Rule-of-thumb to estimate pressure loss for differing pipe diameters

A useful rule-of-thumb to estimate the frictional losses in a system for a different pipe diameter is to multiply your calculated frictional losses for a known diameter by the ratio of diameters to the fifth power.

Darcy-Weisbach tells Us:

At constant volumetric flow velocity changes with pipe area, so substitute

Assuming density, Darcy friction factor, volumetric flowrate, and pipe length are constant we can simplify to:

Applying this to states 1 and 2 with differing pipe diameter and constants as described above:

Warning

For incompressible flow only (i.e. liquids). May be used with caution for compressibles where .

Warning

This relation does not account for static head as it applies to frictional losses only.