Measures & Constants

The metre, the metric base unit of length, is now defined by the distance light travels in a set fraction of a second.

The kilogram, the metric base unit of mass, is now defined using a fixed value of Planck's constant.

The second, the metric base unit of time, is now defined by the precise vibration of a caesium atom.

The Kelvin scale starts at absolute zero and serves as the standard temperature scale across science.

The mole is the base unit for an amount of substance, defined using Avogadro's number of particles.

The newton, the metric unit of force, is named in honor of Isaac Newton.

The watt, the metric unit of power, is named after the engineer James Watt.

The joule, the metric unit of energy, is named after physicist James Prescott Joule.

The pascal, the metric unit of pressure, is named after mathematician Blaise Pascal.

Avogadro's number gives the particle count in one mole of substance, roughly 6.022 times ten to the twenty-third power.

Planck's constant links a photon's energy to its frequency and underlies the foundations of quantum mechanics.

The gravitational constant sets the strength of gravity between masses and was first measured by Henry Cavendish.

The Fahrenheit scale, still used in the United States, sets water's freezing point at 32 degrees.

The Celsius scale, used across most of the world, sets water's freezing and boiling points at 0 and 100 degrees.

The Boltzmann constant links temperature to particle energy and is central to the field of statistical mechanics.

The speed of light, about 299,792 kilometers per second in a vacuum, stands as physics' universal speed limit.

Pi expresses the ratio of a circle's circumference to its diameter and is irrational: its decimal expansion never ends or becomes periodic.

A light-year measures the distance light travels in a year and is used for vast distances between stars.

The astronomical unit, the average Earth-Sun distance, serves as a convenient yardstick within the solar system.