Parallel resistor calculations

This design tip describes a simple equation to compose a desired resistance from two parallel connected resistors.
The formula for parallel connection of two resistors may be found in any electronics textbook and reads:

Req = R1 R2 / (R1 + R2)

In many cases, a theoretical value required in a circuit will deviate somewhat from off the shelf values in the E-series. In analogue filters, for example, maintaining the exact value is a must and even an E96 value is usually too inaccurate.