The easiest equation for two resistors in parallel is the MAD equation. MAD stands for Multiply, Add and Divide. Suppose you have two resistors, one of 50 ohms and another of 20 ohms. Multiplying the two yields 1,000. Adding the two yields 70. Divide the multiplication of 1,000 by the addition of 70. The result is 14.28. Therefore, the end product is 14.28 ohms. Notice that the final product is lower in resistance than either one of the two resistors. This is a common trait of parallelled circuits.
The MAD method works only for two resistors. For more than two, you have to use the reciprocal method. This equation states that one divided by the total resistance is equal to one divided by the first resistor plus one divided by the second resistor, plus one divided by the third resistor, and so on. The form of this equation is 1/R total = 1/R1 + 1/R2 + 1/R3....You have to use a calculator to solve this, since the numbers become quite large.
Because calculating total resistance is complex, use computer programs to do this for you. Enter the value of all the resistors, and the computer displays the result as total resistance. The computer uses the reciprocal method but can do it much faster than you can.
If all the resistors in a circuit are the same value, a simple equation can be used. The total resistance is the resistance value divided by the number of resistors. The form of this equation is R total = R individual / number of resistors. Suppose you have eight 75 ohm resistors wired in parallel. The total resistance of the circuit is 75 divided by 8. The resulting value is 9.375 ohms.