Aquarium Sump Volume Calculator

Enter your sump and display tank sizes. You get the water the sump holds while running, how much drains down in a power cut, and whether the sump can take it.

Units
Measure from the bottom of the sump to the water line with everything running.
Switch the pump off and watch the water line fall. It stops at the overflow teeth or at the siphon-break hole in the return pipe.
Pipes, baffles and glass thickness
This drains down too. Half a gallon (2 L) suits most tanks.
Glass baffles and equipment leave less room for water.
Water in the sump while running12.6 US gal47.7 L
If the pump stopsIt holds3.9 in to spare below the rim

With the pump running, the sump holds 12.6 US gal (47.7 L). When the pump stops, 4 US gal (15.2 L) drains down from the display tank and the pipes, and the sump's water rises 2.9 in (7.3 cm) to 11.9 in (30.2 cm) deep. That leaves 3.9 in (9.9 cm) below the rim, so it is safe.

Water that drains down
4 US gal (15.2 L)
The sump rises by
2.9 in (7.3 cm)
Highest safe running level
11.9 in (30.2 cm)
Sump filled to the brim
23.2 US gal (87.9 L)
  • Test it for real before you trust it: fill the sump to its running level, switch the pump off, and watch. Mark the highest safe running level on the glass.

What to work out next

Aquarium volumeReal water in the display tankWater changeCount the sump water tooSalt mixSalt for the whole system

What a sump has to hold

A sump is a second tank under the main one. Water falls into it through an overflow, and a pump sends it back up. It hides the heater and filter, and it adds water to the system.

It also has one job that matters more than the rest. When the pump stops, water keeps draining down until the display tank's level reaches the overflow teeth, or until the return pipe sucks air. The sump has to catch all of that water. If it cannot, the water ends up on the floor.

So a sump is never run full. The empty space above the water line is there on purpose.

How the sump is worked out

Every step is an area multiplied by a height.

Water in the sump = sump length × width × water depthMeasured inside the glass. Baffles and equipment take a little of the room.
Drain-back = display tank length × width × the drop in its water level, plus the water in the pipesThe drop is how far the display tank's water line falls when the pump is off.
Rise in the sump = drain-back ÷ the sump's water surface

Worked example. A 30 × 12 × 16 inch sump runs with 9 inches of water under a 48 × 18 inch display tank.

  1. The sump holds about 12.6 US gallons (47.7 liters) while running.
  2. When the pump stops, the display tank's level falls 1 inch. Its water surface is about 47 × 17 inches inside the glass, so 1 inch of it is 3.5 gallons. Add half a gallon from the pipes: 4 gallons (15.2 liters) drain down.
  3. Those 4 gallons raise the sump's water by 2.9 inches, to 11.9 inches deep.
  4. The sump is about 15.8 inches deep inside, so 3.9 inches are left below the rim. It holds.

How to measure the drop

Do not guess the drop. Measure it.

  1. Mark the display tank's water line with tape while the pump runs.
  2. Switch the pump off.
  3. Wait until water stops falling into the sump.
  4. Measure from the tape down to the new water line.

Two things set the drop. One is the height of the overflow teeth or weir. The other is the return pipe: water siphons back down it until the outlet, or a small siphon-break hole drilled just below the water line, sucks air. A return outlet deep under water drains far more than one near the surface.

How big should a sump be?

There is no single right size. A bigger sump is easier in every way: more water, more room for equipment, and more space for the drain-back. The hard limit is the one this calculator checks. The sump must hold its running water plus everything that drains down, with a gap to spare.

If the answer is "it overflows", you have three fixes:

  • run the sump at a lower water level;
  • raise the siphon-break hole or the return outlet, so less water drains;
  • use a bigger sump.

Common mistakes

  • Filling the sump to the top while the pump runs. It leaves no room for the water that drains down.
  • Trusting a check valve to stop the drain-back. They clog with algae and snails, and then they fail. The sump must hold the water without one.
  • Guessing the drop. One inch of water in a large display tank is several gallons. Switch the pump off and measure.
  • Forgetting the siphon-break hole. Without one, a submerged return pipe can siphon the tank down to its outlet.
  • Topping up while the pump is off. With the pump off, the display tank looks low and the sump looks full. Top up only while the pump runs, and only to your marked line.

Questions and answers

How do I calculate sump volume?

Multiply the inside length by the inside width by the water depth. In inches, divide by 231 for US gallons. In centimeters, divide by 1,000 for liters. Use the water depth, not the full height, because a sump is never run full.

How much water drains into the sump when the pump stops?

The display tank's length times its width times the drop in its water level, plus the water in the pipes. For a 48 × 18 inch tank that drops 1 inch, that is about 4 US gallons (15 liters).

How full should a sump be?

Low enough that the drain-back still fits with a gap below the rim. This calculator gives the highest safe running level. Mark it on the glass and top up to that line only.

Will my sump overflow in a power cut?

Only if the running water plus the drain-back is more than the sump holds. Enter your sizes above to check, then test it for real: switch the pump off and watch.

Does the sump count toward my tank's water volume?

Yes. The water in the sump is part of the system. Count it when you dose treatments, mix salt, or plan water changes.

Numbers for standard tanks

Have a standard tank? Its size, weight, heater and stocking are already worked out.

Sources

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