Power Calculators

Ω Ohm's Law Calculator (V = IR)

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Pick what you want to find. The box for that quantity is ignored, so you can leave any number in it.

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📘 About the Ohm's Law

Ohm's law links voltage, current and resistance in a conductor: V = IR. Choose which of the three you want, enter the other two, and this Ohm's law calculator works it out along with the power used.

It applies to ohmic conductors such as resistors and metal wires at a steady temperature. Components like diodes and filament bulbs do not follow it exactly.

🧮 Formula

Ohm's law
V = I × R
Current
I = V ÷ R
Resistance
R = V ÷ I
Power
P = V I = I²R = V² ÷ R

Where

  • V = potential difference (volts, V)
  • I = current (amperes, A)
  • R = resistance (ohms, Ω)
  • P = power (watts, W)
Use the same SI units throughout: for mA divide by 1,000 to get A; for kΩ multiply by 1,000 to get Ω.

✏️ Worked examples

Current through a resistor

You enter
FindCurrent (I)
Voltage (V)9 V
Resistance (R)450 Ω
You get
Voltage (V)9
Current (A)0.02
Resistance (Ω)450
Power (W)0.18

A 9 V battery across 450 Ω drives 0.02 A (20 mA) and the resistor uses 0.18 W.

Try this example →

Resistance of a heater

You enter
FindResistance (R)
Voltage (V)230 V
Current (I)8.7 A
You get
Voltage (V)230
Current (A)8.7
Resistance (Ω)26.4368
Power (W)2,001

An 8.7 A heater on 230 V has a resistance of 26.4368 Ω and uses 2,001 W.

Try this example →

🧭 How to use this calculator

What you enter

  • FindPick what you want to find. The box for that quantity is ignored, so you can leave any number in it.
  • Voltage (V) (V)
  • Current (I) (A)
  • Resistance (R) (Ω)

What you get

  • Voltage (V)
  • Current (A)
  • Resistance (Ω)
  • Power (W)

💡 The VIR triangle

Put V at the top of a triangle and I and R at the bottom. Cover the quantity you want: V = I × R, I = V ÷ R, R = V ÷ I.

❓ Frequently asked questions

What is Ohm's law?

At constant temperature, the current through a conductor is proportional to the voltage across it: V = IR.

What is the unit of resistance?

The ohm (Ω). 1 Ω lets a current of 1 A flow when 1 V is applied.

How do I calculate power from voltage and current?

P = V × I. A 230 V appliance drawing 2 A uses 460 W.

Does Ohm's law apply to a bulb?

Only roughly: a filament's resistance rises as it heats up, so its V–I graph is curved.