Compare system flow with the combined listed flow capacity of your suction outlets.
The Pool Main Drain Companion Guide explains suction outlets, covers, sumps, skimmer/main-drain relationships, valves, circulation and common homeowner misunderstandings.
Enter your system flow in GPM, the number of suction outlets, the listed flow rating for each outlet, and the percentage of that combined rating you want to use for comparison. The calculator compares the entered system flow with the adjusted combined listed capacity.
Adjusted combined capacity is the number of outlets multiplied by the listed rating per outlet and the capacity factor you entered. Capacity margin is that adjusted capacity minus your entered system flow.
A positive margin only means the numbers you entered produced a combined listed capacity greater than the entered flow. It is not approval of the drain system.
The fitting at the bottom of a pool is commonly called a main drain, but it functions as a suction outlet connected to the circulation system. Depending on the pool, suction may be shared between bottom outlets and the skimmer and controlled by valves or diverters.
No. Pool designs and circulation systems vary. Some pools rely on skimmers and returns without an active bottom suction outlet.
Not necessarily. Surface skimming is extremely important, and return placement, brushing and overall hydraulic design all affect circulation.
Some systems allow suction to be balanced between the skimmer and main drain. Before changing valve positions, identify the plumbing correctly and understand what each valve controls.
No. This calculator only compares the numbers you enter. Suction-outlet safety depends on the complete system, listed components, installation, manufacturer requirements and applicable codes.
Browse Pool Gal guides, Splash Sheets, and printable pool-care resources made to help homeowners understand and manage their pools.
Browse Pool Care Guides