Aquarium Volume Calculator

Measure the inside, not the outside

Glass and rims add a centimetre or two to every dimension. Measure the internal length, width and height where you can, or subtract the glass thickness from external measurements. For water level, the calculator assumes 90 percent of the height by default, which suits a tank filled to just below the rim with a lid; drop it if you run the water lower to protect a hang-on-back filter or to keep a jumper in.

Shapes that are not rectangles

Bow fronts are handled by the calculator: measure the extra depth at the centre of the curve rather than the total depth.

Cubes and columns are simple to measure and worth a separate warning — they give less swimming length per litre than long tanks, which choosing a goldfish tank covers.

Corner and hexagonal tanks are awkward. Treat a hexagon as a rectangle using the width across the flats and, for a regular hexagon, expect to overestimate by about 15 percent.

Tubs and stock tanks usually carry no stated volume at all, which matters because the tank buying guide recommends them for large single-tails. Measure the inside at the water line rather than the rim, since most tubs taper. For a round one: radius squared, times pi, times the water depth in centimetres, divided by 1,000.

Ponds are an estimate at best, and building a goldfish pond covers why the average-depth method is the only practical one.

Check it once, properly

A measurement error becomes a dosing error, so it is worth confirming the figure the first time. Either fill through a container of known volume and count it, or — easier — do a water change with a marked bucket and see whether the drop in level matches the proportion you expected. The second costs nothing because you were changing water anyway.

Keep the number written somewhere near the tank. Every dose of dechlorinator, medication or aquarium salt is calculated from it, and salt in particular is a case where the concentration is the treatment — dosing against a nominal volume rather than a real one is how people accidentally under-treat or overdose.

What to do with the number

Knowing your true water volume is the foundation for everything else. Dechlorinator, medications and salt are all dosed by volume, and overdosing because you trusted the box is a common way to hurt fish. The same figure feeds the stocking checker, which tells you whether the tank can support the goldfish you have in it, and the tank size guide, which explains why footprint matters more than height.

What this calculator cannot know

It works from the numbers you give it, so its answer is only as good as those. Four things it cannot see:

• How deep your substrate actually is. The displacement field is your estimate, not a measurement. A bed that looks like two centimetres at the front is often four at the back, because goldfish push it there.
• Irregular shapes. Bow fronts are approximated as a half ellipse, a round tub works as a cylinder, and an oval tub is length times width times depth times 0.785. All are estimates, since tubs taper, which is fine for stocking and close enough for dosing, but an estimate rather than a figure.
• Water that is in the system but not in the tank. A sump, a canister body and its hoses hold several litres that are part of the volume a medication disperses into. For dosing, that works in your favour; for the oxygen and waste arithmetic, it does not.
• Whether you measured inside or outside the glass. The calculator cannot tell, and on a 90 cm tank the difference is roughly two percent per dimension.

Where the number matters most — calculating a dose — round it down. Under-dosing a medication is a treatment that works less well; over-dosing one is a common way a treatment becomes the problem, which using medications safely sets out in full.