Since the SI was redefined in May 2019, five of these constants have exact values: they're no longer measured, they're used to define the units themselves. The gravitational constant G is the odd one out. It's still measured, and it's the least precisely known of all the fundamental constants.
| Constant | Symbol | Value | Status |
|---|---|---|---|
| Speed of light in vacuum | c | 299 792 458 m/s | Exact (defines the metre) |
| Planck constant | h | 6.626 070 15 × 10⁻³⁴ J·s | Exact (defines the kilogram) |
| Elementary charge | e | 1.602 176 634 × 10⁻¹⁹ C | Exact (defines the ampere) |
| Boltzmann constant | kB | 1.380 649 × 10⁻²³ J/K | Exact (defines the kelvin) |
| Avogadro constant | NA | 6.022 140 76 × 10²³ mol⁻¹ | Exact (defines the mole) |
| Newtonian constant of gravitation | G | 6.674 30(15) × 10⁻¹¹ m³ kg⁻¹ s⁻² | Measured; relative uncertainty 2.2 × 10⁻⁵ |
| Standard acceleration of gravity | gn | 9.806 65 m/s² | Exact by convention (not the local value) |
The "(15)" after G is standard shorthand for the uncertainty in the last two digits: G = 6.67430 ± 0.00015 × 10⁻¹¹.
Click the book icon at the top of any page to insert one of these into the scientific calculator.
Open the calculatorSpeed of light, c
The metre is defined as the distance light travels in a vacuum in 1/299,792,458 of a second, which makes c exact by definition. It's the speed limit for anything carrying energy or information, and it links mass and energy through E = mc². A useful mental figure: light travels about 30 cm in a nanosecond, which is why signal timing matters in fast electronics.
Planck constant, h
h connects the energy of a photon to its frequency: E = hf. Green light at 540 THz carries 6.626 × 10⁻³⁴ × 5.4 × 10¹⁴ ≈ 3.6 × 10⁻¹⁹ J per photon, or about 2.2 eV. Fixing h is what lets the kilogram be realised with a Kibble balance instead of a metal cylinder.
Elementary charge, e
The charge on one proton (or minus the charge on one electron). One coulomb is about 6.24 × 10¹⁸ elementary charges, so a current of 1 A moves that many electrons past a point every second. It also defines the electronvolt: 1 eV = 1.602176634 × 10⁻¹⁹ J exactly.
On the calculator keypad, e is Euler's number, 2.71828… The elementary charge is inserted as a number from the constants list, so there's no clash.
Boltzmann constant, kB
The bridge between temperature and energy. The average kinetic energy of a gas molecule is (3/2)kBT, so at room temperature (293 K) that's about 6.1 × 10⁻²¹ J. Multiplied by Avogadro's constant it gives the gas constant R = NAkB = 8.314 J/(mol·K), which is also exact now.
Avogadro constant, NA
The number of particles in one mole. It's what lets chemists move between atoms and grams: 12 g of carbon-12 contains (to within a tiny correction since 2019) NA atoms. Since 2019 the mole is defined as exactly 6.02214076 × 10²³ particles.
Gravitational constant, G
G sets the strength of gravity in Newton's law F = Gm₁m₂/r². It's extremely hard to measure because gravity between lab-sized masses is tiny. Different high-precision experiments disagree by more than their stated uncertainties, which is why CODATA's uncertainty for G is thousands of times larger than for most constants.
Standard gravity, gn
9.80665 m/s² is a conventional value used to define units like the kilogram-force and to make calculations comparable. Real gravity varies with latitude and altitude, from about 9.780 m/s² at the equator to 9.832 m/s² at the poles. In the UK it's roughly 9.81 m/s², so gn is a good default.
Source
Values are the CODATA 2022 recommended values, published by the Committee on Data of the International Science Council and maintained by NIST (physics.nist.gov/cuu/Constants).