Almighty Calculator

Ohm's Law Calculator

Ohm's law says voltage = current × resistance, and power = voltage × current; give the calculator any two and it finds the other two.

I know
Only the two values you choose are used; the rest are ignored.
V
A
Ω
W
Power
24W
Voltage12 V
Current2 A
Resistance6 Ω

Show the math

R = V ÷ I = 12 V ÷ 2 A = 6 Ω
P = V × I = 12 V × 2 A = 24 W

Rounded the same way as the result above.

How it works

Ohm's law relates the three basic electrical quantities: voltage (V, volts), current (I, amps), and resistance (R, ohms). Joule's law adds power (P, watts): the rate at which electrical energy is converted to heat, light, or motion.

Any two of the four values fix the other two. The calculator picks the right rearrangement of V = I × R and P = V × I for the pair you choose, so you never have to rearrange them yourself.

V = I × R P = V × I = I² × R = V² ÷ R

Worked example

A 12 V source across a 6 Ω resistor:

  1. I = V ÷ R = 12 V ÷ 6 Ω = 2 A
  2. P = V² ÷ R = (12 V)² ÷ 6 Ω = 24 W
InputValue
I knowVolts and ohms
Voltage12 V
Current2 A
Resistance6 Ω
Power24 W
ResultValue
Voltage12 V
Current2 A
Resistance6 Ω
Power24 W

Assumptions and limits

  • Applies to DC circuits and to AC circuits with purely resistive loads such as heaters and incandescent lamps.
  • For AC motors, transformers, and other reactive loads, real power also depends on power factor, so P = V × I overstates real watts.
  • Resistance is treated as constant; real heaters and lamp filaments change resistance with temperature.

Common questions

How do I convert watts to amps?

Divide watts by volts (I = P ÷ V). A 1,200 W heater on 120 V draws 10 A. For AC motors, divide by volts times power factor as well.

What is the difference between volts, amps, and watts?

Volts are electrical pressure, amps are the flow, and watts are the power that pressure and flow together deliver: watts = volts × amps.

Why are there three formulas for power?

They are the same relationship rearranged with V = I × R, so use whichever pair of values you already have.

Does this work for AC?

For resistive loads, yes, using RMS volts and amps. Reactive loads need power factor; see the power factor and reactance calculators.

Sources

  • Ohm's law (V = IR) and Joule's law (P = VI) as given in standard circuit texts.
  • Independent checks use textbook values such as 12 V across 6 Ω giving 2 A and 24 W.

Updated 2026-09-30