Tank Size Calculator

How this calculator works

It adds up a working range for each fish you enter. Fancy goldfish are given 75 to 100 litres for the first fish and 40 to 50 litres for each additional one. Single-tailed goldfish, which grow far larger, are given 150 to 200 litres for the first and 75 to 100 litres for each additional fish. When you mix both types, the larger single-tail sets the base figure.

The lower figure is the minimum, the higher is what we would call comfortable. Both assume the fish reach adult size, because in decent conditions they do: a fancy goldfish at 15 to 20 cm, a common or comet at 25 to 35 cm, measured with the tail. Buying the tank for the fish you have today, rather than the fish it will become in two years, is a common mistake in goldfish keeping.

Turning litres into a tank you can actually buy

The number this gives you is water, and tanks are sold by nominal volume, which is measured to the brim. A tank labelled 100 litres holds nearer 90 in practice once the gap under the rim is accounted for, and less again after substrate, rocks and equipment displace their own volume. Real water volume is often 10 to 25 percent below the nominal size, which usually means buying one size up from the figure here rather than matching it exactly.

Then convert to a footprint rather than shopping on litres alone. The volume calculator works in the other direction, turning dimensions you are looking at into litres, and choosing a goldfish tank covers the standard sizes, the shapes to avoid, and the two measurements people forget: the space to reach behind the tank, and the route from the front door to the stand.

And check what it will stand on before you buy it. Water weighs a kilogram a litre, so the figure this calculator produces is also, near enough, the weight in kilograms that the floor will carry once furnished — tank placement and floor loading covers when that becomes a real question rather than a formality.

Three worked examples

Two fancy goldfish. 75 to 100 litres for the first, plus 40 to 50 for the second, gives 115 to 150 litres of water. Add the allowance and you are shopping for a tank sold at 130 to 200, ideally long rather than tall. A standard 40-gallon breeder, about 150 litres on the label, is the classic answer.

One common goldfish, bought at 5 cm. 150 to 200 litres, because the calculator is sizing for the 25 to 35 cm adult it becomes rather than the fish in the bag. This is the case where the number looks absurd and is not — how big goldfish get is the page that usually settles the argument.

Three fancies and a comet. The single-tail sets the base at 150 to 200, plus roughly 40 to 50 for each fancy, so 270 to 350 litres. The more useful conclusion is that the combination itself is the problem: a comet outpaces a fancy at every meal, and chasing and bullying covers why that quietly starves the slower fish whatever the volume.

If you cannot meet the figure

Worth saying plainly, because the alternative is that someone reads a number they cannot reach and stops reading. A tank below the comfortable figure is not an emergency; it is a tank that needs more of your attention, and the real levers are fewer fish, more frequent water changes, more filtration and lighter feeding — in that order of effectiveness. Goldfish tank size has a section on running a tank that is smaller than it should be, deliberately and well.

The one thing that does not work is hoping the fish stays small. Goldfish held in too little water do not become naturally compact; they become stunted, and the usual result is a shortened life rather than a healthy small fish. If the gap is large and permanent, rehoming is a legitimate answer rather than a failure.

What the numbers do not capture

Footprint matters more than volume. A long, wide tank gives more swimming room and more surface for gas exchange than a tall one holding the same water, which is why the results suggest standard footprints rather than only litres. Filtration and your water-change routine are the other half of the equation; the stocking checker takes both into account for a tank you already have.

Adult single-tailed goldfish are pond fish. The calculator will give you a tank figure, but a common or comet that reaches 30 cm is happiest in 280 litres or more, or outdoors. The pond guide explains what that involves.