Ohm's Law Calculator
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Results
Resistance 24 Ω and Power 6 W from voltage 12 V and current 500 mA
| Formula | With your numbers | Result |
|---|---|---|
| V = I × R | 500 mA × 24 Ω | 12 V |
| V = P ÷ I | 6 W ÷ 500 mA | 12 V |
| V = √(P × R) | √(6 W × 24 Ω) | 12 V |
| I = V ÷ R | 12 V ÷ 24 Ω | 500 mA |
| I = P ÷ V | 6 W ÷ 12 V | 500 mA |
| I = √(P ÷ R) | √(6 W ÷ 24 Ω) | 500 mA |
| R = V ÷ I | 12 V ÷ 500 mA | 24 Ω |
| R = V² ÷ P | (12 V)² ÷ 6 W | 24 Ω |
| R = P ÷ I² | 6 W ÷ (500 mA)² | 24 Ω |
| P = V × I | 12 V × 500 mA | 6 W |
| P = I² × R | (500 mA)² × 24 Ω | 6 W |
| P = V² ÷ R | (12 V)² ÷ 24 Ω | 6 W |
Ohm’s law applies to resistive loads at a steady current. For AC motors, lamps when cold, LEDs and other non-linear parts, use measured or rated values.
How to use the Ohm's law calculator
- Choose which two values you know: for example voltage and resistance.
- Enter them and pick the unit for each, such as millivolts, milliamps, kilohms or kilowatts.
- The two unknowns appear in the results with the formula used, and the table below them shows all 12 formulas worked through.
Ohm's law formulas
P = V × I · P = I² × R · P = V² ÷ R
V = voltage (V) · I = current (A) · R = resistance (Ω) · P = power (W)
Ohm's law says the current through a resistor is proportional to the voltage across it: double the voltage and the current doubles. The constant of proportionality is the resistance, measured in ohms. Combining it with the power formula P = V × I gives the full "Ohm's law wheel" of 12 formulas, three for each quantity. Because each one can be derived from the others, any two known values fix the whole circuit.
| To find | From V and I or R | From P | Third form |
|---|---|---|---|
| Voltage (volts, V) | V = I × R | V = P ÷ I | V = √(P × R) |
| Current (amps, A) | I = V ÷ R | I = P ÷ V | I = √(P ÷ R) |
| Resistance (ohms, Ω) | R = V ÷ I | R = V² ÷ P | R = P ÷ I² |
| Power (watts, W) | P = V × I | P = I² × R | P = V² ÷ R |
Worked examples
A 24 Ω resistor across a 12 V supply carries I = 12 ÷ 24 = 500 mA and turns P = 12² ÷ 24 = 6 W into heat, so it needs a power rating comfortably above 6 W. Working backwards from a rating plate: a 2 kW kettle on 230 V draws 8.696 A, which means its element is about 26.45 Ω when hot. A 60 W bulb on 120 V draws 500 mA and has a hot resistance of 240 Ω.
Current and power in common resistors
Calculated with I = V ÷ R and P = V² ÷ R. Small through-hole resistors are often rated ¼ W (250 mW), so check the power column before choosing one.
| Resistor | Current at 5 V | Power at 5 V | Current at 12 V | Power at 12 V |
|---|---|---|---|---|
| 100 Ω | 50 mA | 250 mW | 120 mA | 1.44 W |
| 220 Ω | 22.73 mA | 113.6 mW | 54.55 mA | 654.5 mW |
| 470 Ω | 10.64 mA | 53.19 mW | 25.53 mA | 306.4 mW |
| 1 kΩ | 5 mA | 25 mW | 12 mA | 144 mW |
| 2.2 kΩ | 2.273 mA | 11.36 mW | 5.455 mA | 65.45 mW |
| 4.7 kΩ | 1.064 mA | 5.319 mW | 2.553 mA | 30.64 mW |
| 10 kΩ | 500 µA | 2.5 mW | 1.2 mA | 14.4 mW |
Ohm's law in practice: an LED resistor
A light-emitting diode needs a resistor in series to limit its current. The resistor takes whatever voltage the LED doesn't, so R = (supply voltage − LED forward voltage) ÷ LED current, with the forward voltage and current taken from the LED's datasheet. For a 5 V supply, an LED with a 2 V forward voltage and a 20 mA target, the resistor sees 3 V: R = 3 ÷ 0.02 = 150 Ω, and it dissipates P = 3 × 0.02 = 60 mW, comfortably inside a ¼ W rating. If the exact value isn't available, choose the next standard value up so the current falls slightly rather than rises.
Tips and common mistakes
- Convert the prefixes. 20 mA is 0.02 A, and 4.7 kΩ is 4,700 Ω. Most mistakes with Ohm's law are a factor of 1,000 out.
- Use the voltage across the part, not the supply voltage, when other components share the circuit. For series and parallel networks, see the resistor calculator.
- Resistance changes with temperature. A filament lamp's cold resistance is far lower than its hot resistance, which is why bulbs often fail at switch-on.
- AC needs more care. For motors and other inductive loads, voltage × current gives apparent power (VA), not real watts.
- Stay safe. Mains voltage can kill. Electrical work must follow your local code and be done by a qualified person where required.
Related: the voltage drop calculator applies Ohm's law to long cable runs, and the electricity cost calculator turns watts into kWh and cost.
Frequently asked questions
What is Ohm's law?
Ohm's law states that voltage equals current times resistance, V = I × R. Rearranged, current is voltage divided by resistance (I = V ÷ R) and resistance is voltage divided by current (R = V ÷ I). A 12 V supply across 24 Ω drives 0.5 A.
How do I calculate power from voltage and current?
Multiply them: P = V × I. With resistance instead, use P = I² × R or P = V² ÷ R. A 24 Ω resistor on 12 V dissipates 12 × 0.5 = 6 W, and 12² ÷ 24 gives the same 6 W.
How many amps does a 2,000 W appliance draw?
Divide the watts by the voltage: I = P ÷ V. A 2,000 W kettle draws about 8.7 A on 230 V and about 16.7 A on 120 V. That is the current the circuit and plug have to carry.
What are the 12 formulas of the Ohm’s law wheel?
Voltage: V = I × R, V = P ÷ I, V = √(P × R). Current: I = V ÷ R, I = P ÷ V, I = √(P ÷ R). Resistance: R = V ÷ I, R = V² ÷ P, R = P ÷ I². Power: P = V × I, P = I² × R, P = V² ÷ R.
How much current flows through a 1 kΩ resistor at 5 V?
I = 5 V ÷ 1,000 Ω = 0.005 A, which is 5 mA. The resistor dissipates 5² ÷ 1,000 = 0.025 W (25 mW), well within a common ¼ W rating.