Ohm's Law
This free Ohm's law calculator solves for voltage, current, or resistance from any two of the three, and always shows power alongside the result. Nothing you enter is sent anywhere or stored.
How to Use It
- Enter any two of voltage, current, and resistance, and leave the third blank.
- The missing value and power update instantly, no submit button.
- Change any filled field at any time to recalculate.
How It Works
Voltage, current, and resistance describe an electrical circuit the way pressure, flow rate, and pipe narrowness describe water moving through a pipe. Voltage is the electrical pressure pushing charge through a circuit, current is how much charge flows per second, and resistance is how much the circuit opposes that flow. Ohm's law ties all three together in one equation, so knowing any two always determines the third.
Formulas
voltage = current × resistance
current = voltage ÷ resistance
resistance = voltage ÷ current
power = voltage × current
A worked example: a 12 volt supply pushing 2 amps through a circuit means the circuit has 6 ohms of resistance (12 ÷ 2), and is dissipating 24 watts of power (12 × 2). Change any two of the three values and the third, along with power, updates to match.
Everything runs client-side in your browser. Nothing you enter is sent anywhere or stored.
Frequently Asked Questions
What does Ohm's law state?
Ohm's law states that voltage equals current multiplied by resistance (V = I × R). Since it relates three quantities with one equation, knowing any two of them is enough to solve for the third: rearrange the same equation to isolate whichever value you're missing.
What is power (P) and how does it relate to V, I, and R?
Power is the rate energy is used or delivered, measured in watts. Once voltage and current are both known, either entered directly or solved from the other two values, power is simply voltage multiplied by current (P = V × I).
Why can resistance not be zero or negative in this tool?
A real circuit component always has some positive resistance. Zero resistance would mean an ideal short circuit with infinite current, and negative resistance doesn't occur in ordinary passive components, so both are treated as invalid inputs here rather than producing a division by zero or a nonsensical result.
Can you give a real world example?
Sizing a current limiting resistor for an LED is a common one. If a 9 volt supply needs to be dropped to a diode that wants 20 milliamps (0.02 amps), entering 9 volts and 0.02 amps gives a resistance of 450 ohms, the value you would shop for.