How do I size a return pump for my sump?
Pick the flow, then find the pump that delivers it against your plumbing, because the number on the box is the flow with nothing attached. Through the sump the consensus is 5–10 system volumes an hour: a 120 gallon reef with a 40 gallon sump holds about 586 L, so the target is 770–1,550 GPH — call it 1,100 GPH. Now the plumbing: the return outlet sits 3.9 ft above the sump's water surface, through 10 ft of 1 in pipe, two elbows, a check valve, a ball valve and a Loc-Line nozzle. A Sicce Syncra SDC 7.0 (DC) (1,900 GPH, 16.4 ft) on that plumbing delivers 1,200 GPH at 10.1 ft of total head — 62 % of its rating, 7.6× turnover, comfortably over the target with a DC controller to trim it. The same plumbing on a Jebao DCP-4000 (DC) gets 750 GPH: 4.8×, the bottom of the range.
A pump's flow falls along a curve, not a line, and the curve is set by its shut-off head. At 5 ft of head the 16.4 ft Sicce keeps 83 % of its rating; the "10 % per foot" rule says 50 %. At the example's 10.1 ft it still keeps 62 %; the rule says it delivers nothing at all. The rule was written for the old mag-drive pumps with 6–8 ft of shut-off head, where it was roughly right; applied to a modern DC pump with 13–21 ft it undersells the pump by a third or more and sends people out to buy the next size up. The tool draws the curve through the two numbers on the box — H = Hmax·(1 − (Q/Qmax)²), the standard centrifugal shape — and finds where it crosses the plumbing's. Read the maker's published curve if there is one; it will be close.
How much head does my plumbing really add?
Less than the elbow rules say in big pipe and more in small, because friction rises with the flow to the power 1.85 and falls with the pipe's inside diameter to the power 4.87. At 790 GPH, a meter of 1 in PVC costs 9.3 cm of head; a meter of ¾ in costs 30 — 3.2× more for one pipe size. A fitting is worth a length of pipe (a 90° elbow about 30 diameters, a swing check valve 100), so "a foot of head per elbow" is a constant standing in for something that varies by a factor of five across the pipe sizes people actually use. On the example plumbing the Jebao delivers 750 GPH in 1 in and 610 GPH in ¾ in — the pipe size is the lever, and upsizing the return line one step is usually cheaper than upsizing the pump.
| Return line | Per meter of pipe | Per 90° elbow | Swing check valve | Velocity |
|---|---|---|---|---|
| ¾ in | 30 cm | 18.8 cm | 62.7 cm | 2.4 m/s |
| 1 in | 9.3 cm | 7.4 cm | 24.6 cm | 1.5 m/s |
| 1¼ in | 2.4 cm | 2.5 cm | 8.4 cm | 0.9 m/s |
| 1½ in | 1.1 cm | 1.4 cm | 4.7 cm | 0.6 m/s |
Head lost at 790 GPH (3,000 L/h), schedule 40 PVC (Hazen–Williams C = 150; Crane equivalent lengths). Red: over 2 m/s, where the line is loud and the losses steep — go up a size. Flexible hose of the same inside diameter behaves about the same; kinks do not.
Do I need a check valve?
No, and it costs you: on the example plumbing the check valve alone is worth 8.7 ft of pipe, and removing it takes the Jebao from 750 GPH to 780 GPH. What it is meant to prevent — the return line back-siphoning the display into the sump when the power fails — is done for free by a small hole drilled in the return nozzle just below the water line, which breaks the siphon the moment the level drops past it. Check valves also fail the quiet way: a grain of sand or a snail holds the flap open, you find out during the next outage, and the sump has been sized for this failure or the floor has. Size the sump's empty space to swallow the back-siphon down to the hole instead, and leave the valve out. If you must have one, a true union swing check you can open and clean, and count it as three elbows.
Can my overflow drain take that much?
