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How It Works
Ohm's Law (V = I × R) and the power equation (P = V × I) together produce 12 formulas that let you find any one of four electrical quantities — voltage, current, resistance, or power — from any two of the others. Select what you want to solve for, choose which two values you know, and the calculator does the rest.
Example Problem
A 120 V circuit powers a 60 W light bulb. What is the current and resistance?
- Current: I = P / V = 60 / 120 = 0.5 A
- Resistance: R = V / I = 120 / 0.5 = 240 Ω
You can verify: P = I² × R = 0.25 × 240 = 60 W.
Ohm's Law Wheel
The wheel organizes all 12 equations. Each quadrant solves for one variable using any combination of the other two.
Ohm's
Law
Law
VI×RP/I√(P×R)
IV/RP/V√(P/R)
RV/IP/I²V²/P
PV×II²×RV²/R
Each quadrant shows three ways to calculate that variable from any two of the other three.
Frequently Asked Questions
What is Ohm's Law in simple terms?
Ohm's Law says that voltage equals current times resistance (V = I × R). If you push more voltage through a circuit, more current flows. If you increase resistance, less current flows for the same voltage.
How do I calculate power from voltage and resistance?
Use P = V²/R. For example, a 240 V heater with 20 Ω resistance draws 240²/20 = 2,880 W of power.
Does Ohm's Law apply to AC circuits?
Yes, but in AC circuits you replace resistance with impedance (Z), which includes both resistance and reactance. For purely resistive loads like heaters, V = I × R works directly.
What is the relationship between watts and amps?
Watts = Volts × Amps. A 1,500 W space heater on a 120 V circuit draws 1,500/120 = 12.5 A. This is why 15-amp circuits can barely handle one large heater.
Related Calculators
- AC Circuit Calculator — inductive and capacitive reactance.
- Capacitor Design Calculator — capacitance, charge, voltage, and stored energy.
- Inductor Design Calculator — solenoid inductance from coil geometry.
- Power Calculator — calculate electrical power from voltage and current.
- Resistance Unit Converter — convert between ohms, kilohms, and megohms.
Reference: Ohm, Georg Simon. 1827. Die galvanische Kette, mathematisch bearbeitet. Berlin: T. H. Riemann. See also Halliday, Resnick & Walker, Fundamentals of Physics.