Ohm's Law Calculator
Solve for voltage, current, resistance, or power from any two known values, with the exact formula used and the full Ohm's law wheel explained.
How to use this calculator
- 1Choose which two quantities you already know.
- 2Enter their values in volts, amps, ohms, or watts.
- 3The other two are calculated, along with the exact formulas used.
How it works
Ohm's law and the power wheel
V = I × R I = V ÷ R R = V ÷ I P = V × I P = I² × R P = V² ÷ R V = P ÷ I V = √(P × R) I = P ÷ V I = √(P ÷ R) R = V² ÷ P R = P ÷ I²
Ohm's law says current through a resistor is proportional to the voltage across it, with resistance as the constant. Combining that with the definition of power as voltage times current gives twelve formulas in total — any two of the four quantities determine the other two. The relationships that square a term are the important ones in practice: doubling the current through a resistor quadruples the heat it dissipates.
Worked example
A 120 V circuit drawing 5 A has a resistance of 120 ÷ 5 = 24 Ω and dissipates 120 × 5 = 600 W. Run continuously for a day, that is 14.4 kWh of energy.
Ohm's Law Calculator: the complete guide
The water analogy, and where it breaks
Voltage is like pressure, current like flow rate, and resistance like the narrowness of the pipe. Raise the pressure and more flows; narrow the pipe and less does. The analogy makes Ohm's law intuitive and gets most people through their first circuits course.
Where it breaks is power. In the water picture, power feels like it should track flow. In circuits, power dissipated in a resistance goes as the square of the current — double the current and you get four times the heat. That is why undersized wiring fails so suddenly rather than gradually, and it has no clean plumbing equivalent.
Sizing wire, fuses, and heat sinks
Current is what determines wire gauge, because I²R heating in the conductor is what melts insulation. A wire's ampacity rating is a thermal limit, not an electrical one — the same wire carries the same safe current at 12 V as at 240 V.
That is exactly why high-voltage transmission exists. Delivering 1 MW at 1,000 V requires 1,000 A and enormous conductors; delivering it at 100,000 V requires 10 A and a thin one. Transmission losses scale with I²R, so raising voltage by a factor of 100 cuts losses by a factor of 10,000.
For components, the P = I²R relationship is what sets heat sink requirements. A resistor passing twice its expected current dissipates four times the design power and will usually fail open.
Where Ohm's law does not apply
Ohm's law describes ohmic components — resistors, and conductors at constant temperature. Diodes, transistors, LEDs, and fluorescent lamps are all non-ohmic: their resistance changes with the voltage applied, so a single R value does not describe them.
An LED is the clearest example. It has no meaningful resistance and will draw destructive current if connected directly across a supply, which is why it always needs a series current-limiting resistor. To size that resistor, subtract the LED's forward voltage from the supply and divide by the target current — that resistor is ohmic even though the LED is not.
In AC circuits, reactive components — capacitors and inductors — oppose current in a way that depends on frequency and shifts the phase between voltage and current. The generalisation is impedance, and real power then becomes V × I × power factor rather than simply V × I.
Frequently asked questions
How do I calculate amps from watts?
Divide watts by volts: I = P ÷ V. A 600 W device on a 120 V supply draws 5 A. Choose "Power and voltage" above and the calculator returns the current and resistance together.
Why does doubling the current quadruple the heat?
Because power in a resistance is P = I²R. Doubling I multiplies I² by four, so the same resistor dissipates four times the power. This is why overcurrent is so much more dangerous than overvoltage in wiring, and why fuses are rated in amps.
Does Ohm's law work for AC circuits?
For purely resistive AC loads such as heaters and incandescent lamps, yes, using RMS values. For anything with motors, transformers, or switching power supplies, you need impedance rather than resistance, and real power is V × I × power factor.
What resistor do I need for an LED?
Subtract the LED's forward voltage from the supply voltage, then divide by the desired current. A red LED with a 2 V forward drop on a 5 V supply at 20 mA needs (5 − 2) ÷ 0.02 = 150 Ω. Round up to the next standard value for safety.