Sump-to-Display Ratio Guide

Run the Sump Size Calculator and you'll get a 25-40% range. This article covers the part the calculator can't: why 25% is the actual floor, and how to check that your own sump has enough spare capacity for the one moment that matters, when the power cuts out.

Why 25% is the floor, specifically

Bulk Reef Supply's own sump buyer's guide states plainly: "a sump should be no smaller than 25% of your display aquarium's total water volume." The reasoning is specific, not arbitrary. Per the same guide, "most sumps will operate around 50-60% total capacity when the return pump is ON, so when it cuts OFF, there is plenty of room for backflow water to drain into the sump without overflowing." The floor exists to hold water your display and plumbing dump into the sump the instant the pump stops, not to give the sump more room to look impressive.

What eats into that volume

A sump's raw gallon rating isn't all usable space. Baffles need roughly 1 inch of spacing between them and 1 inch of clearance from the bottom to maintain strong flow, per a detailed build breakdown on ReefAquarium.com. Skimmer and refugium chambers each claim their own footprint on top of that. None of this is captured by the sump's headline volume number.

How to actually calculate your drain-back

The same ReefAquarium.com piece walks through a real worked example. The author measured how much the display's water level would drop if the return pump stopped, and calculated that as roughly 15 gallons of drain-back. They built a 55-gallon sump and deliberately kept its operating water level at no more than 40 gallons, reserving the remaining 15 gallons specifically for that drain-back. The formula behind it is simple: length × width × height ÷ 231 = gallons, applied to the water-level drop across your display and plumbing.

What happens when headroom runs short

Hobbyists on Reef2Reef have reported real consequences of underestimating this. One thread describes a sump overflow after a power outage that spilled several gallons onto the floor, eventually fixed by drilling a siphon-break hole in the return line. A broader "scariest sump disaster" thread collects several similar cases: a stuck check valve draining a tank into a room, and an overflow box modification that backfired and dripped from the ceiling. None of these required exotic failures, just insufficient headroom meeting an ordinary power blip.

The bottom line

Treat 25% as a floor you verify, not a number you trust blindly. Calculate your actual drain-back, confirm your sump's operating fill line leaves room for it, and remember that baffles, skimmer, and refugium chambers all compete for space the raw gallon rating doesn't show you.