How to use the Ohm's law calculator
- Enter any two of voltage, current, resistance and power, and pick the units.
- The other two values appear immediately, with the formulas used.
- Press Clear to start again with a different pair.
Ohm's law formula chart (12 formulas)
Ohm's law V = I × R together with the power law P = V × I lets you find the other two values from any two. The table lists all 12 forms by the quantity you need.
| Find | Formula 1 | Formula 2 | Formula 3 |
|---|---|---|---|
| Voltage V | V = I × R | V = P ÷ I | V = √(P × R) |
| Current I | I = V ÷ R | I = P ÷ V | I = √(P ÷ R) |
| Resistance R | R = V ÷ I | R = V² ÷ P | R = P ÷ I² |
| Power P | P = V × I | P = I² × R | P = V² ÷ R |
| Voltage | Resistors | Current | Power |
|---|---|---|---|
| 5 V | 220 Ω | 22.7 mA | 114 mW |
| 12 V | 1 kΩ | 12 mA | 144 mW |
| 12 V | 4.7 kΩ | 2.55 mA | 30.6 mW |
| 24 V | 10 Ω | 2.4 A | 57.6 W |
| 230 V | 529 Ω | 435 mA | 100 W |
| 3.3 V | 330 Ω | 10 mA | 33 mW |
Frequently asked questions
What is the Ohm's law formula?
V = I × R: voltage in volts equals current in amps times resistance in ohms. Rearranged: I = V ÷ R and R = V ÷ I. With power P = V × I, any two values give the other two.
Does Ohm's law work for AC?
For purely resistive loads use RMS values directly. AC circuits with inductance or capacitance use impedance Z instead of R and need the power factor; for motor current use the watts to amps calculator with a power factor.
What power rating should the resistor have?
Work out the dissipation with P = I² × R and allow at least double, so 0.12 W calls for a 1/4 W (0.25 W) resistor; allow more in hot or enclosed spaces.
Are my inputs uploaded?
No. Everything is calculated locally in your browser and nothing is stored; a share link only carries the values you entered.
Results are for design estimates and part selection; for production and safety-related circuits follow component datasheets, applicable standards and measurements.