Constants, units and conversions

2026-09-04

Values follow the 2019 SI definitions and CODATA 2018 recommendations. Quantities marked exact are defined, not measured. Grouped like appendix C of the No bullshit guide.

Fundamental constants of Nature

Constant	Symbol	Value	Unit
speed of light	$c$	$2.997\,924\,58 \times 10^{8}$ (exact)	m/s
Planck constant	$h$	$6.626\,070\,15 \times 10^{-34}$ (exact)	J·s
reduced Planck constant	$\hbar = h/2\pi$	$1.054\,571\,817 \times 10^{-34}$	J·s
elementary charge	$e$	$1.602\,176\,634 \times 10^{-19}$ (exact)	C
Boltzmann constant	$k_B$	$1.380\,649 \times 10^{-23}$ (exact)	J/K
Avogadro constant	$N_A$	$6.022\,140\,76 \times 10^{23}$ (exact)	1/mol
gas constant	$R = N_A k_B$	$8.314\,462\,618$	J/(mol·K)
gravitational constant	$G$	$6.674\,30 \times 10^{-11}$	N·m²/kg²
standard gravity	$g$	$9.806\,65$ (exact by convention)	m/s²
vacuum permittivity	$\varepsilon_0$	$8.854\,187\,8128 \times 10^{-12}$	F/m
vacuum permeability	$\mu_0$	$1.256\,637\,062\,12 \times 10^{-6}$	N/A²
Coulomb constant	$k = 1/4\pi\varepsilon_0$	$8.987\,551\,79 \times 10^{9}$	N·m²/C²
electron mass	$m_e$	$9.109\,383\,7015 \times 10^{-31}$	kg
proton mass	$m_p$	$1.672\,621\,923\,69 \times 10^{-27}$	kg
neutron mass	$m_n$	$1.674\,927\,498\,04 \times 10^{-27}$	kg
atomic mass unit	$u$	$1.660\,539\,066\,60 \times 10^{-27}$	kg
Stefan–Boltzmann constant	$\sigma$	$5.670\,374\,419 \times 10^{-8}$	W/(m²·K⁴)
fine-structure constant	$\alpha$	$7.297\,352\,5693 \times 10^{-3} \approx 1/137$	none

Astronomical quantity	Symbol	Value	Unit
Earth mass	$M_\oplus$	$5.972 \times 10^{24}$	kg
Earth mean radius	$R_\oplus$	$6.371 \times 10^{6}$	m
Moon mass	$M_{\text{moon}}$	$7.346 \times 10^{22}$	kg
Earth–Moon distance		$3.844 \times 10^{8}$	m
Sun mass	$M_\odot$	$1.989 \times 10^{30}$	kg
Earth–Sun distance	au	$1.495\,978\,707 \times 10^{11}$ (exact)	m
light year	ly	$9.460\,730 \times 10^{15}$	m

Units

SI base units:

Quantity	Unit	Symbol	Defined by
time	second	s	caesium-133 hyperfine transition
length	meter	m	$c$
mass	kilogram	kg	$h$
electric current	ampere	A	$e$
temperature	kelvin	K	$k_B$
amount of substance	mole	mol	$N_A$
luminous intensity	candela	cd	$K_{cd}$

Derived units used in mechanics and electromagnetism:

Quantity	Unit	Symbol	In base units
frequency	hertz	Hz	s⁻¹
force	newton	N	kg·m/s²
pressure	pascal	Pa	N/m² = kg/(m·s²)
energy, work	joule	J	N·m = kg·m²/s²
power	watt	W	J/s = kg·m²/s³
charge	coulomb	C	A·s
voltage	volt	V	J/C = W/A
resistance	ohm	Ω	V/A
capacitance	farad	F	C/V
magnetic field	tesla	T	N/(A·m) = kg/(A·s²)
angle	radian	rad	m/m

Prefixes:

Prefix	Symbol	Factor	Prefix	Symbol	Factor
tera	T	$10^{12}$	milli	m	$10^{-3}$
giga	G	$10^{9}$	micro	μ	$10^{-6}$
mega	M	$10^{6}$	nano	n	$10^{-9}$
kilo	k	$10^{3}$	pico	p	$10^{-12}$
hecto	h	$10^{2}$	femto	f	$10^{-15}$
centi	c	$10^{-2}$	atto	a	$10^{-18}$

Other units and conversions

Length:

Unit	Equals
1 inch (in)	$2.54$ cm (exact)
1 foot (ft)	$12$ in $= 0.3048$ m
1 yard (yd)	$3$ ft $= 0.9144$ m
1 mile (mi)	$5280$ ft $= 1609.344$ m
1 nautical mile	$1852$ m
1 ångström (Å)	$10^{-10}$ m
1 light year	$9.4607 \times 10^{15}$ m
1 parsec (pc)	$3.0857 \times 10^{16}$ m $= 3.2616$ ly

Mass and force:

Unit	Equals
1 pound (lb)	$0.453\,592\,37$ kg (exact)
1 ounce (oz)	$28.3495$ g
1 tonne (t)	$1000$ kg
1 atomic mass unit	$1.6605 \times 10^{-27}$ kg
1 pound-force (lbf)	$4.448\,22$ N
1 dyne	$10^{-5}$ N

Time and speed:

Unit	Equals
1 minute	$60$ s
1 hour	$3600$ s
1 day	$86\,400$ s
1 year	$365.25$ days $\approx 3.156 \times 10^{7}$ s
1 km/h	$0.2\overline{7}$ m/s
1 m/s	$3.6$ km/h $= 2.237$ mph
1 mph	$0.447\,04$ m/s
1 knot	$0.514\,44$ m/s

Energy and power:

Unit	Equals
1 electronvolt (eV)	$1.602\,176\,634 \times 10^{-19}$ J (exact)
1 calorie (cal)	$4.184$ J
1 food Calorie (kcal)	$4184$ J
1 kilowatt-hour	$3.6 \times 10^{6}$ J
1 BTU	$1055.06$ J
1 horsepower (hp)	$745.7$ W

Pressure:

Unit	Equals
1 atmosphere (atm)	$101\,325$ Pa (exact)
1 bar	$10^{5}$ Pa
1 mmHg (torr)	$133.322$ Pa
1 psi	$6894.76$ Pa

Temperature:

T_K = T_C + 273.15, \qquad T_F = \tfrac{9}{5} T_C + 32, \qquad T_C = \tfrac{5}{9}(T_F - 32)

Angle, area and volume:

Unit	Equals
1 revolution	$2\pi$ rad $= 360°$
$1°$	$\pi/180$ rad $\approx 0.017\,45$ rad
1 rad	$180°/\pi \approx 57.30°$
1 hectare (ha)	$10^{4}$ m²
1 acre	$4046.86$ m²
1 liter (L)	$10^{-3}$ m³ $= 1000$ cm³
1 US gallon	$3.785\,41$ L
1 imperial gallon	$4.546\,09$ L

References

NIST: CODATA fundamental physical constants (opens in a new tab): the recommended values (this sheet uses CODATA 2018; newer adjustments are published there)
BIPM: The SI Brochure (opens in a new tab): the official definition of the SI, its base and derived units and prefixes
NIST Special Publication 811 (opens in a new tab): guide to using SI units, with conversion factors for non-SI units
Ivan Savov, No bullshit guide to math & physics (opens in a new tab): appendix C, which this sheet's grouping follows