Parallel Resistance Calculator and Formula

This tool calculates the total resistance of resistors connected in parallel. Below, you’ll find the formula used and an example to illustrate its application.

EXAMPLE:

• INPUTS: R1 = 100 Ohm, R2 = 100 Ohm, R3 = 100 Ohm, R4 = 100 Ohm
• OUTPUT: 25 Ohm

Parallel Resistance Formula

As shown in the figure, if two resistors are connected in parallel, the following principles apply:

• Voltage across both resistors will be the same.
• Total current is the summation of currents through them: (I = I1 + I2) which means (V/R) = (V/R1) + (V/R2)
• Total resistance of two resistors in parallel: 1/Req = 1/R1 + 1/R2
• Total resistance of four resistors connected in parallel: 1/Req = 1/R1 + 1/R2 + 1/R3 + 1/R4
• Therefore: Req = 1/(1/R1 + 1/R2 + 1/R3 + 1/R4)

Most manufacturers produce resistors in standard values, including 100 Ohm, 1 KOhm, 10 KOhm, 470 Ohm, 220 Ohm, 330 Ohm, etc. However, some electronic circuits require specific resistance values that aren’t readily available. These values can be achieved by combining resistors in series or parallel.

Popular resistor manufacturers include Vishay, Mouser Electronics, Murata, and Ohmite. The parallel resistance formula can be applied to any number of resistors.

For equal values of resistors, the formula to calculate total resistance simplifies to:

Total resistance, Req = Value of one resistor / Number of resistors

For example, the total resistance of four equal resistors with a value of 10 KOhm is as follows:

• Req = 10 KOhm / 4
• Req = 2.5 KOhm