The pressure drop for liquid or incompressible gas flow in pipe or tube is calculated using the Colebrook-White equation. An iterative method is used to solve the full implicit equation. If the iterative solution fails to converge, the explicit Blasius equation for smooth pipe flow is used. The default roughness is for steel pipe, but other roughness values can be entered. The pressure drop for one velocity head is output and may be multiplied by the sum of the velocity heads for all fittings and valves to account for those components.