Ohm's law states that voltage (V) equals current (I) times resistance (R): V = IR. It describes the relationship between voltage, current, and resistance in an electrical circuit at constant temperature.

How do I calculate electrical power?

Power (P) in watts = V times I (voltage times current). Equivalently: P = I^2 times R, or P = V^2 divided by R. All three forms are equivalent via Ohm's law.

What are the units for voltage, current, and resistance?

Voltage is measured in volts (V). Current is measured in amperes (A). Resistance is measured in ohms (symbol omega). Power is measured in watts (W).

Does Ohm's law apply to all materials?

Ohm's law applies to ohmic materials where resistance is constant regardless of voltage or current. Non-ohmic components like diodes and transistors have nonlinear V-I relationships and do not obey Ohm's law.

Ohm's Law Calculator — V, I, R & Power

How this tool fits your workflow

Ohm's law and the power equations

Ohm's law (V = IR) is the foundation of circuit analysis. From it, four power equations are derived: P = VI, P = I^2 R, P = V^2/R. Knowing any two of the four quantities (V, I, R, P) allows solving for the other two. This calculator accepts any two inputs and outputs the remaining two.

Practical example: a 12V battery drives current through a 100-ohm resistor. I = V/R = 12/100 = 0.12 A. P = VI = 12 times 0.12 = 1.44 W. The resistor must be rated for at least 1.44 watts to avoid overheating.

Series and parallel circuits

In series: resistances add directly (R_total = R1 + R2 + ...). Current is the same through all components; voltage divides proportionally to resistance. In parallel: reciprocals add (1/R_total = 1/R1 + 1/R2 + ...). Voltage is the same across all branches; current divides inversely with resistance.

Ohm's law applies to each component and to the circuit as a whole. For complex circuits with multiple loops, Kirchhoff's laws (sum of voltages around any loop = 0; sum of currents at any node = 0) extend Ohm's law to handle any topology.

Frequently asked questions

What is Ohm's law?: Ohm's law states that voltage (V) equals current (I) times resistance (R): V = IR. It describes the relationship between voltage, current, and resistance in an electrical circuit at constant temperature.
How do I calculate electrical power?: Power (P) in watts = V times I (voltage times current). Equivalently: P = I^2 times R, or P = V^2 divided by R. All three forms are equivalent via Ohm's law.
What are the units for voltage, current, and resistance?: Voltage is measured in volts (V). Current is measured in amperes (A). Resistance is measured in ohms (symbol omega). Power is measured in watts (W).
Does Ohm's law apply to all materials?: Ohm's law applies to ohmic materials where resistance is constant regardless of voltage or current. Non-ohmic components like diodes and transistors have nonlinear V-I relationships and do not obey Ohm's law.