Ohm's Law & Circuit Calculator

Enter any two of voltage, current, and resistance to find the third and calculate power. Switch to resistor network mode to compute equivalent resistance for series and parallel combinations. All calculations run in your browser.

Enter any two values

Leave one field empty to solve for it. Clear a field with the x button.

Voltage (V)

Current (I)SOLVING

Resistance (R)

Results

Voltage (V)

12 V

Current (I)

120 mA

Resistance (R)

100 Ω

Power (P)

1.44 W

Power formulas (all equivalent)

P = IV = 0.12 \times 12

= 1.44 W

P = I^2R = 0.12^2 \times 100

= 1.44 W

P = \frac{V^2}{R} = \frac{12^2}{100}

= 1.44 W

Step-by-step

I = \frac{V}{R}

I = \frac{12}{100}

I = 0.12 \text{ A}

P = I \times V = 0.12 \times 12 = 1.44 \text{ W}

Circuit diagram

Reference Guide

Ohm's Law

Ohm's law relates voltage, current, and resistance in a circuit. If you know any two, you can find the third. Georg Simon Ohm published this relationship in 1827.

The fundamental equation

V = I \times R

Rearranged forms

I = \frac{V}{R}, \quad R = \frac{V}{I}

A helpful memory aid is the "Ohm's Law triangle." Write V at the top and I and R at the bottom. Cover the quantity you want to find, and the remaining two show the formula.

Electrical Power

Power is the rate of energy use in a circuit, measured in watts (W). There are four equivalent formulas that all give the same result.

Power formulas

P = IV = I^2R = \frac{V^2}{R}

Each form is useful depending on which quantities you already know. For example, if you only know voltage and resistance, use $P = V^2/R$ . All four give the same wattage for the same circuit.

Units

1 \text{ W} = 1 \text{ V} \times 1 \text{ A} = 1 \frac{\text{J}}{\text{s}}

Series Circuits

Resistors in series are connected end-to-end so the same current flows through all of them. The total resistance is the sum of all individual resistances.

Equivalent resistance

R_{eq} = R_1 + R_2 + \cdots + R_n

The voltage divides across resistors proportionally to their resistance. The total resistance is always larger than any single resistor in the chain.

Voltage divider

V_k = V_{total} \times \frac{R_k}{R_{eq}}

Parallel Circuits

In a parallel circuit, all resistors share the same voltage across them. The current divides between branches, with more current flowing through lower-resistance paths.

Equivalent resistance

\frac{1}{R_{eq}} = \frac{1}{R_1} + \frac{1}{R_2} + \cdots + \frac{1}{R_n}

The equivalent resistance is always smaller than the smallest individual resistor. A handy special case is two equal resistors in parallel, which gives half the resistance of either one.

Two resistors shortcut

R_{eq} = \frac{R_1 \times R_2}{R_1 + R_2}