I made a really nice chart in Excel but it turns into a mess when copying it here, so here are smaller charts and their explanations: Cross Sectional Area (in square inches) - Pipe sizes vs. their cross sectional area. Useful for dividing flow between pipes. size __area 1/2 = .1963 3/4 = .4418 1.0 = .7854 1.25= 1.227 1.5 = 1.767 2.0 = 3.142 Maximum Gravitational Vertical Flow - This is what we look for when sizing drains or DIY overflows. The flow under the power of gravity reaches a maximum in the same way an object reaches Terminal Velocity as it falls through the air. The gravitational force is countered by the waters viscosity (resistance to flow) and the frictional resistance of the pipe. The viscosity creates a minimum vertical length to acheive the maximum flow. If the vertical pipe length is less than the minimum, the flow rate will be somewhat less than the pipes maximum potential. size _GPM __GPH 1/2 = 2.50 = 150 3/4 = 5.63 = 337.5 1.0 = 10.0 = 600 1.25=15.6 = 937.5 1.5 = 22.5 = 1350 2.0 = 40.0 = 2400 Min. Vertical Length 1/2 = 2" 3/4 = 5" 1.0 = 9" 1.25= 14" 1.5 = 20" 2.0 = 36" Frictional Head Loss per 90 (measured in Feet of Head) size_ head loss 1/2 = .00970 3/4 = .00220 1.0 = .00386 1.25= .00602 1.5 = .00878 2.0 = .00156 Anyway... Each measurement is at the pipes maximum flow rate in the Vertical Flow Rate chart. When calculating the loss for the pump side, these values are slightly more. There is a very conservative rule when estimating head loss. It is one foot of head per fitting. This rule does not take into account flow rates, pressure, actual frictional losses, etc. It does not have to because it is too conservative. If you multiply any of the above head losses by one thousand fittings, you will be very surprised at the actual head loss. (hint: just move the decimal to the right three places.) Gravitational Horizontal Flow This is for horizontal applications that do not rely on a pump, or the force of a vertical pipe. These flow rates are considerably less than the vertical rates because gravity does not offer a significant horizontal force. It is a balance of gravity and the viscosity vs. the cross sectional area of the pipe. size_ GPM 1/2 = 1.563 3/4 = 3.517 1.0 = 6.253 1.25= 9.769 1.5 = 14.07 2.0 = 25.0 In this application, you would use the chart for Vertical Flow. In this application, you would use the chart for Horizontal Flow.