Contents

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 50Î© ADC input impedance)

Enter **R1 = 1 kÎ©**, **Rtotal = 50 Î©**. The answer is **R2 = 52.6 Î©**.

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 Î© to 4 Î©** let’s enter Enter **R1 = 8 Î©**,

**Rtotal = 4 Î©**. The answer is

**R2 = 8 Î©**.

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

**Related Calculators**

- In this calculator we calculate the effective resistance of a bank of any number of resistors in parallel.
- Calculate the total resistance of any number of resistors in series. To increase the impedance in general, a series resistor is added.