The MKS system is a metric system of measurement built on three base units: the meter for length, the kilogram for mass, and the second for time. It replaced an older metric framework called the CGS system (centimeter-gram-second) and eventually became the foundation of the modern International System of Units, known as SI. If you’ve ever measured anything in meters, kilograms, or seconds, you’ve used the core of the MKS system.
The Three Base Units
The name says it all. MKS stands for meter, kilogram, second, and these three quantities anchor the entire system:
- Meter (m) measures length.
- Kilogram (kg) measures mass.
- Second (s) measures time.
Every other unit in the system is derived from combinations of these three. Force, energy, power, pressure, and speed can all be expressed using meters, kilograms, and seconds in various arrangements. For example, speed is simply meters per second. Acceleration is meters per second squared. From there, you can build up to more complex quantities like force and energy.
How MKS Differs From CGS
Before the MKS system became standard, most scientists used the CGS system, which measured length in centimeters, mass in grams, and time in seconds. Both are metric systems, so converting between them involves straightforward powers of ten: one meter equals 100 centimeters, one kilogram equals 1,000 grams.
The practical difference came down to scale. CGS units worked well for laboratory measurements involving small quantities, and the system was popular in physics throughout the 19th and early 20th centuries. But for engineering and everyday applications, centimeters and grams produced awkwardly large or small numbers. A bridge measured in centimeters or a vehicle’s mass in grams isn’t intuitive. The MKS system used units closer to human-scale quantities, which made it more practical for applied sciences and engineering.
The deeper problem was electrical units. The CGS system spawned two competing sets of electrical units, one for electrostatics and another for electromagnetism, and they were mutually contradictory. This created real confusion for anyone working across both domains.
Giovanni Giorgi and the Addition of the Ampere
In 1901, Italian engineer Giovanni Giorgi proposed a solution to the electrical unit problem. His idea was elegant: instead of patching the CGS system, replace it entirely with a new system based on four base units rather than three. The original three mechanical units (meter, kilogram, second) would stay, and one electrical unit would be added. Giorgi initially suggested the ohm as that fourth unit.
His proposal was largely ignored for three decades. It wasn’t until the 1930s that the International Electrical Commission took notice and endorsed the concept, with one change: they chose the ampere as the fourth base unit instead of the ohm. The expanded system became known as the MKSA system (meter-kilogram-second-ampere). Elevating the ampere to a fundamental unit meant that all other electrical quantities, including voltage, resistance, and capacitance, could be derived from it in combination with the three mechanical units.
Derived Units Built From MKS
Once you have the meter, kilogram, and second, you can define nearly every physical quantity used in science and engineering. The most common derived units include:
- Newton (N) for force. One newton is the force needed to accelerate a one-kilogram mass at one meter per second squared. In practical terms, holding an apple in your hand requires roughly one newton of force.
- Joule (J) for energy. One joule is the energy transferred when a force of one newton moves an object one meter. It’s equivalent to one kilogram times one meter squared per second squared.
- Watt (W) for power. One watt equals one joule per second, meaning it measures how quickly energy is used or produced.
- Pascal (Pa) for pressure. One pascal is one newton of force spread over one square meter.
These derived units fit together like building blocks, each one traceable back to meters, kilograms, and seconds. That internal consistency is what made the MKS framework so appealing compared to older systems where unit relationships were less clean.
From MKS to the Modern SI
The MKS system didn’t just influence the modern International System of Units. It became the SI. When the General Conference on Weights and Measures formalized the SI in 1960, it kept the meter, kilogram, and second as three of its base units and added four more: the ampere for electric current, the kelvin for temperature, the candela for light intensity, and the mole for the amount of a substance. That gives the SI seven base units total, but the original MKS trio remains at its core.
For most of its existence, the kilogram was defined by a physical object: a platinum-iridium cylinder kept in a vault near Paris since 1889. That artifact-based definition lasted 130 years. On May 20, 2019, following a decision at the 26th General Conference on Weights and Measures, all seven SI base units were redefined in terms of fundamental physical constants. The kilogram is now tied to a fixed value of the Planck constant, the meter to the speed of light, and the second to the frequency of radiation emitted by cesium atoms. The units themselves didn’t change in size. A kilogram is still a kilogram. But the definitions are now rooted in unchanging properties of nature rather than a single metal cylinder that could, in theory, gain or lose atoms over time.
Where MKS Units Are Used Today
In practice, nearly every country on Earth uses MKS-based SI units as their official measurement standard. Scientific journals worldwide require SI units. Engineering specifications, international trade agreements, and medical dosing all rely on meters, kilograms, and seconds as their starting point. Even the United States, which still uses feet and pounds in daily life, defines those customary units in terms of SI: one inch is legally defined as exactly 25.4 millimeters, and one pound is defined relative to the kilogram.
The CGS system hasn’t disappeared entirely. It still shows up in certain branches of physics, particularly in astrophysics and some areas of electromagnetism, where legacy equations and conventions make CGS units more convenient. But for general science, engineering, and commerce, the MKS framework won. Its units are the ones you encounter in textbooks, on food packaging, in weather forecasts outside the U.S., and in virtually every scientific instrument manufactured today.