Ohms Law Calculator
Enter any two of voltage, current, resistance, or power and this calculator solves for the other two using Ohm's Law.
Example
With 12 volts across a 4 Ω resistor, current is I = V/R = 12/4 = 3 A and power is P = V×I = 12×3 = 36 W.
How it works
Ohm's Law states V = I x R, and electrical power is P = V x I. From any two known quantities the other two follow via the derived forms P = I squared x R and P = V squared / R, so the tool picks the right pair of equations for whatever two fields you fill in.
Good to know
The Ohms Law Calculator solves for voltage, current, resistance, and power when you know any two of those four quantities. It is built for hobbyist electronics, students working through circuit problems, makers wiring LEDs or resistors, and technicians sizing components or sanity-checking a measurement. You type two numbers into the four fields, leave the other two blank, and the remaining values appear instantly without any button press.
Reach for it whenever you have a partial picture of a simple DC circuit. Common cases include finding the series resistor an LED needs from supply voltage and target current, checking whether a resistor's wattage rating is high enough for the power it will dissipate, or working backward from a measured voltage and current to confirm a load's effective resistance. Because the math is exact and immediate, it is also handy as a quick double-check on hand calculations.
Read the result as four matched outputs labeled Voltage (V), Current (A), Resistance (Ω), and Power (W); the two you entered are echoed back and the two derived ones are filled in. Very large or very small results switch to scientific notation, and a field shows a dash when the answer is undefined or physically impossible for your inputs (for example, dividing by a zero current, or a power-and-resistance pair that would require a negative square root).
One caveat worth keeping in mind: Ohm's Law in this form assumes a steady DC circuit and a linear, purely resistive load. It does not account for AC reactance, power factor, temperature-dependent resistance, or non-ohmic devices like diodes and transistors, so treat the output as an idealized estimate rather than a guarantee for real components under load.
Frequently asked questions
Which two values should I enter?
Any two of the four (voltage, current, resistance, power). The calculator picks the correct equations and fills in the remaining two automatically.
Why do I get a dash for resistance when current is zero?
Resistance is V divided by I, and dividing by zero is undefined. Enter a non-zero current (or use a different pair of known values) to get a result.
Is my data uploaded anywhere?
No — this calculator runs entirely in your browser. Your inputs never leave your device, and it works offline once loaded.
Is this calculator free?
Yes, completely free with no sign-up and no limits.
People also ask
What is Ohm's Law in simple terms?
Ohm's Law states that voltage equals current times resistance (V = I x R). It means the current through a conductor is proportional to the voltage across it and inversely proportional to its resistance.
How do you calculate power using voltage and current?
Electrical power equals voltage multiplied by current (P = V x I), measured in watts. If you know resistance instead, you can also use P = I squared x R or P = V squared / R.
What is the Ohm's Law triangle?
It is a memory aid showing V on top with I and R below, so covering the value you want gives its formula: V = I x R, I = V / R, and R = V / I. A larger version adds power for the four-quantity wheel.
How do I find the resistor value for an LED?
Subtract the LED's forward voltage from the supply voltage to get the voltage across the resistor, then divide by the desired LED current. For example, (5V - 2V) / 0.02A gives 150 ohms.
What are the units for voltage, current, resistance, and power?
Voltage is measured in volts (V), current in amperes or amps (A), resistance in ohms (Ω), and power in watts (W). Consistent base units are needed for the formulas to give correct results.
Does Ohm's Law work for AC circuits?
The basic V = I x R relationship applies cleanly to DC and purely resistive AC loads. For AC circuits with inductors or capacitors you must use impedance instead of plain resistance and account for phase and power factor.
How do I rearrange Ohm's Law to solve for current or resistance?
Starting from V = I x R, current is I = V / R and resistance is R = V / I. The calculator applies these rearrangements automatically based on which two values you provide.
What happens if I divide by zero in an Ohm's Law calculation?
Dividing by zero is mathematically undefined, so a zero current or zero resistance in a division makes the result impossible to compute. The calculator shows a dash for that value instead of a number.
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