Fundamental Constants Table
| Constant | Symbol | Value | Status |
|---|---|---|---|
| Speed of light | c | 299,792,458 m/s | exact by definition |
| Planck constant | h | 6.62607015 x 10⁻³⁴ J·s | exact by definition |
| Elementary charge | e | 1.602176634 x 10⁻¹⁹ C | exact by definition |
| Boltzmann constant | k | 1.380649 x 10⁻²³ J/K | exact by definition |
| Avogadro constant | N₀ | 6.02214076 x 10²³ /mol | exact by definition |
| Gravitational constant | G | 6.67430 x 10⁻¹¹ m³/(kg·s²) | measured - least precise |
| Electron mass | mₑ | 9.1093837139 x 10⁻³¹ kg | measured, 2022 CODATA |
| Proton mass | mₖ | 1.67262192595 x 10⁻²· kg | measured, 2022 CODATA |
Since May 2019, the SI system is built on nature instead of objects: five constants - the speed of light, Planck's constant, the elementary charge, Boltzmann's constant and Avogadro's number - are now defined to be exactly what they are, and the kilogram, ampere, kelvin and mole are pinned to them. The table below carries those five exact values plus the most famous measured ones, with NIST's CODATA adjustment as the source of record.
The converter runs the most famous equation in physics on your behalf: type any mass and E=mc2 says how much energy it is worth at the defined speed of light. The numbers come out absurd on human scales - one gram holds about 90 trillion joules, or 25 million kilowatt-hours - which is exactly why nuclear processes dwarf chemical ones, and why the constant c in the table is the hinge the whole modern SI turns on.
How to use
- Read the Status column first: exact values are definitions - they cannot be measured better because they are the ruler - while measured values carry uncertainty and get adjusted every few CODATA releases.
- Type a mass in grams or kilograms and the converter returns its E=mc2 energy in joules and kilowatt-hours.
- Scale intuition: chemical reactions release about a billionth of a mass's rest energy; nuclear fission releases about a tenth of a percent.
Frequently asked questions
Why did the speed of light become exact?
Because we redefined the meter around it: since 1983, the meter is the distance light travels in 1/299,792,458 of a second, so the value 299,792,458 m/s is true by definition. Measuring the speed of light now means measuring the meter. The 2019 revision did the same trick for the other four constants, ending the era of leaking artefact kilograms.
Which constants are exact and which are measured?
Five are exact by SI definition: the speed of light c, Planck's constant h, the elementary charge e, Boltzmann's constant k, and Avogadro's number. Everything else in the table - Newton's gravitational constant G above all - is measured, carries an uncertainty, and gets re-evaluated in NIST's periodic CODATA adjustments, most recently the 2022 set.
Why is G still so imprecise?
Gravity is absurdly weak between lab-sized objects, so every experiment to time how lead balls twist toward each other fights noise from vibration, tides and stray electrostatic forces. Two centuries of measurements agree only to about one part in 100,000 - G's relative uncertainty is a million times worse than most constants', and it has barely improved in decades, making it the least precise famous constant in physics.
How much energy is in one gram of matter?
About 89.9 trillion joules - 25 million kilowatt-hours - from the converter at 1 gram. No process releases all of it: burning hydrogen extracts roughly a billionth, uranium fission about 0.09 percent, and matter-antimatter annihilation is the only 100-percent case, which is why the numbers in the converter feel like science fiction next to any bill you have ever paid.
What is CODATA and why does it matter?
The Committee on Data of the International Science Council runs the periodic adjustment that turns hundreds of worldwide measurements into one recommended value per constant - NIST publishes the set that textbooks and instruments standardize on. When a table shows a constant's digits changing between editions, that is a new CODATA adjustment absorbing better measurements, not physics changing its mind.