Ohm's Law Calculator
Enter any two of voltage, current, resistance and power to find the other two, with unit prefixes from microamps to kilowatts.
Ohm's law says voltage equals current times resistance, V = I x R, and power is voltage times current, P = V x I. Know any two of voltage, current, resistance and power and you can find the other two. A headlight drawing 2.5 A at 12 V has a resistance of 4.8 ohms and uses 30 W.
This calculator is for learning and for planning circuits with resistors, and it is not a substitute for a component's data sheet. It treats parts as ohmic and uses DC magnitudes, and it does not model AC impedance, power factor or whether a resistor's power rating can handle the result.
Fill in any two values and leave the other two blank.
Resistance and power
4.8 Ω, 30 W
- Voltage (V), given
- 12 V
- Current (I), given
- 2.5 A
- Resistance (R)
- 4.8 Ω
- Power (P)
- 30 W
Shown to 4 significant figures. Valid for a resistor or any part whose resistance stays the same at this voltage.
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About the Ohm's Law Calculator
Ohm's law ties together the basic quantities in a circuit. Voltage is the push, in volts; current is the flow of charge, in amperes; resistance is how strongly a part opposes that flow, in ohms, and one ohm is one volt per ampere. Divide the voltage by the resistance and you get the current. Power, in watts, is the rate at which the part turns electrical energy into heat or light, and it equals the voltage times the current. Combining the two gives the other forms: power is also the voltage squared divided by the resistance, or the current squared times the resistance. That is why any two of the four values fix the other two. For example, 9 V across a 330 ohm resistor drives 27.27 mA and turns 245.5 mW into heat, and 100 W into an 8 ohm load resistor takes 3.536 A at 28.28 V. Fill in two fields, choose the units, and the calculator works out the rest.
Frequently asked questions
What are the Ohm's law formulas?+
V = I x R, I = V / R and R = V / I. For power, P = V x I, P = V squared / R and P = I squared x R. The last two combine power with Ohm's law, so they apply to resistors, while P = V x I applies to any part.
Does Ohm's law work for every component?+
No. It holds for ohmic materials, whose resistance stays the same as the voltage changes, such as ordinary resistors and wire. A lamp filament's resistance rises as it heats, and a diode or an LED barely conducts until it reaches its forward voltage, so the calculator describes them only at one operating point.
How do I pick a resistor for an LED?+
Subtract the LED's forward voltage from the supply voltage, then use that voltage and the LED's current. With a 9 V supply, a 2 V LED and 20 mA, enter 7 V and 20 mA to get 350 ohms, then choose the next standard value up, such as 390 ohms in the common E12 series.
Why do the fields have unit choices?+
Circuits span a wide range, so voltage takes mV, V or kV, current takes µA, mA or A, resistance takes ohms, kilohms or megohms, and power takes mW, W or kW. Results are shown with the prefix that keeps them readable, to 4 significant figures.
Ohm's law, the ohm as one volt per ampere and the 12.0 V, 2.50 A headlight at 4.80 ohms: OpenStax, College Physics 2e, section 20.2. Power as P = IV, V squared / R and I squared R, the last two for resistors, and the same headlight at 30.0 W: section 20.4.
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