Ohm’s law ties together the three basic quantities of a circuit: voltage = current × resistance (V = I × R), and power is voltage times current (P = V × I). A 1,500 W space heater on a 120 V outlet draws 1,500 ÷ 120 = 12.5 A, and its element has a resistance of 120² ÷ 1,500 = 9.6 Ω. Pick the two values you know and the calculator works out the other two, with the formula used at each step.
The formula wheel
| You know | Find I | Find R or V | Find P |
|---|---|---|---|
| V and I | – | R = V ÷ I | P = V × I |
| V and R | I = V ÷ R | – | P = V² ÷ R |
| I and R | – | V = I × R | P = I² × R |
| P and V | I = P ÷ V | R = V² ÷ P | – |
| P and I | – | V = P ÷ I, R = P ÷ I² | – |
| P and R | I = √(P ÷ R) | V = √(P × R) | – |
The same appliance at 120 V and 230 V
North America uses 120 V outlets; Europe, the UK and much of the world use 230 V. The same power needs about half the current at 230 V, so the same wiring can deliver about twice the watts.
| Power | Current at 120 V | Resistance at 120 V | Current at 230 V | Resistance at 230 V |
|---|---|---|---|---|
| 60 W | 0.5 A | 240 Ω | 0.26 A | 881.7 Ω |
| 500 W | 4.17 A | 28.8 Ω | 2.17 A | 105.8 Ω |
| 1,000 W | 8.33 A | 14.4 Ω | 4.35 A | 52.9 Ω |
| 1,500 W | 12.5 A | 9.6 Ω | 6.52 A | 35.27 Ω |
| 2,000 W | 16.67 A | 7.2 Ω | 8.7 A | 26.45 Ω |
| 3,000 W | 25 A | 4.8 Ω | 13.04 A | 17.63 Ω |
Add up the currents of everything on one circuit and compare with the breaker’s rating: two 1,500 W heaters on one 120 V circuit draw 25 A, more than a typical 15 A or 20 A breaker allows.
Low-voltage examples
An LED with a 5 V supply that needs 20 mA and drops 2 V itself needs a resistor for the remaining 3 V: R = 3 ÷ 0.02 = 150 Ω, which turns 3 × 0.02 = 0.06 W into heat. Choose “Voltage and current” with 3 V and 0.02 A to see it.
Work on mains wiring belongs to a qualified electrician. The calculator is for understanding and planning, and it works offline once saved.
Frequently asked questions
What is Ohm's law?
The current through a resistor equals the voltage across it divided by its resistance: I = V ÷ R, or V = I × R. A 60 Ω resistor on 12 V carries 0.2 A.
How do I work out amps from watts?
Divide the power by the voltage: I = P ÷ V. A 1,500 W heater draws 12.5 A on 120 V and 6.52 A on 230 V.
How do I find the resistance of a heater or kettle?
R = V² ÷ P. A 2,000 W kettle made for 230 V has a hot element of about 26.45 Ω; one made for 120 V, about 7.2 Ω.
What are the power formulas?
P = V × I = I² × R = V² ÷ R. Any two known values give the other two; the calculator picks the right formula and shows it.
Does Ohm's law work for AC?
For resistive loads such as heaters, kettles and filament bulbs, yes, with the RMS voltage (120 V, 230 V). Motors, chargers and LED drivers also have a power factor, so they draw a little more current than P ÷ V suggests.
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