Ohm’s law calculator
Know two values in a resistive circuit? Find the other two. Choose what you’ve measured, enter the numbers, and see how voltage, current, resistance and power fit together.
12 volts, 2 amps, 6 ohms, 24 watts.
12 V = 2 A × 6 Ω
24 W = 12 V × 2 A
Power is the electrical energy used each second. In an ideal resistor, this becomes heat. A calculated wattage is not a recommended resistor rating; check the part’s datasheet and thermal conditions.
Ohm’s law formulas
| Find | Formula | Using power | Alternative |
|---|---|---|---|
| Voltage (V) | I × R | P ÷ I | √(P × R) |
| Current (A) | V ÷ R | P ÷ V | √(P ÷ R) |
| Resistance (Ω) | V ÷ I | V² ÷ P | P ÷ I² |
| Power (W) | V × I | I² × R | V² ÷ R |
A worked example: 12 V across 6 Ω
Divide voltage by resistance to find current: 12 ÷ 6 = 2 A. Multiply voltage by current to find power: 12 × 2 = 24 W. An ideal 6 Ω resistor connected across 12 V would dissipate 24 W as heat.
For a smaller load, 3.3 V across 10 kΩ gives 0.00033 A (0.33 mA) and 0.001089 W (1.089 mW). The unit selectors convert values to base units before the calculation.
Where the formulas come from
Ohm’s law is V = I × R. Combining it with P = V × I gives the other formulas in the table. See OpenStax’s explanation of Ohm’s law and its electric power reference.
What the result means
Does this work for every electrical load?
These relationships describe an ideal resistive load at an operating point. Motors, LEDs, batteries and switching electronics do not behave like a fixed resistor across all conditions. AC loads with reactance need impedance and power factor calculations.
Why can’t I enter zero?
Some zero-value pairs do not determine all four values. For example, zero volts and zero amps do not tell you resistance. This tool accepts positive magnitudes so every supported pair has one solution.
Can I calculate a battery’s short-circuit current?
A simple voltage divided by resistance estimate does not model a battery fault reliably. Internal resistance changes with temperature and state of charge, and connections and protection devices affect current. Do not use this tool to choose fault protection or test a short circuit.
Is resistance the same as wire voltage drop?
Resistance is a property of the circuit path. Voltage drop depends on both resistance and current: V = I × R. The DC wire tool adds conductor size, material and length so you can estimate the resistance of a cable run.