A clear summary of the seven base units and the most used derived units.
The seven base units of the International System of Units (SI) are the foundation upon which all modern scientific and technical measurement is built. They are: meter (length), kilogram (mass), second (time), ampere (electric current), kelvin (temperature), mole (amount of substance), and candela (luminous intensity). Each measures a fundamental and independent physical quantity, and together they cover virtually all the measurement that science and industry need to perform.
The meter (m) is the unit of length. Since 1983, it has been defined as the distance traveled by light in vacuum in 1/299,792,458 of a second, a definition based on the speed of light that ensures universal precision and reproducibility. From the meter derive practical units such as kilometer (1,000 m), centimeter (0.01 m), and millimeter (0.001 m), used in everyday life and engineering.
The kilogram (kg) is the unit of mass, and since 2019 has been defined by the Planck constant, a fundamental constant of quantum physics — replacing the former physical platinum prototype kept in France. The second (s) is the unit of time, defined by the transition frequency between two energy levels of the cesium-133 atom, an extremely precise atomic measurement. These two units are the most used in everyday life and science.
The ampere (A) is the unit of electric current, defined by the force between current-carrying conductors. The kelvin (K) is the unit of absolute temperature, defined by the Boltzmann constant; its zero is absolute zero (−273.15 °C), the lowest theoretically possible temperature. The kelvin is essential in thermodynamic calculations, while the ampere is the basis of all electrical measurement.
The mole (mol) is the unit of amount of substance, defined by the Avogadro number — approximately 6.022 × 10²³ elementary entities (atoms, molecules, or other particles). It is fundamental in chemistry for relating mass and number of particles. The candela (cd) is the unit of luminous intensity, defined by the luminous efficacy of a monochromatic radiation — used in lighting and photometry.
From these seven base units, all other SI units are derived, in a mathematically coherent way and without arbitrary conversion factors. The newton (N) is the unit of force (kg·m/s²), the joule (J) is the unit of energy (N·m), the watt (W) is the unit of power (J/s), the pascal (Pa) is the unit of pressure (N/m²), and the hertz (Hz) is the unit of frequency (1/s). This coherence is the great advantage of the SI: the units connect through direct mathematical relationships.
For this reason, always prefer the SI in technical and scientific work: it is the international standard, its units are coherent with each other, and it eliminates most conversion errors between different systems. TudoConversor uses the official SI factors and definitions in all its conversions, offering a reliable tool for those just starting to learn about units of measurement and for those who already work with them professionally.
SI prefixes make the system practical across an enormous range of scales. From the nanometer (10⁻⁹ m) used in semiconductor manufacturing to the gigameter (10⁹ m) used in astronomy, the same base unit — the meter — covers 18 orders of magnitude through prefixes alone, without needing different units for each scale. This prefix system (kilo, mega, giga, milli, micro, nano, and so on) applies uniformly across all SI quantities — grams, watts, hertz, pascals — making the system remarkably compact and consistent compared to the imperial system, where each scale tends to have its own named unit.
The 2019 redefinition of the SI base units, based on fundamental constants, has an important practical consequence: the definitions are now immutable and universal, independent of any physical object that could deteriorate. Before 2019, the kilogram was defined by a physical cylinder kept in Sèvres, France, which showed microgram variations over time in comparisons with its official copies — an instability unacceptable for a global standard. With the new definition based on the Planck constant, any laboratory equipped with a Kibble balance can reproduce the kilogram with equivalent precision, without depending on the original object.
For the beginner, the most useful practical rule when working with the SI is: always use prefixes in powers of 1,000 (kilo, mega, giga upward; milli, micro, nano downward), and avoid non-standard units like kilometer per hour in scientific contexts — prefer the meter per second, which is the coherent SI unit. This coherence means the units connect through direct mathematical relationships without arbitrary conversion factors: 1 newton = 1 kg × 1 m/s², 1 joule = 1 N × 1 m, 1 watt = 1 J/s. This property, called dimensional coherence, is the SI's greatest advantage over non-decimal systems.