Ohm's Law Calculator
Fill in any two of the four boxes. The other two are worked out at once, with the formula that was used.
Enter exactly two values and leave the other two empty. Clear a box to change which values are used.
Worked examples
Heater element
230 V and 2300 W
10 A and 23 ohms
Current = 2300 / 230 and resistance = 230 squared / 2300.
Torch bulb
12 V across 4 ohms
3 A and 36 W
Current = 12 / 4 and power = 12 x 3.
About Ohm's Law Calculator
Ohm's law is the first rule of electronics. It says the voltage across a resistor equals the current through it multiplied by its resistance, and the power it uses is the voltage multiplied by the current. This calculator lets you enter any two of voltage, current, resistance and power and returns the other two, so you never have to rearrange the formula by hand or remember which version to use.
Each box has its own unit menu, so you can type milliamps, kilohms or kilovolts as they appear on a data sheet or a meter. The answer is shown in the unit you chose for that box, and a line underneath names the exact formula that was applied. It is handy for checking homework, choosing a resistor, working out how much current a heater draws, or deciding whether a small part will run too hot.
How to use Ohm's Law Calculator
- Decide which two values you know, for example the supply voltage and the resistance.
- Type them into their boxes and make sure the other two boxes are empty.
- Choose the unit beside each value if it is not the default, such as milliamps or kilohms.
- Read the two missing values, and the formula shown under the results, then clear a box to try another pair.
Common uses
- Finding the current a 12 volt supply pushes through a 4 ohm load
- Checking the power rating needed for a resistor in a circuit
- Working out the resistance of a heating element from its voltage and wattage
- Homework and exam practice with the V, I and R triangle
Frequently asked questions
What is Ohm's law?
Ohm's law states that the current through a conductor is proportional to the voltage across it, so voltage equals current times resistance (V = I x R). Together with the power formula P = V x I, it lets you find any of the four quantities from two others.
Which two values can I enter?
Any two of voltage, current, resistance and power. The six possible pairs are all handled, and the page tells you which formulas it used. If you fill in three or more boxes it asks you to leave two empty.
Does Ohm's law work for alternating current?
It works for alternating current when the load is purely resistive, such as a kettle or a filament lamp. Motors, transformers and chargers also have reactance, so for them you need impedance and the power factor, not just resistance.
What units does it use?
Volts, amps, ohms and watts, with a menu on each box for prefixes such as milli, kilo and mega. The answer is converted back to the unit you picked for the empty box.
Why is my answer a very small or very large number?
Check the unit menus. Typing 5 into a box set to millivolts instead of volts, or kilohms instead of ohms, changes the result by a factor of a thousand.