SI Base Units and Prefixes: Full Table and Practical Guide

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 SI Base Units

Every SI quantity is either a base unit, a product of powers of base units, or a named combination of them. Since the 2019 redefinition, each base unit is fixed by an exact value of a constant of nature rather than by a physical artefact.

A few notes that matter in practice:

• The kilogram is the only base unit whose name already contains a prefix. Prefixes therefore attach to the gram: 1 mg = 10⁻⁶ kg, not a “microkilogram”.
• The second is the SI unit of time. Minutes, hours and days are accepted for use with SI but are not themselves SI units.
• The kelvin has the same step size as the degree Celsius but starts at absolute zero — see the temperature scales guide.
• The mole counts entities — atoms, molecules, ions, electrons. One mole is exactly 6.022 140 76 × 10²³ of them, which is why chemists use it to count particles far too small to see.
• The candela underpins the lumen and the lux used in lighting calculations.

SI Prefix Table: Quecto to Quetta

Prefixes scale a unit by an exact power of ten. There are 24 official SI prefixes, running from quecto (10⁻³⁰) to quetta (10³⁰). Above kilo, every step is a factor of 1,000; the small set around the unit itself (hecto, deca, deci, centi) fills in the powers of ten in between.

The prefix list has grown in stages. The original 1960 set ran from tera to pico. Femto and atto were added in 1964, peta and exa in 1975, and zetta, zepto, yotta and yocto in 1991. The newest four — ronna, quetta, ronto and quecto — were adopted by the 27th General Conference on Weights and Measures (CGPM) in November 2022, driven mainly by data science, where global storage is heading beyond yottabytes, and by particle physics, where masses are tiny fractions of a yoctogram.

Named Derived Units You Already Use

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

The full list of all 22 named derived units, with step-by-step derivations, is in the SI derived units in base units guide.

How to Convert With Prefixes

Converting between prefixed units is a matter of replacing each prefix with its power of ten and subtracting the exponents:

1. Write the value with the first prefix as a power of ten: 3.2 km = 3.2 × 10³ m.
2. Divide by the target prefix: 3.2 × 10³ m ÷ 10⁻³ = 3.2 × 10⁶ mm.
3. Sanity-check the direction: a smaller unit always needs a bigger number.

For squared and cubed units, the prefix is raised to the same power: 1 km² = (10³ m)² = 10⁶ m², and 1 cm³ = (10⁻² m)³ = 10⁻⁶ m³ = 1 mL.

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.
• “Kilo” is k, not K. Capital K is the kelvin. Write 1 km, 1 kW, 1 kg.
• Prefixes do not stack. 10⁻⁶ m is a micrometre, not a “millimillimetre”. Only one prefix is allowed per unit.
• Kelvin has no degree symbol. Write 300 K, not 300 °K.
• Spaces matter. SI style separates the number and the unit with a space: 10 m, not 10m. The exceptions are the plane-angle signs: 30°.
• Prefixes are not used with every unit. They do not attach to the minute, hour, day, or the degree of angle. They are used with the litre, tonne and electronvolt, which are accepted for use with the SI.
• Decimal vs binary prefixes. In computing, “kilo” historically meant 1,024 rather than 1,000. 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:

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 the 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.
• Check the result against the order of magnitude you expect.

Related Converters and Guides

• SI derived units in base unitsAll 22 named units: N, J, W, C, V, Ω and more.
• Metric vs. imperialHow SI compares with US customary and imperial units.
• KB vs KiBWhere SI prefixes meet the IEC binary ones.
• Kilometres to milesConvert km to mi with a table.
• Kilograms to poundsConvert kg to lb instantly.
• Celsius to kelvinAdd 273.15 — with examples.

To convert any other quantity right now, the ConvertMe.org converter has categories for length, mass, temperature, energy, power, frequency, pressure and more, and the conversion index lists every dedicated converter page.

Frequently Asked Questions

How many SI base units are there?

There are seven SI base units: the second (s) for time, metre (m) for length, kilogram (kg) for mass, ampere (A) for electric current, kelvin (K) for temperature, mole (mol) for amount of substance and candela (cd) for luminous intensity. Every other SI unit is derived from these seven.

Which SI base unit is used to count microscopic objects such as atoms and molecules?

The mole (mol). One mole contains exactly 6.022 140 76 × 10²³ specified entities — atoms, molecules, ions or electrons — so chemists can count particles that are far too small to handle individually by weighing them in bulk.

Which prefix means one tenth of the base unit?

Deci (d) means 10⁻¹, one tenth. A decilitre is 0.1 litre. One hundredth is centi (c), and one thousandth is milli (m). Do not confuse deci (d) with deca (da), which means ten.

Which SI prefix means 1,000,000 times the base unit?

Mega (M) means 10⁶, one million: 1 MW = 1,000,000 W. One billion (10⁹) is giga (G), and one billionth (10⁻⁹) is nano (n).

What is the difference between capital M and lowercase m as a prefix?

Capital M is mega (10⁶) and lowercase m is milli (10⁻³). They differ by a factor of one billion, so 5 MW is a power station while 5 mW is a laser pointer. SI symbols are always case-sensitive.

Why is the kilogram the only base unit with a prefix?

For historical reasons. The gram was the original unit of mass in the 1790s, but a gram was too small to make a practical standard, so the physical reference was made at 1,000 grams. When the SI was formalised in 1960 the kilogram became the base unit, but prefixes still attach to the gram — you write mg, not µkg.

Do SI prefixes apply to any unit, not just metres?

Yes. Any SI prefix can be combined with any SI unit, base or derived: kilogram, megawatt, nanosecond, millivolt, gigapascal. They are also used with the litre, tonne and electronvolt. They are not used with the minute, hour, day or the angle degree.

What is the difference between an SI unit and an SI prefix?

A unit names what is measured (metre, second, joule). A prefix is a multiplier placed in front of a unit to scale it by a power of ten. “km” is the unit metre with the prefix kilo, meaning 1,000 metres. A prefix never appears on its own.