The 12 Basic Formulas
There are 12 basic formulas that are used regularly. The formulas are for calculating voltage, current, resistance, and power. With any two values you can calculate the other two.
Voltage V =  I × R 

√ (P × R)  result in volts V  
Current I = 



result in amperes A  
Resistance R = 



result in ohms Ω  
Power P =  V × I  R × I^{ 2} 

result in watts W 
Formulas
Energy Lost in a Resistor
TODO: Find the actual formulas for this.
A 12 V battery is connected in series with a resistance of 50 ohm. The power consumed in the resistor can be calculated as
P = (12 V)^{2} / (50 ohm)
= 2.9 W
Electrical Motor Efficiency when Shaft Output is measured in Horsepower
If power output is measured in horsepower (hp), efficiency can be expressed as
η_{m} = P_{out} × 746 / P_{in}
where
η_{m} = efficiency
P_{out} = shaft power out (horsepower, hp)
P_{in} = electric power in to the motor (Watt, W)
Electrical Motor Efficiency when Shaft Output is measured in Watts
If power output is measured in watts (W), efficiency can be expressed as
η_{m} = P_{out} / P_{in}
where
η_{m} = motor efficiency
P_{out} = shaft power out (watts, W)
P_{in} = electric power in to the motor (watts, W)
Conductance
G = 1 / R
G = I / V
where
G = siemens (S)
R = resistance (Ω)
I = electric current through the object
V = voltage (electrical potential difference) across the object
IC 555 Timer Formulas
f = 1 / [ ln(2) x C x (R_{1} + 2R_{2}) ]
f = 1.44 / C x (R_{1} + 2R_{2}) … where … 1 / ln(2) = 1.44
T_{P} = ln(2) x C x (R_{1} x R_{2})
T_{P} = 0.693 x C x (R_{1} x R_{2}) … where … ln(2) = 0.693
T_{N} = ln(2) x C x R_{2}
T_{N} = 0.693 x C x R_{2} … where … ln(2) = 0.693
where
C = value of capacitor in farads
R_{1} and R_{2} = value of the input resistance
f = frequency in kHZ
T_{P} = positive or high time from each pulse
T_{N} = negative or low time from each pulse
Temperature Conversions
T_{°F} = Degrees Fahrenheit (°F)
T_{°C} = Degrees Celsius (°C)
T_{K} = Kelvins (K)
Degrees Fahrenheit (°F) to Degrees Celsius (°C)
T_{°C} = (T_{°F} − 32) × 5/9
T_{°C} = (T_{°F} − 32) / (9/5)
T_{°C} = (T_{°F} − 32) / 1.8
Degrees Celsius (°C) to Degrees Fahrenheit (°F)
T_{°F} = T_{°C} × 9/5 + 32
T_{°F} = T_{°C} × 1.8 + 32
Degrees Fahrenheit (°F) to Kelvins (K)
T_{K} = (T_{°F} + 459.67) × 5/9
Degrees Celsius (°C) to Kelvins (K)
T_{K} = T_{°C} + 273.15
Kelvins (K) to Degrees Fahrenheit (°F)
T_{°F} = T_{K} × 9/5 − 459.67
Kelvins (K) to Degrees Celsius (°C)
T_{°C} = T_{K} − 273.15
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International System of Units (SI) Units
SI Prefixes
Prefixes are added to unit names to produce multiples and submultiples of the original unit, All multiples are integer powers of ten, and above a hundred or below a hundredth all are integer powers of a thousand.
Prefix  Symbol  Value  

yocto–  y  10^{−24}  = 0.000 000 000 000 000 000 000 001 
zepto–  z  10^{−21}  = 0.000 000 000 000 000 000 001 
atto–  a  10^{−18}  = 0.000 000 000 000 000 001 
femto–  f  10^{−15}  = 0.000 000 000 000 001 
pico–  p  10^{−12}  = 0.000 000 000 001 
nano–  n  10^{−9}  = 0.000 000 001 
micro–  µ  10^{−6}  = 0.000 001 
milli–  m  10^{−3}  = 0.001 
centi–  c  10^{−2}  = 0.01 
deci–  d  10^{−1}  = 0.1 
—  —  10^{0}  = 1 
deca–  da  10^{1}  = 10 
hecto–  h  10^{2}  = 100 
kilo–  k  10^{3}  = 1 000 
mega–  M  10^{6}  = 1 000 000 
giga–  G  10^{9}  = 1 000 000 000 
tera–  T  10^{12}  = 1 000 000 000 000 
peta–  P  10^{15}  = 1 000 000 000 000 000 
exa–  E  10^{18}  = 1 000 000 000 000 000 000 
zetta–  Z  10^{21}  = 1 000 000 000 000 000 000 000 
yotta  Y  10^{24}  = 1 000 000 000 000 000 000 000 000 
SI Units
Common NonSI Units
The SI is capable of describing most useful and measurable physical quantities, but many nonSI units still appear in the scientific, technical, and commercial literature. Some units are deeply embedded in history and culture. The Comité International des Poids et Mesures (International Committee for Weights and Measures) recognized and acknowledged such traditions by compiling a list of nonSI units accepted for use with SI. The units that are defined in BIPM are marked ^{[BIPM]}.
Unit  Symbol  Measures  Definition  Notes 

