Ohm's Law Calculator
Enter any two of voltage, current, resistance and power to get the other two
Any Two Values
e.g. 230 V, or the drop across a component
e.g. 10 A
e.g. an element's or a conductor's resistance
e.g. a 2400 W heater
Result
Enter any two of voltage, current, resistance and power to work out the other two.
How It's Calculated
Two equations link the four quantities:
- Ohm's law: V = I × R
- Power: P = V × I
Substituting one into the other gives a formula for each quantity from any pair of the others:
- V = I × R = P / I = √P × R
- I = V / R = P / V = √P / R
- R = V / I = V² / P = P / I²
- P = V × I = V² / R = I² × R
Worked Example
A resistive load draws 10 A from a 230 V supply:
- R = 230 / 10 = 23 Ω
- P = 230 × 10 = 2,300 W
- A 2400 W heater on 230 V: I = 2400 / 230 = 10.43 A, R = 230² / 2400 = 22.04 Ω
Frequently Asked Questions
Ohm's Law Questions Answered
What is Ohm's law?
The current through a conductor is proportional to the voltage across it: V = I × R, where V is in volts, I in amps and R in ohms. Combined with the power equation P = V × I, any two of the four quantities give the other two.
How do I calculate current from voltage and resistance?
Divide the voltage by the resistance: I = V / R. 230 V across a 23 Ω element drives 10 A.
How do I calculate resistance from power and voltage?
Square the voltage and divide by the power: R = V² / P. A 2400 W heater rated at 230 V has a hot resistance of about 22 Ω and draws about 10.4 A.
What are the twelve Ohm's law formulas?
V = I × R = P / I = √(P × R); I = V / R = P / V = √(P / R); R = V / I = V² / P = P / I²; P = V × I = V² / R = I² × R. The calculator picks the right one for the two values you enter.
Does Ohm's law apply to AC circuits?
Exactly, for resistive loads like heaters and incandescent lamps. For motors, transformers and electronics, replace the resistance with the impedance Z and include the power factor in the power: P = V × I × cos φ.
Why does a heater draw more current when it is cold?
The resistance of most metals rises with temperature, so a cold element has a lower resistance and draws more current for the moment it takes to warm up. Calculations from the rated power give the hot, running values.
Reviewed by Ahmed Tayeh, electrical engineer at CableHero. The calculator is a design aid: the designer remains responsible for checking the installation against the standards.
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Put It to Work on a Cable
The voltage drop calculator applies the same law to a real cable run, with the AS/NZS 3008 impedances.