Ohm's Law Calculator

Last updated: 2026-05-18

Ohm's Law

Ohm's Law states that the current through a conductor between two points is directly proportional to the voltage across them, with resistance as the constant of proportionality: V = I × R. Voltage (V) is the electric potential difference across a component, current (I) is the rate of charge flow through it, and resistance (R) is the component's opposition to that flow. Georg Simon Ohm published the relationship in 1827, and it is the basic equation of circuit analysis.

This calculator solves for any one of the three quantities — voltage, current, or resistance — given the other two. The power dissipated by the component is reported alongside the result in every mode.

Why the quantities relate this way

A water-pipe analogy makes the three quantities concrete. Voltage corresponds to water pressure, the force pushing water through the pipe. Current corresponds to the flow rate, the volume passing a point each second. Resistance corresponds to the narrowness of the pipe, which restricts the flow. Raising the pressure increases the flow; narrowing the pipe reduces it. For an ohmic material, the flow is exactly proportional to the pressure divided by the restriction, which is what V = I × R expresses.

Formulas

The single relationship rearranges into three forms, one for each unknown. Power is computed from the same quantities.

The three power forms are equivalent through substitution of V = IR; P = I²R is convenient when current and resistance are known, and P = V²/R when voltage and resistance are known.

Worked example

A typical red LED has a forward voltage of 2 V and a rated current of 20 mA (0.020 A). Driven from a 5 V supply, the series resistor must drop 5 − 2 = 3 V at 20 mA:

R = V ÷ I = 3 V ÷ 0.020 A = 150 Ω

The power the resistor dissipates is:

P = I²R = (0.020)² × 150 = 0.06 W

A standard ¼ W resistor handles this comfortably. Setting the mode to "Find resistance" and entering 3 V and 0.020 A reproduces the result.

Power and component ratings

Resistors and wires carry a maximum power rating (⅛ W, ¼ W, ½ W, 1 W, and so on). When the dissipated power exceeds the rating, the component overheats, which is why power is checked alongside resistance when sizing a circuit. A 100 Ω resistor carrying 0.1 A dissipates P = I²R = 0.01 × 100 = 1 W, so it requires a resistor rated above 1 W.

When Ohm's Law does not apply

Ohm's Law holds exactly only for ohmic (linear) components at constant temperature, where R is independent of voltage and current. It does not describe several common cases:

• Diodes — current rises exponentially with voltage rather than linearly.
• Thermistors and tungsten filaments — resistance changes significantly with temperature, so R is not constant.
• Transistors in saturation or cut-off — these operate far outside the linear region.
• High-frequency circuits — parasitic capacitance and inductance add reactance, which complex impedance Z describes; Ohm's Law alone does not.

For ordinary resistors at typical temperatures and frequencies, the relationship is effectively exact.