ångström^{[BIPM]}  Å  length  1 Å = 10^{−10} m 1 Å = 10^{−8} cm 1 Å = 10^{−4} μm 1 Å = 0.1 nm 1 Å = 100 pm 

bar^{[BIPM]}  bar  pressure  1 bar = 0.1 MPa 1 bar = 100 kPa 1 bar = 10^{5} Pa 

barn^{[BIPM]}  b  area  1 b = 100 fm^{2} 1 b = (10^{−12} cm)^{2} 1 b = 10^{−28} m^{2} 
The barn is a unit of area employed to express cross sections in nuclear physics. 
bel  B  logarithmic ratio quantities  
day^{[BIPM]}  d  time  1 d = 24 h 1d = 86 400 s 

decibel  dB  logarithmic ratio quantities  1 dB = 0.1 B  
degree^{[BIPM]}  °  plane angle  1° = (π/180) rad  
dyne^{[BIPM]}  dyn  force  1 dyn = 10^{−5} N  
erg^{[BIPM]}  erg  energy  1 erg = 10^{−7} J  
gal^{[BIPM]}  Gal  acceleration  1 Gal = 1 cm s^{−2} 1 Gal = 10^{−2} m s^{−2} 
The gal is a special unit of acceleration employed in geodesy and geophysics to express acceleration due to gravity. 
gauss^{[BIPM]}  G  magnetic flux density  1 G = 1 Mx cm^{−2} 1 G = 10^{−4} T 

hectare^{[BIPM]}  ha  area  1 ha = 1 hm^{2} 1 ha = 10^{4} m^{2 } 

hour^{[BIPM]}  h  time  1 h = 60 min 1 h = 3600 s 

knot^{[BIPM]}  kn  speed  1 kn = (1852/3600) m/s  
litre^{[BIPM]}  L, l  volume  1 L = 1 l = 1 dm^{3} = 10^{3} cm^{3} = 10^{−3} m^{3} 

maxwell^{[BIPM]}  Mx  magnetic flux  1 Mx = 1 G cm^{2} 1 Mx = 10^{−8} Wb 

millimetre of mercury^{[BIPM]}  mmHg  pressure  1 mmHg ≈ 133.322 Pa  
minute^{[BIPM]}  ′  plane angle  1′ = (1/60)° 1′ = (π/ 10 800) rad 

minute^{[BIPM]}  min  time  1 min = 60 s  
nautical mile^{[BIPM]}  M  distance  1 M = 1852 m  
neper  Np  logarithmic ratio quantities  
œrsted^{[BIPM]}  Oe  magnetic field  1 Oe ≜ (10^{3} / 4π) A m^{−1}  The symbol ≜ means “is defined as” or “is equal by definition to”.^{[WIKI1]} 
phot^{[BIPM]}  ph  illuminance  1 ph = 1 cd sr cm^{−2} 1 ph = 10^{4} lx 

poise^{[BIPM]}  P  dynamic viscosity  1 P = 1 dyn s cm^{−2} 1 P = 0.1 Pa s 

second^{[BIPM]}  ″  plane angle  1″ = (1/60)′ 1″ = (π/ 648 000) rad 

stilb^{[BIPM]}  sb  luminance  1 sb = 1 cd cm^{−2} 1 sb = 10^{4} cd m^{−2} 

stokes^{[BIPM]}  St  kinematic viscosity  1 St = 1 cm^{2} s^{−1} 1 St = 10^{−4} m^{2} s^{−1} 

tonne^{[BIPM]}  t  mass  1 t = 10^{3} kg 
Formula Symbols
Formula Symbol  It Measures  SI Unit  SI Unit Symbol  Notes  

B 

tesla = weber per square metre 
T 


C  electric capacitance  farad  F  F = C/V = A·s/V = s/Ω  
D  electric flux density  coulomb per square meter  C/m^{ 2}  
E  electric field strength  volt per meter  V/m  
E_{V}  illuminance and luminous emittance 
lux  Ix  Ix = Im/m^{ 2}  
G  electric conductance  siemens  S  S = 1/Ω  
H  magnetic field strength  ampere per meter  A/m  
I  electric current  ampere  A  
I_{V}  luminous intensity  candela  cd  
J  current density  ampere per square meter  A/m^{ 2}  
L  inductance  henry  H  H = Wb/A = V·s/A = Ω·s  
L_{V}  luminance  candela per square meter  cd/m^{ 2}  
P  power  watt, joule per second, newtonmeter per second 
W, J/s, Nm/s  P = W / t  
Q  electric charge  coulomb  C  amperesecond A·s = C  
R  electric resistance DC  ohm  Ω  Ω = V/A  
T  period  second  s  T=1 / f  
T  temperature  Kelvin  K  0 K = −273,15 °C  
V  electric voltage, electric potential difference 
volt  V  V = W/A  
W  work W = energy E  wattsecond, Joule, newtonmeter 
Ws, J, Nm  W = P × t  
Z  electric impedance AC  ohm  Ω  Ω = V/A  
Θ  magnetomotive force  ampereturn  A  uppercase theta U+0398  
Φ  magnetic flux  weber  Wb  voltsecond V·s = Wb
uppercase phi U+03A6 

Φ_{V}  luminous flux  lumen  Im  uppercase phi U+03A6  
ρ  electrical resistivity  ohm times meter or ohm times square millimeter divided by meter 
Ω · m Ω · mm^{ 2}/m 
lowercase rho U+03C1  
Notes
References
[BIPM] Bureau International des Poids et Mesures [International Bureau of Weights and Measures]. “The International System of Units (SI), 8th Edition”. PDF. 2006. ISBN 9282222136. Accessed August 14, 2017. Link goes to English language version.
[THOM] Thompson, Ambler; Taylor, Barry N. “Guide for the Use of the International System of Units (SI) (Special Publication 811)”. PDF. 2008. Gaithersburg, MD: National Institute of Standards and Technology. Accessed February 14, 2018.
[WIKI1] Wikipedia contributors. “List of Mathematical Symbols”. Wikipedia. February 10, 2018. Accessed February 14, 2018.