Below is a reference chart giving all the equations deriving from Ohm's Law. The parameters E, I, R and P are shown in the central area, each occupying one of the four quadrants of the pie. To solve for a given parameter, find that parameter at the center of the chart and choose the equation in its quadrant that defines the quantity in terms that you have measured or know.

• I = Electrical current in amperes
• E = Electrical potential in volts
• R = Resistance in ohms (Also sometimes "Z" or Impedance in ohms)
• P = Power in watts (Also sometimes "W")

NOTE: Equations referring to power and impedance are describing a Power Factor (PF), rather than pure DC power. This quantity accounts for the reactance of the load and the AC signal.

Ohms Law (DC Current):

Current in amps = Voltage in volts / Resistance in ohms = Power in watts / Voltage in volts

Current in amps = (Power in watts / Resistance in ohms)

Voltage in volts = Current in amps × Resistance in ohms

Voltage in volts = Power in watts / Current in amps

Voltage in volts = (Power in watts × Resistance in ohms) Power in watts = (Current in amps)² × Resistance in ohms

Power in watts = Voltage in volts × Current in amps

Power in watts = (Voltage in volts)² / Resistance in Ohms

Resistance in ohms = Voltage in volts / Current in amps

Resistance in ohms = Power in watts / (Current in amps)²

Ohms Law (AC Current):

In the following AC Ohms Law formulas, q is the phase angle in degrees by which current lags voltage (in an inductive circuit) or by which current leads voltage (in a capacitive circuit). In a resonant circuit (such as normal household 120VAC) the phase angle is 0º and Impedance = Resistance.

Current in amps = Voltage in volts / impedance in ohms

Current in amps = (Power in Watts / Impedance in ohms × cosq)

Current in amps = Power in Watts / (Voltage in volts × cos q)

Voltage in volts = Current in amps × Impedance in ohms

Voltage in volts = Power in Watts / (current in amps × cos q)

Voltage in volts = ([Power in watts × Impedance in ohms] / cos q)

Impedance in ohms = Voltage in volts / Current in amps

Impedance in ohms = Power in watts / (Current amps² × cos q)

Impedance in ohms =(Voltage in volts² × cos q) / Power in watts

Power in watts = Current in amps² × Impedance in ohms × cos q

Power in watts = Current in amps × Voltage in volts × cos q

Power in watts = ([Voltage in volts]² × cos q) / Impedance in ohms

ELECTRONIC CIRCUIT EQUATIONS: 

Resonant frequency in hertz (where X_L= X_C) = 1 / (2p ×[Inductance in henrys × Capacitance in farads])

Reactance in ohms of an inductance is X_L
X_L = 2p(frequency in hertz × Inductance in henrys)

Reactance in ohms of a capacitance is X_C
X_C = 1 / (2p[frequency in hertz × Capacitance in farads] )

Impedance in ohms (series) = (Resistance in ohms² + (X_L-X_C)²)

Impedance in ohms (parallel) = (Resistance in ohms × Reactance) / (Resistance in ohms² + Reactance²

Resistors in Series (values in Ohms):

Total Resistance = Resistance₁ + Resistance₂ + ... Resistance_n

Two Resistors in Parallel (values in Ohms):

Total Resistance = Resistance₁ × Resistance₂ / Resistance₁ + Resistance₂

Multiple Resistors in Parallel (values in Ohms):

Total Resistance = 1 / (1 / Resistance₁] + 1 / Resistance₂ + ... 1 / Resistance_n])

Capacitors in Parallel (values in microfarads):

Total Capacitance in Parallel (values in any farad) = Capacitance₁ + Capacitance₂ + .... Capacitance_n

Capacitors in Series (values in microfarads):

Total Capacitance in Series (values in any farad) = Capacitance₁ × Capacitance₂ / Capacitance₁ + Capacitance₂

Multiple Capacitors in Series (values in farads) = 1 / ([1 / Capacitance₁] + [1 / Capacitance₂] + ...... [1 / Capacitance_n])

LCR Series Time Circuits:
Time in seconds = Inductance in henrys / Resistance in ohms
Time in seconds = Capacitance in farads × Resistance in ohms 

SINE WAVE VOLTAGE AND CURRENT:

Effective (RMS) value = 0.707 × Peak value

Effective (RMS) value = 1.11 × Average value

Average value = 0.637 × Peak value

Average value = 0.9 × Effective (RMS) value

Peak Value = 1.414 × Effective (RMS) value

Peak Value = 1.57 × Average value

DECIBELS:

db = 10 Log₁₀ (Power in Watts #1 / Power in Watts #2)

db = 10 Log₁₀ (Power Ratio)

db = 20 Log₁₀ (Volts or Amps #1 / Volts or Amps #2)

db = 20 Log₁₀ (Voltage or Current Ratio)

Power Ratio = 10^(db/10)


Voltage or Current Ratio = 10^(db/20) If impedances are not equal: db = 20 Log₁₀ [(Volt₁ [Z₂]) / (Volt₂ [Z₁])]

Frequency and Wavelength

Frequency in kilohertz = (300,000) /wavelength in meters

Frequency in megahertz = (300) / wavelength in meters

Frequency in megahertz = (964) / wavelength in feet

Wavelength in meters = (300,000) / frequency in kilohertz

Wavelength in meters = (300) / frequency in megahertz

Wavelength in feet = (964) / frequency in megahertz

Wavelength = speed of sound (ft/sec or m/sec) / frequency Speed of sound = 1130 ft/sec

Antenna Length:

Quarter-wave antenna: (ordinary wire, velocity factor = 0.95)
Length in feet = 234 / frequency in megahertz

Half-wave antenna: (ordinary wire, velocity factor = 0.95)
Length in feet = 466 / frequency in megahertz

70 Volt Loudspeaker Matching Transformer Primary Impedance = (Amplifier output volts)² / Speaker Power

WIRE GAUGE DIAMETERS
A.W.G. Diameter (mm) 000 10.41 00 9.27 0 8.26 2 6.54 4 5.19	A.W.G. Diameter (mm) 6 4.12 8 3.26 10 2.59 12 2.05 14 1.63	A.W.G. Diameter (mm) 16 1.29 18 1.02 20 .81 22 .64 24 .51