Flow Merging and Dividing

You can start here until you learn enough to refine your question.

you should be a little more specific if you are looking for detailed calculations and formulas, however, pressure losses from a substance in motion can be calculated using EQUIVALENT LENGTH(EL).

This term refers to how; as the pipe grows longer in distance there will be more friction between the pipe walls and the fluid, and therefore pressure losses. Pressure loss through fittings(couplings, tees, y, valves, etc) are measured as the "EL" of such fitting, you can find this information from reference articles from the scientific associations that research your field.

Exmpl. 3/4" gate valve= 2 ft EL,

Risers within a piping circuit may also be calculated as the equivalent length.

1 Vertical Foot of 1/2" PVC Schedule 40= 4 Feet of EL.

Obviously this is not accurate data, it is only example.