Defining constants
Since the 2019 SI redefinition most of these are exact by fiat: the number is the definition, and no future experiment can change it. The site's exactness badge depends on which side of that line a constant falls.
| Symbol | Quantity | Value used | Status | Copy |
|---|---|---|---|---|
| pi | π | 3.14159265358979323846264338328… | Truncated | |
| tau | 2π | 6.28318530717958647692528676656… | Truncated | |
| e | e | 2.71828182845904523536028747135… | Truncated | |
| c | speed of light, m/s | 299792458 | Exact | |
| h | Planck constant, J·s | 0.000000000000000000000000000000000662607015… | Exact | |
| kB | Boltzmann constant, J/K | 0.00000000000000000000001380649 | Exact | |
| NA | Avogadro constant, 1/mol | 602214076000000000000000 | Exact | |
| qe | elementary charge, C | 0.0000000000000000001602176634 | Exact | |
| g0 | standard gravity, m/s² | 9.80665 | Exact | |
| atm | standard atmosphere, Pa | 101325 | Exact | |
| au | astronomical unit, m | 149597870700 | Exact | |
| ly | light-year, m | 9460730472580800 | Exact | |
| me | electron mass, kg | 0.000000000000000000000000000000910938371393… | Truncated | |
| amu | atomic mass unit, kg | 0.00000000000000000000000000166053906892… | Truncated |
Any of these names can be typed straight into a converter's value box:
pi/4, c*3 and 2^10 are all valid input, evaluated
in the same exact arithmetic as the conversion itself.
What "exact" means here
Exact constants are definitions. The speed of light is exactly 299 792 458 m/s because the meter is defined from it, not because anyone measured it that precisely. The same now applies to the Planck constant, the elementary charge, the Boltzmann constant and the Avogadro constant.
Truncated constants are either irrational (π, e) or measured (the electron mass, the atomic mass unit). π and e are carried to 75 decimal places, which is well beyond the 50-digit display limit, but a truncation is still a truncation — so any conversion touching one is reported as rounded rather than exact.
Conventional values
Standard gravity (9.806 65 m/s²) and the standard atmosphere (101 325 Pa) are neither measurements nor natural constants — they are agreed conventions. They are exact in the same sense a definition is, which is what makes the pound-force, psi and kgf convert without approximation.