Basic Units and derived units
time, seconds, s
length, meter, m
mass, kilogram, kg
electric current, ampere, A
temperature, kelvins, K
amount, mole, mol
luminous intensity, candela, cd

Energy, joule, J = kg•m²/s²
Charge, coulomb, C = As
Capacitance, farad, F = s⁴A²/kg•m²
Force, newton, N = kg•m/s²
Pressure, pascal, Pa = kg/m•s²
Inductance, henry, H = kg•m²/s²A²

Mass/Weight
US short ton = 2000 lb = 907.2 kg
US long ton = 2240 lb = 1016 kg (aka Imperial ton)
metric ton = 1000 kg = 2205 lb
electron mass = 9.109e-31 kilograms
1 avoirdupois ounce = 28.3 g
1 troy ounce = 31.1 g
one grain = 1/480 troy ounce = 64.79891 mg
1 GeV/c² = 1.783×10^−27 kg
! amu = 1 Da = 1.660e−27 kg


Power/Energy
A ton of TNT = 4.184 GJ = 4.184e9 J
A megaton of TNT = 4.184 PJ = 4e15 J
one ton HVAC capacity = 12,000 BTU/hour = 3517 watts
  assuming a COP of 3, that means AC power needed is 1200 W
  or an EER or 12, which is BTU/h / AC input
1 HP = 746 watts = 550 ft-lb/sec = 33,000 ft-lb/min = 2547 BTU/h
1 BTU per hour = 0.293 watts
1 eV = 1.6022×10^−19 J
1 GeV = 1.6022×10^−10 J

BTU is defined as heat required to raise the temperature of one
  1 pound of liquid water by 1°F at a constant pressure of one
  atmosphere. BTU/h is the power needed to raise the temperature
  of one 1 pound of liquid water by 1 °F at a constant pressure of
  one atmosphere in one hour.
tons of refrigeration: It is the amount of power needed to melt one
  short ton of ice (2000 lb) in 24 hours
foot-pounds per second is the amount of power to raise one pound
  against gravity by one foot in one second.

1 therm = 100,000 BTU = 105.5 MJ or 29.3 kW•hour
Gasoline contains about 35 MJ/L (132 MJ/US gal).

In physics, energy (Ancient Greek: ἐνέργεια energeia "activity,
operation") is an indirectly observed quantity that is often
understood as the ability of a physical system to do work on other
physical systems. Since work is defined as a force acting through a
distance (a length of space), energy is always equivalent to the
ability to exert pulls or pushes against the basic forces of nature,
along a path of a certain length.


Time
1 time constant is 0.37
6 time constants is 0.0026
Seconds per Julian year, 3600 x 24 x 365.25 = 31,557,600 sec
  Gregorian year is 365.2425 days or 31,556,952 sec
  Sidereal year is 365.256363004 days or 31,558,149.8 sec


Pressure
1 Pa = 1 N/m²
1 torr = 1/760 of a standard atmosphere
1 standard atmosphere = 101.325 kPa = 1.01325 bar = 14.7 PSI
    = 10.33 meters of water = 29.92 inch of mercury
1 bar = 100 kPa = 14.5 PSI
1 torr = 0.001316 atm
1 torr = 1 mm Hg (slightly off)
760 torr = 1 atm
1 PSI (lb/in²) = 6,894.8 Pa (N/m²) = 6.895x10^-3 N/mm² = 6.895x10^-2 bar
Vacuum, 100 micron = 13 Pa = 0.1 mm Hg


Length
Ångström (symbol Å) = 0.1 nm or 1e−10 metres
The parsec (parallax of one arcsecond) = 31e12 kilometres
  or 19.2e12 miles or 206e3 AU or 3.26 light-years.
1 US inch = 25.4 mm
1 foot = 12 inch = 305 mm
1 yard = 36 inch = 914 mm
1 mile = 5280 ft = 1609 m
1 nautical mile = 1,852 m

1 light-year = 9460730472580800 metres (exactly)
  ≈ 5.878625 trillion miles
  ≈ 63241.077 astronomical units
  ≈ 0.306601 parsecs


Area
The hectare is a unit of area defined as 10,000 square metres.
  or 2.471 acre
640 acres = 1 square mile
1 acre = 43,560 square feet = 4,050 square meters


Volume
US dry gallon = 4.405 L
Imperial (UK) gallon = 4.546 L
1 UK fluid ounce = 28.4 ml = 1.733 in³
1 US fluid ounce = 29.6 mL = 2 tablespoons = 6 teaspoons
  = 1.805 cubic inch
  = 1.041 avoirdupois ounce of water
  = 1.041 imperial (UK) fluid ounces
1 US gill = 4 US fluid ounce
1 US cup = 8 US fluid ounce
1 US pint = 16 US fluid ounce
1 US quart = 32 US fluid ounce
1 US gallon = 128 US fluid ounce
  = 3.785 L = 3785 cm³
1 Liter = 33.8 US fluid ounce
1 m³ = 1000 L = 264 US gallon
  = 35.3 cubic feet = 1.307 cubic yard
1 acre-ft = 325800 US gal = 1233.5 m³

