An eccentric reducer is a fitting used in piping systems between two pipes of different diameters. The same fitting can be used in reverse as an eccentric increaser or expander. The eccentric design means the reduction in pipe diameter in offset rather than centered, which is essential in applications where the pipe needs to be kept close to a wall for the proper drainage of liquids.

In concentric reducer the reduction of the pipe size is achieved by decreasing the diameter of the fitting at a constant rate over a specified length, maintaining symmetry around the fitting. Eccentric reducers have minimum turbulence envelopment and require minimum space.