English-Metric Conversions
inch (in) = 2.54 cm (exactly)
meter (m) = 39.37008 in (approximately)
mile (mi) = 1.609344 km (exactly)
kilometer (km) = 0.6213712 mi (approximately)
pound (lb) = 453.6 g = 0.4536 kg
kilogram (kg) = 2.205 lb

(Perhaps Unfamiliar) Units
atmosphere (unit of pressure) = average atmospheric pressure at the surface of the Earth
= 14.7 pounds per square inch ~ 2120 pounds per square foot ~ one tonne per square foot
bar (unit of pressure) = 10 Newtons per square centimeter (~ one atmosphere)
Pascal (unit of pressure) = 1 Newton per square meter = 1/100,000th of a bar

Distances
Astronomical Unit (AU) = 149,597,870.691 km = 92,955,807.267 mi
Semi-major axis of Earth's orbit = 149,597,890 km = 92,955,819.3 mi = 1.00000013 AU
Light year (LY) = 9.46053 trillion km = 5.87850 trillion mi = 63,239.7 AUs = 0.30660 pc
Parsec (pc) = 1296000 / 2 π AUs = 206264.8062 AUs = 30.856776 trillion km = 19.1735 trillion mi
Parsec (pc) = 3.2616 LY

Time Units
Rotational periods of the Earth (all increasing by 0.002 seconds/day/century)
Relative to fixed stars = 23 hr 56 m 4.099 s
Mean sidereal day (relative to Vernal Equinox) = 23 hr 56 m 4.091 s
Mean solar day (relative to Sun, average over a year) = 24 hr 00 m 0.000 s in 1900

Orbital periods of the Earth
Sidereal year (Relative to fixed stars) = 365.256363 Mean solar days
Tropical year (Relative to Vernal Equinox) = 365.242190 days = 365d 05h 48m 45.2s = 31,556,925.2 s
Anomalistic year (Relative to perihelion) = 365.259635 days
Eclipse year (Relative to Moon's ascending node) = 346.620075 days in 1999 (+0.000001 days/year)

Astronomical Bodies
equatorial radius of the Earth (R_Earth) = 6378.1 km = 3963.2 mi
mass of the Earth (M_Earth) = 5.9736 x 10²⁴ kg
equatorial radius of the Sun (R_☉) = 696000 km = 432000 mi = 109 R_Earth
mass of the Sun (M_☉) = 1.989 x 10³⁰ kg = 333000 M_Earth
luminosity of the Sun (L_☉) = 3.85 x 10²⁶ watts = 5.9 million watts/square foot
surface temperature of the Sun (T_☉) = 5780 Kelvin = 9950 Fahrenheit

Fundamental Physical Constants
(Units: s = second, J = joule, K = Kelvins, kg = kilograms, m = meters, W = watts)
Avogadro constant (N_A) = 6.0221420 x 10²³ mol^-1
Boltzmann constant (k) = 1.380650 x 10^-23 J K^-1
elementary charge (e) = 1.602176462 x 10^-19 Coulombs
electron volt (eV) = 1.602176462 x 10^-19 J
molar gas constant (R) = 8.31447 J mol^-1 K^-1
Newtonian gravitational constant (G) = 6.67 x 10^-11 m³ kg^-1 s^-2
Planck constant (h) = 6.6260688 x 10^-34 J s
Planck length (l_P) = 1.616 x 10^-35 m
Planck time (t_P) = 5.3906 x 10^-44 s
Speed of light in a vacuum (c) = 299,792.458 km/sec (exactly) =186,282.397 mi/sec (approximately)
Stefan-Boltzmann constant (σ) = 5.6704 x 10^-8 W m^-2 K^-4

Masses of fundamental particles
atomic mass unit (m_u) = 1.6605387 x 10^-27 kg
electron mass (m_e) = 9.1093819 x 10^-31 kg
proton mass (m_p) = 1.6726216 x 10^-27 kg = 1836.152668 m_e
alpha-particle mass (m_α) = 6.6446560 x 10^-27 kg = 3.9725997 m_p