100000 L = 100 m³
10000 L = 10 m³
1000 L = 1 m³
100 L = 0.1 m³
10 L = 10 dm³ = 10000 cm³ = 0.01 m³
1 L = 1 dm³ = 1000 mL = 1000 cm³ = 0.001 m³
0.1 L = 0.1 dm³ = 100 mL = 100 cm³ = 0.0001 m³
0.01 L = 10 mL = 10 cm³ = 0.00001 m³
0.001 L = 1 mL = 1 cm³ = 0.000001 m³

1 mL = 1000µL = 1000 mm³ = 1 cm³ = 0.000001 m³
0.1 mL = 100µL = 100 mm³ = 0.1 cm³
0.01 mL = 10µL = 10 mm³ = 0.01 cm³
0.001 mL = 1µL = 1 mm³ = 0.001 cm³

1 µL = 1000 nL = 1 mm³ = 100000 µm³
0.1 µL = 100 nL = 0.1 mm³ = 10000 µm³
0.01 µL = 10 nL = 0.01 mm³ = 1000 µm³
0.001 µL = 1 nL = 0.001 mm³ = 100 µm³


Temperature
C = (5/9)(F–32)
C = K − 273.15
K = (5/9)(F + 459.67)
K = C + 273.15
F = (9/5)C + 32
F = (9/5)K − 459.67
F = R – 459.67
R = F + 459.67

For temperature intervals rather than specific temperatures,
1 °F = 1°R = (9/5)°C = (9/5)K
1 °C = 1 K = (5/9)°F = (5/9)°R


deciBell
dB = 10 log P or 20 log V
Power ratio = 10^(dB/10)
Voltage ratio = 10^(dB/20)

. . . . .power . . voltage
0.1 dB . 1.02 .. 1.01
0.2 dB . 1.05 .. 1.02
0.5 dB . 1.12 .. 1.05
1 dB .... 1.26 .. 1.12
2 dB .... 1.54 .. 1.26
3 dB .... 2.0 .... 1.41
4 dB .... 2.51 .. 1.58
6 dB .... 4.0 .... 2.0
8 dB .... 6.31 .. 2.51
10 dB . 10 ...... 3.16
20 dB . 100 .... 10
30 dB . 1000 .. 31.6
40 dB . 10k . . 100
50 dB . 100k .. 316
60 dB . 1M .... 1000
70 dB . 10M .. 3160
80 dB . 100M . 10k
etc.

dB references

Voltage
dBu or dBv, reference is 0.775 VRMS
dBV, reference is 1 volt RMS
dBmV, reference is 1 mV across 75 ohms
dBµV or dBuV, reference is 1µV

Acoustic Power
dB(SPL) (Sould Pressure Level), reference to 20 µPa
dB(PA), reference to 1 Pz
dB SIL (Sound Intensity Level), reference to 10^-12 W/m²
dB SWL (sound Power Level) referenced to 10^-12 W
dB(A), dB(B), dB(C). these are dB SIL but with different
   frequency weighing filters.
dB HL (Hearing Level) is used in audiograms as a measure of
   hearing loss

Electrical Poewr
dBm is db referenced to 1 mW
dBW is db referenced to 1 watt
they differ by a factor of 1000 or 30 dB

dBFS (Full Scale) reference is the max signal the amplifier
   can handle
dBTP (True peak) reference is the max peak signal the amplifier
   can handle



Electron
1 C = 6.242e18 electrons
charge on electron 1.602e−19 C
electron mass = 9.10938188e-31 kilograms



Denier

Denier is a unit of measure for the linear mass density of
fibers. It is defined as the mass in grams per 9,000 meters.
8 dernier = 8 g/9000 meter

The conversion formula is
d = √(dernier•1.415e-6 / ρ)
where d is diameter in cm
ρ is material density in g/cm³


R-Value

The conversion between SI and US units of R-value is
1 h·ft²·°F/Btu = 0.176110 K·m²/W, or
1 K·m²/W = 5.678263 h·ft²·°F/Btu.



SI (metric) prefixes
da (deca) = x10 (rarely used)
h (hecto) = x100 (rarely used)
k (kilo) = x1000  = e3
M (mega) = x1000000  = e6
G (giga) = x1000000000  = e9
T (tera) = x1000000000000 = e12
P (peta) = x1000000000000000 = e15
E (exa) = x1000000000000000000 = e18
Z (zetta) = x1000000000000000000000 = e21
Y (yotta) = x1000000000000000000000000 = e24

d (deci) = /10 = e-1 (rarely used)
c (cent) = /100  = e-2 (rarely used except for cm)
m (milli) = /1000  = e-3
µ (micro) = /1000000  = e-6
n (nano) = /1000000000 = e-9
p (pico) = /1000000000000 = e-12
f (femto) = /1000000000000000 = e-15
a (atto) = /1000000000000000000 = e-18
a (zepto) = /1000000000000000000000 = e-21
a (yocto) = /1000000000000000000000000 = e-24

SI binary prefixes
Ki (kibi) = x1024
Mi (mebi) = x1024²
Gi (gibi) = x1024³
Ti (tebi) = x1024⁴
Pi (pebi) = x1024⁵
Ei (exbi) = x1024⁶
Zi (zebi) = x1024⁷
Yi (yobi) = x1024⁸