vs m).
• Convert by replacing the prefix with its power of ten and doing arithmetic in the base unit.
• If the calculation involves ratios or absolute values (gas laws, efficiency), use absolute units — kelvin for temperature, base SI for everything else.
• Double-check results against the orders of magnitude your physical intuition expects.

Where to Go From Here

To convert specific quantities right now, the home page has dedicated categories for length, mass, temperature, energy, power, frequency, pressure and more. For the relationship with non-SI systems, the metric vs. imperial guide covers the practical side. For storage units in particular, the bits, bytes and KiB guide explains where the SI prefixes coexist with the IEC binary ones.