Ohm's Law Calculator
Ohm's Law calculator solves for any two of the four fundamental electrical quantities — voltage (V), current (I), resistance (R), and power (P) — when you provide the other two. Named after German physicist Georg Ohm, the law states that voltage equals current times resistance. This calculator is useful for electronics students, electricians, and anyone working with circuits, batteries, or electrical components.
How to Use
- Enter any two of the four electrical values: Voltage, Current, Resistance, or Power.
- Leave the remaining two fields blank. Those blank quantities are the unknowns the calculator will solve.
- Click Calculate to find the two unknown values using Ohm's Law and the power formula.
- The result cards show all four values, separating the values you entered from the two that were calculated.
- For example, enter Voltage = 12 V and Current = 2 A to get Resistance = 6 Ω and Power = 24 W.
Ohm's Law Formula
V = I x R
I = V / R
R = V / I
P = V x I = I^2 x R = V^2 / R
Ohm's Law is written as V = I x R. V is voltage in volts, I is current in amperes, and R is resistance in ohms. The same relationship can be rearranged to find current with I = V / R or resistance with R = V / I. Electrical power is connected through P = V x I, which also becomes P = I^2 x R and P = V^2 / R. These four formulas form a complete set: any two values determine the other two in a simple ohmic circuit.
In practical terms, higher resistance means lower current for the same voltage. Doubling the voltage doubles the current if resistance is constant. This linear relationship is what defines an ohmic conductor; most metal wires and resistors behave this way when temperature is stable. Non-ohmic devices such as LEDs, diodes, lamps warming up, and transistors do not keep the same voltage-current ratio across all operating points, so their data sheets and safe operating limits matter.
Worked Example
A 12 V car battery is connected to a headlight with resistance 6 Ω. Given V = 12 V and R = 6 Ω, current is I = V / R = 12 / 6 = 2 A. Power is P = V x I = 12 x 2 = 24 W. This means 24 watts of electrical power are consumed by the headlight.
In a second scenario, a 60 W household bulb on 230 V draws I = P / V = 60 / 230 = 0.26 A. The equivalent hot resistance is R = V / I = 230 / 0.26 = 884.6 Ω. A 100 W bulb draws more current: I = 100 / 230 = 0.43 A. Real bulb filaments are temperature-dependent, so their cold resistance and operating resistance can differ substantially.
Practical Circuit Notes
Ohm's Law is most reliable when the part being measured behaves like a resistor. A fixed resistor, a length of copper wire, or a heating element near a stable temperature usually fits the model well. Batteries, motors, speakers, LEDs, solar panels, and semiconductor circuits need extra context because their voltage, current, and resistance are not always linked by one constant value. For example, a motor may draw a large inrush current when it starts because it is not yet spinning and has little back electromotive force. A diode may pass almost no current below its forward voltage and then increase current rapidly once it turns on.
Power is the safety check many beginners miss. A resistor can have the correct resistance but still fail if the wattage rating is too low. If a 100 Ω resistor has 10 V across it, current is 0.1 A and power is 1 W. A common quarter-watt resistor would overheat in that circuit. Choose a part rated above the expected power and leave margin for ventilation, ambient temperature, and manufacturing tolerance. When designing LED circuits, use the supply voltage, LED forward voltage, and target current to calculate the series resistor, then confirm the resistor power rating before building the circuit.
Common Electrical Values and Applications
| Device | Voltage | Current | Resistance | Power |
|---|---|---|---|---|
| LED | 2-3.3 V | 20 mA | 100-150 Ω | 0.04-0.066 W |
| Phone charger | 5-20 V | 1-3 A | Varies | 5-60 W |
| Laptop adapter | 19 V | 3.5 A | About 5.4 Ω | About 65 W |
| Ceiling fan | 230 V | 0.4 A | 575 Ω | 75-100 W |
| Hair dryer | 230 V | 8.7 A | 26.4 Ω | 2000 W |
| Car battery crank | 12 V | 100-600 A | Load dependent | High surge |
| Solar panel | 17-21 V | 5-8 A | Operating point changes | 100-200 W |
| AA battery | 1.5 V | 1-2 A max | Internal resistance varies | Short bursts |
FAQ
What is Ohm's Law and who discovered it?
Ohm's Law is the relationship between voltage, current, and resistance in an ohmic conductor: voltage equals current multiplied by resistance. It is named after Georg Simon Ohm, the German physicist who published the relationship in the nineteenth century after studying how electric current changed through different conductors. The law became one of the foundations of circuit analysis because it lets you calculate one electrical quantity from the other two. It applies best to components with a nearly constant resistance over the operating range, such as fixed resistors and many metal conductors.
What is the difference between voltage, current, and resistance?
Voltage is electrical potential difference, often described as the push that drives charge through a circuit. Current is the rate of flow of electric charge, measured in amperes. Resistance is opposition to that flow, measured in ohms. A simple water analogy is pressure, flow, and pipe restriction: higher pressure can drive more flow, while a narrow pipe reduces flow. The analogy is imperfect, but it helps beginners see why increasing voltage raises current when resistance is fixed, and why increasing resistance lowers current when voltage is fixed.
What is electrical power and how is it measured in watts?
Electrical power is the rate at which electrical energy is converted into heat, light, motion, sound, or another form of energy. It is measured in watts. In direct current circuits, power is voltage multiplied by current, so a device drawing 2 A from a 12 V supply consumes 24 W. Power matters because it determines battery drain, heat generation, appliance running cost, and component safety. A resistor, wire, switch, relay, or power supply may fail if the wattage or current rating is exceeded even when the voltage looks acceptable.
Why do some devices not follow Ohm's Law?
Some devices are non-ohmic because their current is not proportional to voltage. LEDs, diodes, transistors, lamps, motors, batteries, and many sensors change behavior with temperature, operating point, chemistry, or semiconductor junction effects. An LED, for example, does not act like a fixed resistor; below its forward voltage it passes little current, and above that point current can rise sharply. A filament bulb has low resistance when cold and higher resistance when hot. For these devices, Ohm's Law may still help analyze a small part of the circuit, but the device data sheet and ratings are essential.
How do I use Ohm's Law to choose the right resistor for an LED?
Start with the supply voltage, subtract the LED forward voltage, and divide the remaining voltage by the desired LED current. For a 5 V supply, a red LED with about 2 V forward voltage, and a target current of 20 mA, the resistor is (5 - 2) / 0.02 = 150 Ω. Then calculate resistor power: P = I^2 x R = 0.02^2 x 150 = 0.06 W. A common quarter-watt resistor has enough margin. If the supply voltage or LED current changes, repeat the calculation instead of reusing the same resistor blindly.