- V
- Voltage in volts (V)
- I
- Current in amps (A)
- R
- Resistance in ohms (Ω)
- P
- Power in watts (W)
The 12 Ohm's law formulas
| Formula | Finds | From | Example |
|---|---|---|---|
| V = I × R | voltage (V) | current and resistance | 2 A × 6 Ω = 12 V |
| V = P ÷ I | voltage (V) | current and power | 24 W ÷ 2 A = 12 V |
| V = √(P × R) | voltage (V) | resistance and power | √(24 W × 6 Ω) = 12 V |
| I = V ÷ R | current (A) | voltage and resistance | 12 V ÷ 6 Ω = 2 A |
| I = P ÷ V | current (A) | voltage and power | 24 W ÷ 12 V = 2 A |
| I = √(P ÷ R) | current (A) | resistance and power | √(24 W ÷ 6 Ω) = 2 A |
| R = V ÷ I | resistance (Ω) | voltage and current | 12 V ÷ 2 A = 6 Ω |
| R = V² ÷ P | resistance (Ω) | voltage and power | (12 V)² ÷ 24 W = 6 Ω |
| R = P ÷ I² | resistance (Ω) | current and power | 24 W ÷ (2 A)² = 6 Ω |
| P = V × I | power (W) | voltage and current | 12 V × 2 A = 24 W |
| P = I² × R | power (W) | current and resistance | (2 A)² × 6 Ω = 24 W |
| P = V² ÷ R | power (W) | voltage and resistance | (12 V)² ÷ 6 Ω = 24 W |
Every example uses the same circuit: 12 V across 6 Ω draws 2 A and uses 24 W. Each formula gives the same answers from a different pair of values.
Heating element resistance
| Element | Resistance at 230 V (Ω) | Current at 230 V (A) | Resistance at 120 V (Ω) | Current at 120 V (A) |
|---|---|---|---|---|
| 250 W | 211.6 | 1.087 | 57.6 | 2.083 |
| 500 W | 105.8 | 2.174 | 28.8 | 4.167 |
| 750 W | 70.53 | 3.261 | 19.2 | 6.25 |
| 1,000 W | 52.9 | 4.348 | 14.4 | 8.333 |
| 1,200 W | 44.08 | 5.217 | 12 | 10 |
| 1,500 W | 35.27 | 6.522 | 9.6 | 12.5 |
| 2,000 W | 26.45 | 8.696 | 7.2 | 16.67 |
| 2,500 W | 21.16 | 10.87 | 5.76 | 20.83 |
| 2,750 W | 19.24 | 11.96 | 5.236 | 22.92 |
| 3,000 W | 17.63 | 13.04 | 4.8 | 25 |
To test an element, switch off, disconnect it and measure across its two terminals with a multimeter. A working element reads close to the figure for its rating, often a little lower when cold. No reading (OL, open circuit) means it has failed.
About this chart
Voltage, current, resistance and power are linked, so any two give you the other two. The wheel shows all 12 ways: find what you want in the middle, then use the formula next to it that needs the two values you know. Each one is worked through below with the same example, 12 V across 6 Ω.
Choose Heating elements above the chart for the resistance a working element should measure, to check one with a multimeter. For your own values, use the Ohm's law calculator.
How it's worked out:
- Ohm's law: V = I × R. Rearranged: I = V ÷ R and R = V ÷ I
- Power: P = V × I. Putting Ohm's law into it gives P = I² × R and P = V² ÷ R, and the rest of the wheel
- Heating elements: R = V² ÷ P and I = P ÷ V, for the element's rated power at its rated voltage
Values are to 4 significant figures, like the calculator. These hold exactly for heaters, kettles and other resistive loads; motors and electronics behave differently on AC.
Last checked 31 October 2026 · Last rev. 31 October 2026 · 31c5