The drain is the real ceiling, and the loud one. A single 1 in Durso standpipe carries about 350 GPH before it gurgles; a 1 in full-siphon drain — Herbie or Bean Animal — about 1,200 GPH with 20 % of its capacity left for the gate valve to tune with. Run the pump above what the drain can pass and the water finds the emergency pipe, or the floor. The example's 1,200 GPH needs a siphon drain of 1 in or a Durso of 1½ in, and the tool checks whatever you have against whatever the pump will deliver. Pipe size on the drain side is free flow; on the return side it is free head. Go big on both.
| Drain pipe | Durso / open standpipe | Full siphon (Herbie, Bean Animal) | Siphon, with headroom |
|---|---|---|---|
| ½ in | 120 GPH | 400 GPH | 320 GPH |
| ¾ in | 220 GPH | 800 GPH | 640 GPH |
| 1 in | 350 GPH | 1,500 GPH | 1,200 GPH |
| 1¼ in | 600 GPH | 2,400 GPH | 1,900 GPH |
| 1½ in | 950 GPH | 3,200 GPH | 2,550 GPH |
Schedule 40 PVC with about 4 ft of drop, from aquaculator.com's table and corroborating Reef Central / Reef2Reef measurements. A Durso is quiet well under its figure; a siphon drain is tuned to sit at about 80 % of full capacity so the valve has range. Per drain — a Bean Animal's open channel adds a little, its emergency pipe nothing until it is needed.
Pumps obey the affinity laws: flow goes with speed, head with speed squared, power with speed cubed. Turn a DC pump down to 92 % to hit the example's 1,100 GPH target and it draws about 77 % of full power. Close a ball valve on an AC pump to get the same flow and the pump still runs at full speed against a higher head — it draws most of its rating and turns the difference into heat in the sump. Oversize the DC pump a little, then, and run it slow: quieter, cheaper, and headroom for the day the plumbing is dirtier. The electricity cost calculator takes the watts at the speed you run, and credits them to the heater, since every one of them ends up in the water.
Which return pump — example curves at real heads
Four current pumps, from their published zero-head flow and shut-off head, at the heads a sump actually presents. Read across: the difference between a 4 m pump and a 6.5 m pump is not the rating but how little the rating falls.
| Pump (box numbers) | at 1 m / 3.3 ft | at 1.5 m / 4.9 ft | at 2 m / 6.6 ft | at 3 m / 9.8 ft |
|---|---|---|---|---|
| Eheim compactON 3000 (AC) 3,000 L/h, 8.8 ft | 630 GPH | 530 GPH | 400 GPH | — |
| Jebao DCP-4000 (DC) 4,000 L/h, 13 ft | 920 GPH | 840 GPH | 750 GPH | 530 GPH |
| Sicce Syncra SDC 7.0 (DC) 1,900 GPH, 16.5 ft | 1,700 GPH | 1,600 GPH | 1,450 GPH | 1,200 GPH |
| EcoTech Vectra M2 (DC) 2,000 GPH, 21.5 ft | 1,800 GPH | 1,750 GPH | 1,650 GPH | 1,450 GPH |
Parabolic fit through the two published numbers (mid-2026 listings — models change; the maker's own curve beats this where one exists). Total head, i.e. lift plus friction, not lift alone.
The other way around: to put 1,100 GPH through the example plumbing (10.1 ft of total head) you need a zero-head rating of 2,250 GPH from a 13.1 ft pump, 1,750 GPH from a 16.4 ft pump or 1,500 GPH from a 21.3 ft pump. The tool prints that line for your target and your head.
My return pump is rated for far more than I'm getting — why?
In order of likelihood: the pipe is smaller than the pump's outlet (a 1 in pump reduced to ¾ in hose is the single commonest mistake, and the table above says what it costs); the check valve, or a partly closed valve someone tuned a year ago; the lift measured from the pump rather than from the sump's water surface, which is the only level that matters; a long horizontal run with a dozen fittings that were "only elbows"; and a pump six months past its last impeller clean, which is worth a size on its own. If the overflow gurgles or the emergency drain runs, it is the opposite problem — the pump is winning — and the DC controller, not a valve, is the fix. And turnover through the sump is not flow in the tank: the powerhead calculator sizes the circulation that the filter page separates out.