# Parallel Resistor Calculator in Reverse

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

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

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.

## Related Calculators

In this calculator we calculate the effective resistance of a bank of any number of resistors in parallel.