Ohms Law Calculator

Ohm's Law

Ohm's law states that the current flowing through a conductor is proportional to the voltage across it, as long as its temperature and other physical conditions stay the same. The constant of proportionality is the resistance. The German physicist Georg Simon Ohm published the relationship in 1827 after experiments with wires of different lengths and thicknesses.

In practical terms: double the voltage across a resistor and the current doubles; double the resistance at the same voltage and the current halves. With 110 V across a component drawing 0.2 A, its resistance must be 110 ÷ 0.2 = 550 Ω.

Ohm's law holds very well for metal wires and ordinary resistors, which are called ohmic. Components such as diodes, transistors, light bulb filaments as they heat up, and gas discharge lamps do not have a constant resistance, so the simple proportion only applies to them at a given operating point. In alternating-current circuits with capacitors or inductors, resistance is replaced by impedance, but the same form, V = I × Z, still applies.

V = I × R

V

voltage in volts

I

current in amperes

R

resistance in ohms

Rearranged: I = V ÷ R and R = V ÷ I.

Electrical Power

Electrical power is the rate at which electrical energy is converted into heat, light, motion or other forms. It is measured in watts; one watt is one joule per second. In a direct-current circuit, or an AC circuit with a purely resistive load, power equals voltage multiplied by current.

Substituting Ohm's law gives two more forms that are often handier: P = I² × R and P = V² ÷ R. The first explains why power lines run at high voltage: for the same delivered power, a higher voltage means less current, and the heat lost in the wires falls with the square of the current. The second shows why a 120 V appliance would draw four times the power, and likely burn out, if connected to 240 V.

The calculator also gives power in horsepower (1 hp ≈ 745.7 W) and BTU per hour (1 W ≈ 3.412 BTU/h), units still used for motors, heaters and air conditioners.

Ohm's Law Formula Wheel

The formula wheel collects all twelve ways of finding voltage, current, resistance or power from any two of the others. The four quantities sit in the center, and each quarter of the outer ring lists the three formulas for one of them. Find the quarter for the quantity you want, then choose the formula that uses the two values you know.

For example, to find the current drawn by a 60 W bulb on a 120 V circuit, use I = P ÷ V = 0.5 A; its hot resistance is then R = V² ÷ P = 240 Ω.

Frequently asked questions