Physics calculator
Ohm's Law Calculator
Calculate voltage, current, resistance, or electrical power from the values you know.
Calculated current
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Enter two or more compatible values to calculate.
What is Ohm's Law?
Ohm's Law describes the relationship between voltage, current, and resistance in an electrical circuit. It helps you find any one of these values when the other two are known.
Voltage is measured in volts (V), current in amperes (A), and resistance in ohms (Ω).
Ohm's Law formulas
Rearrange the main equation to solve for each quantity:
Voltage, current, and resistance
Voltage is the electrical potential difference that pushes charge through a circuit. Current is the rate at which charge flows. Resistance opposes that flow. With resistance held constant, increasing voltage increases current; with voltage held constant, increasing resistance decreases current.
Electrical power formulas
Power measures how quickly electrical energy is transferred. Power is measured in watts (W), and it can be calculated using voltage, current, and resistance:
Worked examples
To find current from 12 V and 4 Ω:
To find power from 2 A and 10 Ω:
To find resistance from 120 V and 60 W:
Unit conversions
The calculator converts entered values to base SI units before calculating. For example, 4 kΩ is 4,000 Ω, 12 V ÷ 4,000 Ω is 0.003 A, and that is 3 mA. It also converts mV, kV, µA, MΩ, mW, and kW automatically.
Frequently asked questions
What does V = IR mean?
It means voltage equals current multiplied by resistance. The equation is the central form of Ohm's Law.
Can I calculate power with this calculator?
Yes. Choose Power as the unknown and enter any compatible pair, such as voltage and current, current and resistance, or voltage and resistance.
What happens if I enter zero resistance?
Resistance must be greater than zero for these calculations. Zero resistance can make division undefined, so it is treated as invalid input.
Can voltage or current be negative?
Yes. Signed voltage and current can represent direction or polarity. Resistance must be positive and power cannot be negative.
Why is no result shown?
The entered values may be blank, invalid, non-finite, or not a compatible pair for the selected unknown. Enter two compatible known quantities and check that resistance is greater than zero.