Common Electronic & Electricity Formulas

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 × cos q)
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 XL= XC) = 1 / (2p[Inductance in henrys × Capacitance in farads])

Reactance in ohms of an inductance is XL

XL = 2p(frequency in hertz × Inductance in henrys)

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

Impedance in ohms (series) = Resistance in ohms² + (XL-XC)²

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

Resistors in Series (values in Ohms)

Total Resistance = Resistance1 + Resistance2 + … ResistanceN

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 / ResistanceN])

Capacitors in Parallel (values in microfarads)

Total Capacitance = Capacitance1 + Capacitance2 + … CapacitanceN

Capacitors in Series (values in microfarads)

Total Capacitance = Capacitance1 × Capacitance2 / Capacitance1 + Capacitance2

Multiple Capacitors in Series (values in farads)

Total Capacitance = 1 / ([1 / Capacitance1] + [1 / Capacitance2] + … [1 / CapacitanceN])

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

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 = 1130ft/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

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