Ohm's Law Calculator
Enter any two of voltage, current, resistance, or power. The other two solve out automatically.
Solution
Voltage
—
V
Current
—
A
Resistance
—
Ω
Power
—
W
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The Ohm's law + power formulas
Ohm's law states voltage equals current times resistance. The power formula adds watts to the picture. Combining them gives twelve equivalent formulas — known as the "Ohm's law wheel" — letting you solve any unknown from any two known values.
V = I × R I = V / R R = V / I
P = V × I P = I² × R P = V² / R
Worked examples
- Resistive heater on 120 V drawing 10 A: Resistance R = 120/10 = 12 Ω. Power P = 120 × 10 = 1,200 W (= 1.2 kW).
- 1500 W toaster on 120 V: Current I = 1500/120 = 12.5 A. Resistance R = 120²/1500 = 9.6 Ω.
- 10 Ω resistor, 5 A through it: Voltage drop V = 5 × 10 = 50 V. Power dissipated P = 5² × 10 = 250 W.
AC caveats
For purely resistive AC loads (heaters, incandescent bulbs, electric water heaters), Ohm's law works with RMS values just like it does on DC. For reactive loads (motors, transformers, electronics) you have impedance (Z) instead of pure resistance, and the relationship between current, voltage, and power involves power factor:
Real power (W) = V × I × power factor
Apparent power (VA) = V × I
Three-phase real power (W) = √3 × V_line × I × pf
Frequently asked questions
What is Ohm's Law?
V = I × R. Voltage equals current times resistance.
Watts from volts and amps?
P = V × I. 120 V × 10 A = 1200 W.
Resistance from volts and amps?
R = V / I. 120 V / 10 A = 12 Ω.
Does this work for AC?
For resistive loads, yes — use RMS values. Reactive loads need impedance and power factor.