Ohm's law calculator
Ohm's law links voltage, current and resistance: volts = amps × ohms, and power is watts = volts × amps. A load drawing 10 A on 120 V has a resistance of 12 Ω and uses 1,200 W. Enter any two values to get the other two.
Enter any two values. The two you changed last are used; the other two are calculated.
- Voltage
- 120 V
- Current
- 10 A
- Resistance
- 12 Ω
- Power
- 1,200 W
How it is calculated
- R = V ÷ I, P = V × I
How to calculate with Ohm's law
Ohm's law says the current through a resistance is the voltage divided by the resistance; with power, any two of volts (V), amps (I), ohms (R) and watts (P) give the other two. That makes twelve formulas, three for each unknown. The worked numbers use the calculator's default: 120 V and 10 A, which is 12 Ω and 1,200 W.
Step 1: voltage
V = I × R
V = P ÷ I
V = √(P × R)
Example 10 A × 12 Ω = 120 V
Example 1,200 W ÷ 10 A = 120 V
Example √(1,200 W × 12 Ω) = 120 V
Step 2: current
I = V ÷ R
I = P ÷ V
I = √(P ÷ R)
Example 120 V ÷ 12 Ω = 10 A
Example 1,200 W ÷ 120 V = 10 A
Example √(1,200 W ÷ 12 Ω) = 10 A
Step 3: resistance
R = V ÷ I
R = V² ÷ P
R = P ÷ I²
Example 120 V ÷ 10 A = 12 Ω
Example 120² ÷ 1,200 W = 12 Ω
Example 1,200 W ÷ 10² = 12 Ω
A resistance worked out this way is the resistance of the load while it runs, at its working temperature.
Step 4: power
P = V × I
P = I² × R
P = V² ÷ R
Example 120 V × 10 A = 1,200 W
Example 10² × 12 Ω = 1,200 W
Example 120² ÷ 12 Ω = 1,200 W
P = V × I is exact for DC and for resistive AC loads. Motors and electronics on AC also have a power factor, so their watts are lower than volts × amps.
Current and power by resistance at 120 V and 240 V
A 12 Ω load draws 10 A and uses 1,200 W at 120 V; at 240 V the same resistance draws 20 A and uses 4,800 W, because power grows with the square of the voltage.
| Resistance, Ω | Amps at 120 V | Watts at 120 V | Amps at 240 V | Watts at 240 V |
|---|---|---|---|---|
| 1 | 120 | 14,400 | 240 | 57,600 |
| 2 | 60 | 7,200 | 120 | 28,800 |
| 5 | 24 | 2,880 | 48 | 11,520 |
| 10 | 12 | 1,440 | 24 | 5,760 |
| 12 | 10 | 1,200 | 20 | 4,800 |
| 20 | 6 | 720 | 12 | 2,880 |
| 50 | 2.4 | 288 | 4.8 | 1,152 |
| 100 | 1.2 | 144 | 2.4 | 576 |
- 120 V
- 240 V
Why a measured value can differ
Ohm's law is exact; the difference comes from the numbers put into it. The resistance of a heating element, a lamp filament or a wire changes with its temperature, so the ohms read with a meter on a cold, disconnected element differ from volts ÷ amps while it runs. Measure the volts and amps under load, or use the ratings on the nameplate, to work out the resistance at working temperature.
The voltage at an outlet is rarely exactly the nominal 120 or 240 V, and it can dip when a large load switches on, so the current and power of the same load move with it. Long or thin wires add their own resistance in series with the load: that is the voltage drop the wire size calculator checks.
On AC, motors, transformers and electronics have a power factor, and their current is not set by resistance alone. For those loads use the watts, volts and power factor in the watts to amps calculator, or the amps on the nameplate.