Use the free Aquarium Overflow Calculator below, right in your browser. No sign-up, no software to install, and nothing is sent to a server.
An aquarium overflow moves water from the display tank down to a sump by gravity, and its capacity is set mainly by the size of the drain pipes. If the return pump pushes more water up than the drains can carry down, the display tank level rises and the sump can run dry or flood. This calculator estimates gravity flow from pipe diameter so you can match a pump to your plumbing before you buy anything.
Enter the inside diameter of one drain pipe in millimetres and the number of drains in your system. The tool estimates the flow each drain can carry, adds the drains together for total capacity, and then suggests a return pump rated at about eighty percent of that total. That twenty percent margin covers bends, valves, algae film and the small differences between one plumbing layout and another.
The estimate uses a conservative rule of about two point two litres per hour for each square millimetre of pipe cross section. A twenty five millimetre drain therefore comes out near one thousand three hundred and seventy five litres per hour, while a forty millimetre drain comes out near three thousand five hundred and twenty litres per hour. Real flow also depends on the height of the drop and on how the pipe is routed, so treat the number as a planning figure, not a laboratory measurement.
Start by measuring the pipe itself rather than trusting a label. Plumbing sold as one inch often has an inside diameter near twenty five millimetres, but fittings and pipe standards vary between brands and countries. If you are unsure, measure across the inside of the pipe opening with calipers or a ruler, and use the smaller figure when two measurements disagree, because a narrow section limits the whole drain.
Count every drain that genuinely carries water during normal running. A second drain that sits higher as an emergency backup should not be counted at full value, because it only starts flowing when the water level rises above its intake. Count it separately in your head as safety capacity, and size the pump against the main drains only. That way a blocked main drain still leaves you with a backup path.
The suggested pump figure is deliberately below total overflow capacity. A pump rated exactly at the drain limit will overflow the tank as soon as a snail, a plant leaf or a film of algae narrows the pipe slightly. Pumps also lose flow as they push water higher, so check the pump curve at your real head height rather than the headline figure on the box. A valve on the return line lets you trim the flow down, but it cannot make a weak drain carry more water.
Pipe layout matters as much as diameter. Long horizontal runs, tight elbows and valves all slow the flow, and a drain that ends below the sump water level can gurgle and surge as air gets trapped. Keep runs as short and straight as practical, use swept bends instead of sharp elbows where you can, and give air somewhere to escape at the top of a standpipe. A quiet drain is usually a drain that is not running near its limit.
If the calculator shows your planned pump is too strong, you have three honest options. You can choose a smaller pump, add another drain, or fit a larger drain pipe. Adding a drain is often the safest fix on a new build, because it adds redundancy as well as capacity. On an existing tank, a smaller pump with a valve is usually cheaper and less disruptive than drilling new holes.
Test the real system before adding fish. Fill the sump, start the pump, and watch the display level for at least thirty minutes. Then unplug the pump and check that the sump can hold all the water that drains back down. Repeat the test with the return valve fully open, because that is the worst case. If the sump nearly overflows during the power cut test, lower the sump water level or fit a larger sump.
Clean the drains on a regular schedule, because overflow capacity falls as pipes narrow with growth. Check strainers and standpipe slots every week, and look inside the pipes every few months. A drain that was fine in January can be a flood risk by summer if it slowly clogs. Write down your pipe sizes and the calculated flow, so future pump replacements start from known numbers instead of guesses.
Versión en español: Caudal de rebosadero para acuarios – Según diámetro de tubo