With Hydraulica, you can simply calculate the headless or diameter, corresponding the pipe parameters you’ve entered. So you can calculate the headloss based a flow and diameter, or the other way: around calculate the diameter based on a flow and and headloss.
Hydraulica is based on the Colebrook-White equation and Darcy-Weisbach friction factor and only suitable for fully filled water pipes.
This version is available in English and Dutch.