Parallel Resistor Calculator in Reverse (with Examples)

Here we calculate the value of a resistor used in parallel with another known resistor to achieve a desired total effective resistance.

Calculator

Enter

  • One resistor value (R1) in a parallel set of resistors
  • Target resistance (Rtotal) for the combination

The tool will calculate the value of the other resistor

Formula

R2 = (R1*Rtotal)/(R1-Rtotal)

Note that the effective resistance Rtotal is always equal to or lower than the smallest resistor of the two.

  • Rtotal < R1
  • Rtotal < R2

Example Calculation for RF and Digital Systems

What resistor should be added in parallel to a 1 kilohm resistor to create an effective resistance of 50 ohm? (This may be used to create a 50Ω ADC input impedance)

Enter R1 = 1 kΩ, Rtotal = 50 Ω. The answer is R2 = 52.6 Ω.

In practice you might not be able to find this value. In that case, pick a value that’s close.

Example Calculation for Audio Systems

How to convert 8 ohm to 4 ohm impedance?

To reduce the impedance from 8 Ω to 4 Ω let’s enter Enter R1 = 8 Ω, Rtotal = 4 Ω. The answer is R2 = 8 Ω.

Therefore we need to connect an 8 ohm resistor in parallel to reduce the total effective resistance to 4 ohm.

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