299,792,458 is a number quiz setters love, and since 1983 it has been more than a trivia fact: it is the figure that fixes the length of the metre. The United States National Institute of Standards and Technology (NIST) publishes the exact values behind all seven base units of the International System of Units (SI), the modern metric system. The pages used here were read on 28 October 2026.
The seven numbers that fix the seven units
NIST says the units in the revised SI are based completely on seven unchanging quantities, or universal constants, including the speed of light, the electric charge of an electron and the Planck constant. Its definitions page gives each fixed value:
| Unit (symbol) | Quantity held fixed | Fixed value |
|---|---|---|
| Second (s) | Caesium-133 hyperfine transition frequency | 9,192,631,770 Hz |
| Metre (m) | Speed of light in vacuum, c | 299,792,458 m/s |
| Kilogram (kg) | Planck constant, h | 6.62607015 x 10^-34 J s |
| Ampere (A) | Elementary charge, e | 1.602176634 x 10^-19 C |
| Kelvin (K) | Boltzmann constant, k | 1.380649 x 10^-23 J/K |
| Mole (mol) | Avogadro constant, NA | 6.02214076 x 10^23 per mole |
| Candela (cd) | Luminous efficacy at 540 x 10^12 Hz | 683 lm/W |
NIST spells the unit “meter” and “cesium”; the spellings above are the British forms.
Four units changed on one date
NIST states that on 20 May 2019 four of the seven base units, the kilogram, kelvin, ampere and mole, were redefined in terms of constants of nature, while the other three, the second, metre and candela, were already based on universal constants. A vote to adopt the change took place on 16 November 2018 at Versailles, France, at the 26th General Conference of Weights and Measures.
The effect, in NIST’s words, was that for the first time all measurement units would be defined by natural phenomena rather than by physical artifacts. NIST adds that consumers and most industries would not notice immediate impacts, and that scientists expect the change ultimately to inspire new technologies and reduce calibration costs.
The older changes: the second in 1967, the metre in 1983
The second and the metre had already moved to natural definitions. NIST’s 2018 news release says the second is defined as 9,192,631,770 natural oscillations of microwave radiation released by the element caesium, and the metre as the distance travelled by light in vacuum in 1/299,792,458th of a second. It dates those revised definitions to 1967 and 1983 respectively and says they were necessary for the invention of GPS and many other modern technologies.
Because the metre depends on the second, the chain of definitions is worth remembering for a tiebreaker. The metre rests on the speed of light and the second. The kilogram rests on the Planck constant plus the metre and the second.
The kilogram: from a metal cylinder to a constant
The kilogram has the most colourful backstory. According to NIST’s mass page, it was originally defined as the mass of one cubic decimetre of water at the temperature of maximum density, and was known as the Kilogram of the Archives. After the International Metric Convention of 1875 it was replaced by the International Prototype Kilogram, which became the unit of mass without reference to the water definition.
In 2018, NIST described that artefact, known as Le Grand K, as a platinum-iridium object secured in a vault outside Paris and used to calibrate all measures of mass throughout the world. Like any alloy, NIST said, it may change over time by absorbing molecules from the air or losing them through cleaning, and even tiny changes mean it is no longer accurate enough for anticipated advanced research. Under the 2019 definition the kilogram is fixed by the Planck constant. NIST describes an instrument called a Kibble balance, named after its inventor Bryan Kibble, that uses an electric current in a coil to produce a magnetic field strong enough to balance a mass of one kilogram.
Two further facts are standard quiz material. NIST says the kilogram is the only SI base unit whose name and symbol, for historical reasons, include a prefix: “kilo”, the SI prefix for 1,000. And it notes that the primary standard of mass for the United States is United States Prototype Kilogram 20, a platinum-iridium cylinder kept at NIST.
Mass is not weight
NIST’s mass page draws the distinction that trips up many answers. The mass of a body is a measure of its inertial property, or how much matter it contains. Its weight is a measure of the force exerted on it by gravity, or the force needed to support it. In science and technology, NIST adds, the SI unit of weight defined as a force is the newton (N), while the SI unit of mass is the kilogram. NIST also says that in everyday usage, “weight” is often a synonym for mass, and that the verb “to weigh” means “to determine the mass of”, but that using weight in place of mass should be phased out.
The bottom line
Three of the seven SI base units, the second, metre and candela, were already tied to constants before 2019; the kilogram, kelvin, ampere and mole joined them on 20 May 2019. NIST gives the exact values for all seven, and those exact values, rather than rounded textbook figures, are the safest basis for a quiz answer.
Sources
- Definitions of SI Base Units, NIST, page updated August 2025, read 28 October 2026 (nist.gov)
- SI Redefinition, NIST, read 28 October 2026 (nist.gov)
- Historic Vote Ties Kilogram and Other Units to Natural Constants, NIST news release, 16 November 2018, read 28 October 2026 (nist.gov)
- SI Units – Mass, NIST Office of Weights and Measures, page updated December 2025, read 28 October 2026 (nist.gov)
Leave a Reply