How to Calculate Resistors in Parallel

Combine resistors in series and in parallel.
••• Comstock/Comstock/Getty Images

Figuring total resistance for resistors in parallel is a chore confronted by early students of electronics. The general method that works for any situation is to take the reciprocal of each resistance, add these together, and take the reciprocal of the result. A couple of tricks can cut this task down to size. If all the resistors have the same value, divide the resistance of one resistor by the number of resistors. If you’re finding the value of two resistors in parallel, divide the product of their resistances by their sum.

General Case

    Take the reciprocal of each resistance. Example: For three resistors in parallel, 15, 20 and 25 ohms. The reciprocals are 1/15, 1/20 and 1/25.

    Add the reciprocals together. Example: 1/15 + 1/20 + 1/25 = .157

    Take the reciprocal of the result. This gives the total resistance of the parallel combination. Example: 1/.157 = 6.4 ohms

All the Same Value

    Determine the resistance. Example: Three resistors in parallel, all 300 ohms. The resistance to divide is 300 ohms.

    Count the resistors. Example: 3

    Divide the resistance by the count. This give the total resistance. Example: 300/3 = 100 ohms.

Pair of Resistors

    Multiply the resistances. Example: Two resistors in parallel, 100 and 200 ohms. 100 x 200 = 20,000

    Add the resistances. Example: 100 + 200 = 300

    Divide the result in Step 1 by the result in Step 2. This gives you the total resistance. Example: 20,000 / 300 = 66.7 ohms.

Related Articles

How to Calculate a Voltage Drop Across Resistors
How to Calculate Output Voltage
How to Find Wattage With Voltage & Frequency
What is the Identity Property of Multiplication?
How to Convert Ohms to Kilowatts
How to Use a 9-Volt Battery to Power LEDs
How to Calculate 30 KW to Amps
How to Add Parallel Resistors
How to Calculate a Sigma Value
How to Reduce Voltage With Resistors
How to Calculate a Cumulative Numerical Average
How to Calculate Potential Difference
How to Wire a Battery in Series
How to Calculate Load Current
How to Divide Polynomials By Monomials
How to Calculate KVA to MVA
How to Wire 50 LEDs Together
How to Factorise a Quadratic Expression
The Use of a Series & Parallel Circuit Connection
How to Convert 12 Volt to 6 Volt