Source: http://teslacoils4christ.org/TCFormulas/TCFormulas.htm#capreac

Ohm’s Law

E = volts
I = current in amps
Z = impedance or resistance in ohms
P = power in watts

Transformer Input and Output

E_PI_P = E_SI_S

E_P = primary voltage
I_P = primary current in amps
E_S = secondary voltage
I_S = secondary current in amps

Capacitive Reactance

X_C = capacitive reactance in ohms
F = frequency in hertz
C = capacitance in farads

Inductive Reactance

X_L = inductive reactance in ohms
F = frequency in hertz
L = inductance in henrys

Resonant Circuit Formula

F = frequency in hertz
L = inductance in henrys
C = capacitance in farads

Spiral Coil Inductance

L = inductance of coil in microhenrys (µH) R = average radius of the coil in inches N = number of turns W = width of the coil in inches

Helical Coil Inductance

L = inductance of coil in microhenrys (µH) N = number of turns R = radius of coil in inches (Measure from the center of the coil to the middle of the wire.) H = height of coil in inches

Inverse Conical Coil Inductance

L = inductance of coil in microhenrys (µH) L1 = helix factor L2 = spiral factor N = number of turns R = average radius of coil in inches H = effective height of the coil in inches W = effective width of the coil in inches X = rise angle of the coil in degrees

Secondary Coil Dimensions

T = AH

L = length of wire in feet
D = outer diameter of coil form in inches
H = height of windings in inches
A = number of turns per inch
T = total number of turns
B = thickness of wire in inches

Medhurst

C = self capacitance in picofarads
R = radius of secondary coil in inches
L = length of secondary coil in inches

Toroid Capacitance

C = capacitance in picofarads
D₁ = outside diameter of toroid in inches
D₂ = diameter of cross section of toroid in inches
This equation courtesy Bert Pool.

Sphere Capacitance

C = capacitance in picofarads
R = radius in inches

Plate Capacitors

C = capacitance in microfarads
K = dielectric constant
A = area of each plate in square inches
N = number of plates
D = distance between plates in inches (thickness of dielectric)

Leyden Jar Capacitors

C = capacitance in microfarads
K = dielectric constant
D = diameter of jar in inches
H = height of jar in inches
T = thickness of jar in inches

AC RMS and Peak Voltage

E_RMS = 0.7071·E _P

E_RMS = RMS voltage
E_P = peak voltage

Rotary Spark Gap Firings per Second

F = firings per second (hertz)
R = motor RPM rating
E = number of rotary electrodes

Rotary Spark Gap Electrode Speed

S = electrode speed (MPH)
R = motor RPM rating
D = diameter of electrode placement circle (inches)

Energy for L and C

J = joules of energy stored
V = peak charge voltage
I = peak current
C = capacitance in farads
L = inductance in henries

Peak values of V and I are stated in order to emphasize not to use RMS values. The energy stored at any given time is of course: J(t) = 0.5 [V(t)]² C and J(t) = 0.5 [I(t)]² L.