In short
Ohm's law says voltage equals current times resistance, written V = I x R. Rearranged, current is voltage divided by resistance and resistance is voltage divided by current. The power formula, P = V x I, adds watts. With any two of volts, amps, ohms and watts you can find the other two, which is what the calculator below does.
Fill in any two boxes, leave the rest blank, then press Solve.
The formulas
| To find | Use | Example |
|---|---|---|
| Voltage (V) | V = I x R | 2 A through 60 ohms = 120 V |
| Current (I) | I = V / R | 120 V across 10 ohms = 12 A |
| Resistance (R) | R = V / I | 240 V at 20 A = 12 ohms |
| Power (P) | P = V x I | 120 V at 12 A = 1,440 W |
| Power (P) | P = I x I x R | 10 A through 12 ohms = 1,200 W |
| Current (I) | I = P / V | 1,800 W at 120 V = 15 A |
Worked examples from real work
Will this heater trip the breaker?
A 1,500 watt space heater on a 120 volt circuit draws 1,500 / 120 = 12.5 amps. On a 15 amp circuit that leaves little room for anything else.
What resistance does the element have?
At operating temperature, that heater works out to 120 / 12.5 = 9.6 ohms. Checked cold with a meter it may read a little lower. A reading of infinite ohms means the element is open (broken).
Sizing a load at 240 volts
A 4,800 watt water heater element at 240 volts draws 4,800 / 240 = 20 amps.
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Straight answers
What is Ohm's law in simple terms?
Voltage pushes current through resistance. Double the voltage across the same resistance and you double the current.
What is the Ohm's law triangle?
A memory aid with V on top and I and R on the bottom. Cover the value you want, and the other two show the formula.
Is Ohm's law on apprenticeship tests?
Some electrical aptitude tests and almost all first-year apprenticeship classes use it. The IBEW aptitude test itself focuses on algebra, which is the same skill.