Physical Constants & SI Prefixes
A quick reference for the numbers every physics and chemistry student needs: speed of light, Planck constant, charge of an electron, gas constant and many more, plus the full list of SI prefixes with a converter. Tap any row to copy its value.
Physical constants
| Constant | Value | Unit |
|---|
exact means the value is exact by definition. Other values are rounded to 8 digits or fewer from the CODATA recommended values (the usual small measurement uncertainty applies). Use the values your textbook gives for exams.
SI prefixes
| Prefix | Symbol | Power of ten | Meaning |
|---|
Prefix converter
How prefixes work
A prefix is a short word in front of a unit that means “times a power of ten”. kilo means ×1000, so 1 km = 1000 m. milli means ×0.001, so 1 mm = 0.001 m. To change from one prefix to another, count how many powers of ten apart they are. From kilo (103) to milli (10−3) is 6 steps, so 1 kg = 1,000,000 mg.
Scientific notation
Very big and very small numbers are written as a number between 1 and 10 times a power of ten. For example the speed of light, 299,792,458 m/s, is 2.99792458 × 108 m/s, and the charge of an electron is 1.602176634 × 10−19 C. On a scientific calculator the ×10x key (or EXP) types this.
FAQ
What is the value of the speed of light?
299,792,458 m/s, which is 2.99792458 x 10^8 m/s. It is an exact value by definition, and 3 x 10^8 m/s is used in most calculations.
What is the value of the Planck constant?
h = 6.62607015 x 10^-34 joule-seconds. It is exact by definition since 2019. The reduced Planck constant h-bar is h divided by 2 pi, about 1.0546 x 10^-34 J s.
What is the charge of an electron?
The elementary charge is e = 1.602176634 x 10^-19 coulomb. An electron has charge minus e and a proton has plus e.
What are the SI prefixes in order?
From largest to smallest: quetta, ronna, yotta, zetta, exa, peta, tera, giga, mega, kilo, hecto, deca, deci, centi, milli, micro, nano, pico, femto, atto, zepto, yocto, ronto, quecto.
How do I convert between prefixes?
Subtract the powers of ten. To go from kilo (10^3) to milli (10^-3), multiply by 10^(3 - (-3)) = 10^6. To go the other way, divide.
Which value of g should I use?
Use 9.8 or 9.81 m/s^2 for most school problems, or 10 m/s^2 if your question says so. The standard value is exactly 9.80665 m/s^2.