Why is the sump only 25-40% of the display volume?

The sump only needs to hold the water that drains down during a power outage, plus room for equipment and a refugium. Oversizing wastes salt and heating, while undersizing risks an overflow when the return pump stops and water siphons back.

What turnover rate should I aim for?

Most reef and marine tanks target 5 to 10 times the display volume per hour through the sump, with the in-tank powerheads handling the rest of the flow. Seven times per hour is a common balanced default.

Why is the recommended pump bigger than my target flow?

Return pumps lose flow as they push water vertically. A pump rated at zero head might deliver only half its rating after four feet of lift, so the tool uplifts the rated GPH to compensate for that loss.

How is head loss estimated?

Small aquarium return pumps lose roughly 10 to 12 percent of their flow per foot of vertical head in their usable range. The tool applies about 11 percent per foot and floors the result so you do not undersize the pump.

Does this include plumbing friction losses?

Only the vertical head is modeled. Long horizontal runs, elbows, and valves add friction that further reduces flow, so round up to the next pump size if your plumbing is complex.

Aquarium Sump Volume & Return Pump Calculator

A sump turns the cabinet under your aquarium into hidden filtration, heating, and equipment space, but it only works if it is sized so an outage cannot overflow it and the return pump still delivers real turnover after fighting gravity. This calculator sizes both for any display tank.

How it works

Two independent calculations run from your inputs:

sump capacity = display volume × sump ratio
target flow   = display volume × turnover per hour
derate        = 1 − 0.11 × head (feet), floored at 0.15
rated pump GPH = target flow / derate

The sump ratio (25-40%) leaves headroom for drain-down water and gear. The return pump must be bought at a rated (zero-head) flow higher than your target, because every foot of vertical lift strips flow away.

Example and tips

A 75-gallon display at a 30% ratio needs about a 22-gallon sump. Targeting 7× turnover means 525 GPH at the tank. With 4 feet of head the pump keeps only about 56% of its rating, so you should buy a pump rated for roughly 940 GPH at zero head. Keep the sump no more than three-quarters full at running level, mark your maximum drain-down line, and test it by cutting power to confirm the sump holds the back-siphoned water without spilling.