I = Current Units = Amps (A),
V = Voltage Units = Volts (V)
R = resistance Units = Ohms
P = Power Units = Watts (W)
C = Capacitance Units = Farads (F)
L = Inductance Units = Henries (H)
F = Frequency Units = Hertz (Hz)

p = Pico   0.000000000001
n = Nano  0.000000001
µ = Micro 0.000001
m = Milli   0.001

Ohms law - V = I x R
P = I² x R       P = V² / R       P = I x V

Resistors In Series - Rt = R1 + R2  + R3 + R4 ...
Resistors In Parallel - Rt = 1 / ((1 / R1) + (1 / R2) + (1 / R3))

Inductors In Series - Lt = L1 + L2 + L3 ...
Inductors In Parallel - Lt = 1 / ((1 / L1) + (1 / L2) + (1 / L3))

Capacitors In Parallel - Ct = C1 + C2
Capacitors In Series - Ct = 1 / ((1 / C1) + (1 / C2) + (1 / C3))

Powering a LED, resistor value selection:

R = (V-0.7) / I    or    R = (V-0.3) / I

Standard (silicon) LEDs drop the voltage by 0.7 of a volt, Germanium LEDs drop it by 0.3
Typical value for I is 20mA or 0.02A. Typical voltage used 5 volts DC. thus:

R = (5-0.7) / 0.02    =>    R = 215 (220) ohms

Wire Guage Table

Select your resistor colour code below: Black Brown Red Orange Yellow Green Blue Violet Gray White Black Brown Red Orange Yellow Green Blue Violet Gray White Black Brown Red Orange Yellow Green Blue Violet Gray White Gold Silver      Gold Silver None