By David Robinson | Updated Mar 24, 2022

Electrical calculations involving resistance, voltage, current, or power typically rely on Ohm’s Law. Discovered in 1827 by Georg Simon Ohm, it states that the current flowing through a conductor is directly proportional to the applied voltage and inversely proportional to its resistance. Because power (in watts) is the product of voltage and current, we can rearrange the relationships to compute resistance when only power and voltage are known.

See the Wikipedia entry on Ohm’s Law for more details.

Step 1: Determine Current from Power and Voltage

Use the formula I = P ÷ V. Enter the power (in watts) and divide by the voltage (in volts). Record the resulting current for the next step.

Example: If P = 100 W and V = 50 V, then I = 100 W ÷ 50 V = 2 A.

Step 2: Calculate Resistance

Rearrange Ohm’s Law: R = V ÷ I. With the voltage from Step 1 and the current you just calculated, divide to find resistance in ohms.

Using the example above: R = 50 V ÷ 2 A = 25 Ω.

Step 3: Verify Your Result

To ensure accuracy, calculate power again using P = V² ÷ R. If the computed power matches the original value, your calculations are correct; otherwise, review the steps for errors.

TL;DR

Use a simple online calculator or follow the three‑step method to compute resistance from power and voltage.

Safety Notice

Always double‑check calculations before applying them to any electrical project. Incorrect values can lead to fire, electric shock, or even death.