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Ohm's Law Calculator

Type in any two of voltage, current, resistance and power. The calculator finds the other two with Ohm's law and the power law, and shows the equation it used.

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How to use the Ohm's law calculator

  1. Enter the voltage if you know it, and pick mV, V or kV.
  2. Enter the current in µA, mA or A.
  3. Enter the resistance in mΩ, Ω, kΩ or MΩ.
  4. Enter the power in mW, W or kW.
  5. Fill in exactly two of the four boxes and leave the other two empty. The answers appear as you type, with the working and the full formula wheel underneath.

How Ohm's law works

Ohm's law says that the current through a resistor is proportional to the voltage across it. Add the definition of electrical power and you can get from any two quantities to the other two:

V = I × R (Ohm's law) P = V × I (power law) V = voltage in volts (V) I = current in amperes (A) R = resistance in ohms (Ω) P = power in watts (W)

These are the SI units. One ohm is one volt per ampere, and one watt is one volt-ampere, or one joule per second. Substituting one equation into the other gives the twelve versions on the formula wheel. For example, P = I² × R comes from replacing V with I × R, and I = √(P ÷ R) is that equation solved for current.

Worked example

The calculator starts with 12 V across a 6 Ω resistor, the current and power left blank:

  • Current: I = V ÷ R = 12 ÷ 6 = 2 A
  • Power: P = V² ÷ R = 144 ÷ 6 = 24 W
  • Check with the power law: P = V × I = 12 × 2 = 24 W
  • Conductance: 1 ÷ 6 Ω = 0.1667 S (166.7 mS)
  • In one hour the resistor turns 24 W × 3,600 s = 86,400 J (0.024 kWh) into heat
  • In US horsepower that is 24 ÷ 745.7 = 0.0322 hp

Now clear the voltage and resistance and type 0.5 A and 60 W, the figures for an old 60-watt bulb. The calculator gives V = 60 ÷ 0.5 = 120 V and R = 60 ÷ 0.5² = 240 Ω.

The formula wheel

To find Formula 1 Formula 2 Formula 3
Voltage V I × R P ÷ I √(P × R)
Current I V ÷ R P ÷ V √(P ÷ R)
Resistance R V ÷ I V² ÷ P P ÷ I²
Power P V × I I² × R V² ÷ R

Common mistakes

  • Mixing prefixes. 20 mA is 0.020 A, not 20 A. A 4.7 kΩ resistor on 12 V passes 2.553 mA, and getting the prefix wrong changes the answer a thousand times over. Choose the prefix in the dropdown next to each box and the calculator converts for you.
  • Forgetting the power rating. A resistor turns P = I² × R into heat. Pick one rated for at least twice that figure: the 24 W in the example would need a large power resistor, not a ¼-watt part.
  • Using it on reactive loads. For DC and for heaters or bulbs on AC, use RMS voltage and current and the results hold. Motors, transformers and many power supplies also have reactance, so the real power is V × I × power factor. A 1,500 W heater on a 120 V circuit draws 12.5 A and has a hot resistance of 9.6 Ω; a 1,500 W motor draws more current than that.

For long cable runs, the wire's own resistance matters too: the voltage drop calculator works that out. To read a resistor's value from its color bands, use the resistor calculator.

Frequently asked questions

What is Ohm's law in simple terms?

Voltage pushes current through a resistance. Double the voltage across the same resistor and the current doubles; double the resistance at the same voltage and the current halves. In symbols, V = I × R.

How do I calculate amps from watts and volts?

Divide the power by the voltage: I = P ÷ V. A 1,200 W microwave on 120 V draws 1,200 ÷ 120 = 10 A. On a 230 V supply the same power needs only 5.2 A.

Why do I need to leave two boxes empty?

The four quantities are linked by two equations, so two known values fix the other two. If you fill in three, they might contradict each other, so the calculator asks you to clear one.

Does Ohm's law apply to LEDs and diodes?

Not directly. An LED's current rises steeply once the voltage passes its forward voltage, so its resistance isn't fixed. You can still use Ohm's law for the series resistor: R = (supply voltage − LED voltage) ÷ the current you want.

Related calculators

  • Voltage Drop Calculator — Voltage drop, percent drop and voltage at the load for copper or aluminum wire, AWG or mm², single- or three-phase.
  • Resistor Color Code Calculator — Read 4-, 5- and 6-band resistor color codes, or find the color bands for any resistance value.
  • Electricity Cost Calculator — Energy use in kWh and running cost per day, month and year for any appliance, with typical wattages.

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