How to Find Resistance With Power & Voltage

Resistance is a function of voltage and current.
••• Polka Dot Images/Polka Dot/Getty Images

Most electrical calculations involving resistance, voltage, current or power are solved using Ohm's law. Ohm's law, discovered in 1827 by Georg Simon Ohm, states that the current in a conductor is proportional to the voltage and inversely proportional to the resistance. Since power, measured in watts, is a function of voltage and current, and current is a function of voltage and resistance, it is possible to calculate resistance from power and voltage. The calculations are simple but an understanding of simple math is advantageous.

    Enter the power, in watts, into the calculator. Divide the wattage by the voltage. The result is the current because Ohm's law states that current = power / voltage. Record the current value for later use. For example if the power is 100 watts and the voltage is 50 volts, the current is 100 / 50, or 2 amps.

    Calculate the resistance, in Ohms, by dividing the voltage by the current. Ohms law states that voltage = current x resistance, so by rearranging the formula resistance = voltage / current. Using the example in Step 1, with a voltage of 50 volts and a current of 2 amps, the resistance is 50 / 2, or 25 ohms.

    Check for errors in the calculation. Divide the voltage squared (voltage x voltage) by the calculated resistance. If the result does not equal the power, in watts, there was an error in the calculations.


    • Use an online calculator to simplify the Ohms law calculations.

      Ohms law states that V=IR, where "V" is voltage, "I" is current and "R" is resistance. If you can remember that Ohms law is a "Very Important Rule," you have remembered V, I and R in the correct order.


    • Always double-check your results before applying the values to an electrical project. Using incorrect values may result in fire, electric shock and death.

Related Articles

How to Convert mW to mA
What Is Ohm's Law & What Does It Tell Us?
How to Calculate a KVA Rating
How to Calculate Millivolts to Amps
How to Calculate a Current From the HP & Voltage
How to Convert Watts to Volts
How to Calculate DC Voltage
How to Simplify Monomials
How to Calculate Battery Voltage
How to Convert Ohms to Kilowatts
How to Calculate 30 KW to Amps
How to Calculate Millivolts from Microsiemens
How to Integrate the Cube Root of X
How Do I Convert Amps to Kw 3 Phase?
How to Figure kW Rating of a 3 Phase Electric Motor
How to Do Exponents Outside of the Parenthesis
How to Change Electrical Amps to Watts
How to Convert 12 Volt Alternator to 120 Volts
How to Convert HP to Amps & Volts
How to Calculate Voltage Across a Resistor