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Parallel Resistance

This calculator requires the use of Javascript enabled and capable browsers. Begin by entering the resistance values into the resistance input box. Please be sure to click on the "Enter Resistance" button after each entry followed by clicking on the resistance entry box to prepare for the next resistor value entry. There is no reasonable limitation as to the number of entries as long as you follow each entry by clicking on the "Enter Resistance" button and again on the entry box to allow for the next entry. After the last resistance value has been entered, click on the "Calculate Resistance" button for the returned parallel resistance value and the number of resistor values entered. Parallel resistance is NOT the same as averaging. It uses the formula RT = 1 / ((1 / R1) + (1 / R2) + (1 / R3)). There are other formulae for doing the resistance of only 2 resistors but this works with an unlimited number of resistors, including 2. In the formula given, RT is the total (parallel) resistance of the circuit, R1 is the resistance value of the first resistor, R2 is the resistance value of the second resistor, R3 is the resistance value of the third resistor, and on until the last resistor. Please remember that RT is with no exception, always smaller than the lowest value resistor in parallel. Input value is assumed to be ohms and returns in the same input value. See the information below the calculator for explanation of the formula. To calculate series resistance, use our Series Resistance Calculator.

Total Parallel Resistance Is  From   Resistors

An example (and test) of the formula is to use three resistors, 100 ohms, 200 ohms and 300 ohms, all in a circuit in parallel. The total (parallel) resistance would be 1 / (1/100 (.01) + 1/200 (.005) + 1/300 (.00333)) or about 54.555 ohms. Accuracy is good to the capability of your browser, generally 18 characters in the display window. NaN is an indication of an error in data entry or calculation; do the process again paying close attention to your data entry.
Updated 8.12.11

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