Aquarium Sump Backflow Calculator
Enter the display, return plumbing, overflow box, and usable sump dimensions to estimate how far the sump rises when the return pump stops. The result also reverse-solves a recommended running water depth that leaves your selected reserve.
Maximum recommended operating depth (in)13.63
- Effective display drain-down depth (in)
- 0.75
- Display drain-down volume (US gal)
- 2.81
- Overflow-box drain-down (US gal)
- 0.42
- Return-pipe drain-down (US gal)
- 0.11
- Total estimated backflow (US gal)
- 3.83
- Predicted shutdown water rise (in)
- 1.37
- Required empty height (in)
- 2.37
- Available backflow capacity (US gal)
- 19.64
- Operating-level margin (in)
- 4.63
- Estimated shutdown status
- Reserve available: 4.63 in below the recommended operating depth.
Uses inside dimensions and assumes every listed source can drain to the sump during a power outage.
Confirm the result with a supervised power-off test after plumbing and equipment are installed.
How to use this calculator
- Measure display and overflow-box dimensions on the inside, using the normal running waterline as the reference.
- Enter the overflow stop depth and the return outlet or siphon-break depth that limits display drain-down.
- Include every return line that can empty above the sump and add known drainage from attached equipment.
- Measure only the sump chambers that communicate while water rises, then enter their usable footprint and lowest safe maximum height.
- Compare the entered operating depth with the recommended depth, then verify it with a supervised power-off test.
How sump backflow is calculated
When the return pump stops, water continues to drain from the display until the overflow stops and any return siphon admits air. This calculator separates that display drain-down from water in an overflow box, drainable return plumbing, and other attached equipment. It then converts the total into the height it adds to the part of the sump that can actually share the rise.
The return-siphon stop depth is the return outlet depth when there is no siphon-break hole. With a hole, it is the shallower of the outlet and hole depths: a hole below the outlet cannot stop the siphon before the outlet admits air. The display drain-down depth is the greater of that result and the overflow stop depth.
Using inside dimensions in inches, the main volume calculation is:
Vtotal = (Ld x Wd x D + Lo x Wo x Ho + N x pi x (Di / 2)^2 x Lp) / 231 + Vother
- Ld, Wd, and D are the display footprint and effective drain-down depth.
- Lo, Wo, and Ho are the overflow-box dimensions and its drain-down depth.
- N, Di, and Lp are the return-line count, actual inside diameter, and drainable length per line.
- Vother is measured or estimated drainage from other equipment, in US gallons.
There are 231 cubic inches in a US gallon. If Ls x Ws is the effective sump footprint, shutdown rise is R = 231 x Vtotal / (Ls x Ws). The calculator adds your reserve height to that rise, then solves the safe running depth as Hmax = Hs - R - Hr, where Hs is the usable sump height and Hr is the reserve. A positive operating-level margin means the entered running level is below that recommendation.
What moves the result most
The display footprint multiplies every inch of drain-down, so a small change in overflow or return outlet height can add a meaningful volume. A shallower, clean siphon-break hole can reduce display drainage, but it must work when the pump stops. The effective sump footprint matters just as much: baffles, isolated chambers, filter socks, and equipment can prevent the full sump area from accepting the shutdown rise.
Return plumbing is usually a smaller contribution, but it is easy to omit. Use the pipe's real inside diameter, count parallel lines, and include only the length that can drain above the sump waterline. Treat different pipe sizes or paths separately and add their measured volume under other equipment backflow.
What this calculator leaves out
This is a geometric planning model. It does not predict weir flow, trapped air, flexible-hose routing, a fouled siphon-break hole, changing plumbing, water held in media, or the exact behavior of a check valve. Aqueon's setup instructions call for a check valve or siphon break and a shutdown test; neither device removes the need to make the sump passively capable of containing drain-down.
Worked example
Consider a 48 by 18 inch display with a 0.75 inch overflow stop depth, a 1.5 inch-deep return outlet, and a 0.5 inch siphon-break hole. The effective display drop is 0.75 inches, because the overflow takes longer to stop than the return siphon. The display contributes 48 x 18 x 0.75 / 231 = 2.81 US gallons.
With a 12 by 4 by 2 inch overflow box, one 0.824 inch inside-diameter return pipe draining 48 inches, and 0.5 gallons from other equipment, total backflow is about 3.83 gallons. In a 36 by 18 inch effective sump footprint, that produces a 1.37 inch shutdown rise. With a 16 inch usable height, a 9 inch running depth, and a 1 inch reserve, the recommended maximum operating depth is 13.64 inches, leaving about 4.64 inches of margin.
Common questions
How much empty space should I leave in an aquarium sump?
Leave enough height to contain the measured or estimated shutdown rise plus a practical reserve. This calculator uses the display, overflow box, return plumbing, and other equipment to estimate that rise, then subtracts it and the reserve from the sump’s usable height. Confirm the final level with a power-off test because the true drain-down depends on the completed system.
Does a siphon-break hole reduce sump backflow?
It can reduce backflow by letting air into the return line sooner than a submerged outlet would. Its depth only helps if it is shallower than the return outlet, and the hole must remain clear and actually admit air. Plan for a failed or clogged hole by testing the system and retaining enough passive capacity where practical.
Should I count water inside the return plumbing?
Yes, count the part of each return pipe that can empty into the sump after shutdown. Use the actual inside diameter and the drainable length above the sump waterline. If several lines differ, calculate each pipe volume separately and add the extras under other equipment backflow.
Why is the return outlet depth more important than the overflow height?
The display keeps draining until both paths have stopped. The overflow stops at its crest or standpipe level, while a return siphon stops when the outlet or a working siphon-break hole admits air. The deeper of those two stop depths sets the display drain-down used by this calculator.
How do sump baffles affect available backflow capacity?
Only chambers that can freely share the rising shutdown water count toward the effective footprint. A baffle, high divider, filter section, or sealed chamber can keep a portion of the sump from contributing. Measure the area that will actually rise together rather than assuming the entire sump footprint is available.