Ohm's Law formula
| Symbol | Quantity | SI unit | Meaning |
|---|---|---|---|
| V | Voltage | V | Potential difference across the component |
| I | Current | A | Electric current through it |
| R | Resistance | Ω | Electrical resistance |
What is Ohm's Law?
Ohm's Law: The current through a conductor is directly proportional to the voltage across it, with resistance as the constant of proportionality. — Georg Simon Ohm, 1827
Ohm's law is the starting point of circuit analysis. For ohmic conductors (most metals and resistors at constant temperature), the voltage–current graph is a straight line whose slope is the resistance. The calculator also reports the power dissipated, P = VI = I²R = V²/R.
Ohm's law is the electrical analogue of Fick's law of diffusion and Fourier's law of heat conduction. In each case a flux (current, mass, heat) is proportional to a driving gradient (voltage, concentration, temperature).
How to use this calculator
- Choose the quantity you want to find in the Calculate menu. It is highlighted as the result field.
- Enter the known values. Scientific notation such as
1.6e-11is accepted. - Select a unit for each value. Units are converted to SI automatically.
- The answer updates as you type. Open Show calculation steps to see the working, or use Copy result to share a link with your inputs.
Worked example
A 24 Ω heater element carries 0.5 A.
- V = 0.5 × 24 = 12 V.
- P = 12 × 0.5 = 6 W.
Frequently asked questions
Does Ohm's law apply to all components?
No. Diodes, transistors, gas discharges and filament lamps (whose resistance rises as they heat up) are non-ohmic. R = V/I still defines an instantaneous resistance, but it is not constant.
How do I pick a resistor for an LED?
R = (V_supply − V_LED) / I_LED. For example, (5 V − 2 V) / 0.02 A = 150 Ω. Select "Resistance" and enter the voltage across the resistor and the desired current.