What Ohm's law says
Ohm's law relates voltage, current and resistance: V = I·R. A 2 A current flowing through a 10 Ω resistor produces 20 V across it. Voltage (V) is in volts, current (I) in amperes, and resistance (R) in ohms (Ω). It is the foundation of basic circuit analysis.
Read it as a statement about one component, not a whole circuit: V is the potential difference across that element, and I is the current through that same element. Applying it piece by piece is what lets you analyse series and parallel networks. Not all of the electrical power drawn does useful work, and the efficiency calculator shows how much survives.
How to use this calculator
Enter the current in amperes and the resistance in ohms to get the voltage in volts. Make sure values are in base units — convert milliamps to amps and kilohms to ohms first so the result is correct.
Those conversions matter: 250 mA is 0.25 A and 4.7 kΩ is 4,700 Ω. Entering the prefixed numbers directly produces answers wrong by a factor of a thousand in either direction. The relationship is linear, so doubling either input doubles the voltage. For motors it is often easier to think in mechanical terms, so convert the watts with the horsepower calculator.
Rearrangements and power
The same law rearranges three ways: V = I·R, I = V / R, and R = V / I. Electrical power follows directly as P = V·I — with these numbers, P = 20 × 2 = 40 W. Work out wattage with the power calculator.
Substituting Ohm's law into the power equation gives two more useful forms, P = I²·R and P = V² / R. These are what you use to check whether a resistor can handle the heat it will dissipate: 40 W through a component rated for a quarter of a watt will destroy it, even though the voltage and current look modest.
Frequently asked questions
- What is Ohm's law?
- It states that voltage equals current times resistance: V = I·R.
- How do I find current from voltage and resistance?
- Rearrange to I = V / R. For 20 V across 10 Ω, the current is 2 A.
- How do I find resistance?
- Use R = V / I. With 20 V and 2 A, the resistance is 10 Ω.
- How is electrical power related?
- Power is P = V·I. At 20 V and 2 A the power is 40 watts.
- Do all components obey Ohm's law?
- No. Ohm's law holds for ohmic components such as resistors and most wires, where resistance stays constant. Diodes, LEDs, transistors and filament lamps are non-ohmic — their resistance changes with voltage or temperature, so a single V = I·R calculation does not describe them.