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.