Ohm's Law Calculator

What Is Ohm's Law?

Ohm's law describes how voltage, current and resistance relate in an electrical circuit. It states that the current through a conductor between two points is directly proportional to the voltage across it, with resistance as the constant of proportionality. It holds for many materials over a wide range of voltages and currents — in particular for circuits made of resistive elements, whether the source is steady (DC) or alternating (AC). Enter any two known values into this Ohm's law calculator and it works out the others, including electrical power.

The Ohm's Law Formulas

• V = I × R — voltage (volts) equals current (amperes) times resistance (ohms).
• I = V ÷ R — current equals voltage divided by resistance.
• R = V ÷ I — resistance equals voltage divided by current.

A 12 V battery across a 4 Ω resistor drives 3 A of current. Double the resistance to 8 Ω and the current drops to 1.5 A.

Electrical Power

Power is the rate at which a circuit transfers energy, measured in watts (W). Joule's law gives P = V × I, and combining it with Ohm's law yields two more useful forms: P = I² × R and P = V² ÷ R. That 12 V, 4 Ω circuit dissipates 12 × 3 = 36 W, almost all of it as heat in the resistor.

The Ohm's Law Formula Wheel

Many electricians use a "wheel" with V, I, R and P and the twelve equivalent formulas arranged around it: pick the quantity you want in the center and use whichever pair of known values appears in its section. The calculator does the same algebra for you, so you can skip memorizing the wheel.

Where It Is Used

• Choosing the right resistor to limit current through an LED.
• Checking whether a wire or fuse can carry a given load safely.
• Estimating the power rating a component needs so it does not overheat.
• Working out battery life from the current a device draws.

Ohm's law does not apply as written to components like diodes and transistors, whose resistance changes with voltage. For the underlying mechanics, see the Work and Energy Calculator.