Direct comparison
Base units vs derived units
Base units are the seven fundamental SI units from which all measurement starts; derived units are the many units built by combining those seven.
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Side-by-side comparison
| Dimension | Base units | Derived units |
|---|---|---|
| What it is | A fundamental SI unit for one physical quantity, defined in its own right. | A unit formed by combining base units through multiplication or division. |
| How many | Exactly seven. | Unlimited — every other SI unit, with many named for convenience. |
| Examples | Second, metre, kilogram, ampere, kelvin, mole, candela. | Newton, joule, watt, pascal, volt, hertz, metre per second. |
| Defined by | A fixed constant of nature since the 2019 redefinition. | An algebraic combination of base units. |
| Independence | Independent — none is built from the others. | Dependent — each can be rewritten in base units. |
| Example breakdown | The kilogram stands alone as the unit of mass. | The newton is kg·m·s⁻², the joule is kg·m²·s⁻². |
| Special names | Each has its own name and symbol from the outset. | Twenty-two carry special names; the rest are written out. |
| Role in the SI | The minimal foundation of the whole system. | The bulk of practical units used in science and engineering. |
| Coherence | Fixed reference points for the system. | Coherent — formed with no extra numerical factors. |
Why the SI keeps base units to seven
The International System of Units deliberately rests on just seven base units, because a small, coherent foundation is what lets the whole system fit together without contradiction. Each base unit covers a distinct physical quantity — time, length, mass, current, temperature, amount of substance and luminous intensity — and none can be derived from the others. Every remaining unit is a derived unit, written as a product or ratio of these seven. The newton, joule, watt, volt and pascal all dissolve into base units when written out fully, which is exactly the property that makes results comparable in any laboratory.
Common questions
FAQ
How many SI base units and derived units are there?+
There are exactly seven SI base units: the second, metre, kilogram, ampere, kelvin, mole and candela. The number of derived units is effectively unlimited, since any combination of base units forms one. Twenty-two of them have special names and symbols — such as the newton, joule and watt — while the rest are simply written out in base units.
Is the newton a base unit or a derived unit?+
The newton is a derived unit. It is defined as the kilogram metre per second squared (kg·m·s⁻²), the force needed to accelerate one kilogram at one metre per second squared. It combines three base units — the kilogram, metre and second — so although it has its own name, it is built entirely from base units rather than standing on its own.
Why are derived units useful if they reduce to base units?+
Special names make communication and calculation far easier. Saying a force is "five newtons" is clearer than "five kilogram metres per second squared", even though the two mean the same thing. Named derived units give familiar handles for common quantities — pressure in pascals, energy in joules, frequency in hertz — while still reducing to base units whenever a calculation needs checking.
Going deeper
