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Ohm’s Law Calculator

Ohm’s law links voltage, current and resistance; the power law adds watts. Enter any two of the four quantities and the calculator works out the other two, whichever pair you start from.

Quick examples
V
Ω
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Current (I)0.05 A
Voltage (V)
12 V
Resistance (R)
240 Ω
Power (P)
0.6 W

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    Results you calculate here are kept on this device so you can come back to them.

      Formula

      Ohm’s law: V = I × R
      Power: P = V × I = I² × R = V² ÷ R
      From voltage and power: I = P ÷ V, R = V² ÷ P
      From resistance and power: I = √(P ÷ R), V = √(P × R)

      How to use it

      1. Choose which two values you know.
      2. Enter them in volts, amps, ohms or watts.
      3. Read the other two. Convert milliamps to amps first: 20 mA is 0.02 A.

      Worked examples

      12 V across a 240 Ω resistor

      Current (I)
      0.05 A
      Power (P)
      0.6 W

      A 60 W bulb on a 120 V supply

      Current (I)
      0.5 A
      Resistance (R)
      240 Ω

      2 A flowing through 10 Ω

      Voltage (V)
      20 V
      Power (P)
      40 W

      A 100 Ω resistor dissipating 4 W

      Current (I)
      0.2 A
      Voltage (V)
      20 V

      What the letters mean

      Voltage (V, in volts) is the electrical pressure pushing charge around a circuit. Current (I, in amps) is how much charge flows. Resistance (R, in ohms, Ω) is how strongly the circuit opposes that flow. Power (P, in watts) is the rate at which energy is used or turned to heat.

      Household examples

      On a 120 V US circuit, a 1,500 W space heater draws 12.5 A. A standard 15 A circuit can supply 1,800 W, and for loads that run for hours the usual rule is to stay at or under 80% of that, or 1,440 W. At 230 V, the same 1,500 W heater draws only 6.5 A.

      Where it applies

      Ohm’s law is exact for resistors and DC circuits. For AC circuits with motors, transformers or capacitors, real power also depends on power factor, and devices such as LEDs and diodes do not have a fixed resistance. This tool is for learning and estimating, not a substitute for a qualified electrician on mains wiring.

      Questions people ask

      How many amps does a 60-watt bulb draw at 120 volts?

      0.5 A, because 60 ÷ 120 = 0.5. Its resistance when lit is 240 Ω.

      How do I calculate current from voltage and resistance?

      Divide volts by ohms. 12 V across 240 Ω gives 0.05 A, or 50 mA.

      How many watts is 2 amps through 10 ohms?

      40 W: P = I² × R = 4 × 10. The voltage across it is 20 V.

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