This free aquarium sump size calculator tells you what sump volume and return-pump flow a display tank needs for reef and large setups. It runs fully in your browser, so nothing you type is sent to a server, and you need no account and no sign-up to use it.
Aquarium problems usually build slowly. A tank that looks fine today can drift over weeks when feeding, stocking, or water changes stay unmeasured. Small numbers, checked early, stop most trouble before fish show stress. This tool turns a guess into a number you can act on, and that number is easier to trust than a rough rule of thumb from a shop assistant.
To use the tool, type in Display tank size (litres). Press the calculate button and the tool shows the result at once. If you change any number, press calculate again to see the new result right away. Try a few values to see how sensitive the result is to each input.
Example: a 200 litre display tank needs a sump of about 40 litres and a return pump moving about 1000 litres per hour.
Leave headroom in the sump for a power cut. When the pump stops, water drains back down, so a sump filled to the brim will flood the floor the first time power fails.
Size the pump for head height, not just the label. A pump rated at 1000 litres per hour at zero height may deliver far less after lifting water a metre to the display tank.
Keep the return section steady. An auto top-up in the return section stops the pump running dry as water evaporates, which is the most common sump failure.
Common question: Why use a sump at all? Because it hides equipment, adds water volume for stability, and gives space for a skimmer, heater, and refugium away from the display fish.
Common question: How much turnover does a reef need? About five times display volume per hour through the sump is a solid starting point, with extra flow inside the tank from powerheads.
Common question: Can a sump be too big? Rarely, as long as it fits the cabinet and the pump matches. A larger sump adds stability, but it must still leave drain-back room for power cuts.
Keep a short notebook by the tank. Write the date, the numbers you entered, the result, and anything you changed afterwards. That short record turns a vague feeling that something changed into clear evidence. After a month you can look back and see which choices helped and which ones did not, and that makes the next decision much easier.
The most common mistake is changing too much at once. When several things change in the same week, you cannot tell which change caused the result you see. Change one thing, wait, watch the fish and the water, and only then change the next thing. Slow, single changes keep a tank stable, and stable tanks keep fish healthy.
Recheck the numbers whenever anything changes in the tank. New fish, a new filter, a different food, or a change in room temperature can all shift the right answer. A quick recheck takes less than a minute and stops an old number from quietly becoming wrong. Bookmark this page so the recheck is easy.
This tool is free to use as often as you like. Bookmark it, recheck the numbers when your tank changes, and keep the daily routine steady. Small, steady habits matter far more for an aquarium than any single perfect number, and a calm routine is what fish respond to best. If a number surprises you, run the tool twice with different inputs before you act on it, and trust the steadier result.