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.

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