How to work out your aquarium's volume in gallons or litres

The formula for a rectangular tank

Almost every figure you'll need downstream of this — how much medication to dose, how much water to change, whether a filter or heater is sized correctly — ultimately depends on a single number: how much water your tank actually holds. For the common case of a rectangular tank, that's straightforward geometry rather than anything specific to fishkeeping[1]:

231 is simply the number of cubic inches in one US gallon, and dividing by 1,000 converts cubic centimetres to litres, since a litre is defined as 1,000cm³. If you're working in UK (imperial) gallons instead of US gallons, divide the litre figure by 4.546 rather than the inch figure by 231 — the two gallon definitions aren't the same size, and mixing them up is a common source of a wrong answer that still looks plausible.

A worked example

Take a tank measuring 24 inches long, 12 inches wide and 16 inches tall, measured to the inside of the glass:

24 × 12 × 16 = 4,608 cubic inches

4,608 ÷ 231 ≈ 19.95 US gallons

That's a genuine "20 gallon" tank, and it's typical for the calculated figure to land close to but not exactly on the round number printed on the box — manufacturers round to a marketable size, and small variations in glass thickness between models shift the true internal dimensions slightly. Measure to the inside face of the glass rather than the outside of the frame for the most accurate result; on a larger tank with thick panes, the difference between inside and outside measurements is enough to matter.

Bowfront, cylinder and other shapes

The length × width × height formula only applies to a true rectangular box, and a cylindrical tank uses an entirely different formula based on radius rather than width (π × radius² × height, then divided by 231 or 1,000 the same way). A bowfront tank is the hardest of the three to pin down from simple external measurements, since its curved front panel doesn't reduce to a single width figure the way a flat-sided tank does. Our free Aquarium Calculator's bowfront mode uses the same length × width × height approximation as its rectangular mode, taken from the tank's widest point — it doesn't calculate the curve's own geometry, so treat the figure it gives you as a reasonable estimate rather than an exact, curve-integrated volume. Hexagonal, corner and custom-cut tanks each have their own geometry again, and none of these shapes reduce cleanly to a hand calculation. Whatever shape you're working from, the actual usable water volume will typically run below any calculated figure once shape, glass thickness, substrate, decor and equipment displacement are all accounted for — covered next.

Why "gross volume" isn't what's actually in the tank

Every calculation above answers "how much water would this tank hold, filled completely, with nothing in it" — the tank's gross volume. That's rarely the number you actually want. In practice:

• The water line usually sits below the rim, often by an inch or two, both to leave room for filter flow and to avoid splashing over the top.
• Substrate displaces water — a couple of inches of gravel or sand across the base measurably reduces the water volume, proportionally more in a smaller tank than a large one.
• Rock, wood and hardscape displace more again, and the amount varies enormously by how heavily aquascaped the tank is.

For rough purposes — sizing a filter, getting a general sense of stocking — the gross figure is a reasonable starting point. For anything dosed by volume, it isn't precise enough on its own; see below.

Why the exact number matters more for some things than others

How much precision you need depends entirely on what you're using the figure for:

• Filter and heater sizing tolerate a reasonable estimate, since both are sold in bands rather than an exact match to one specific litre figure. See our aquarium heater wattage guide and the free Filter Turnover Calculator.
• Medication dosing is much less forgiving. Under-dosing risks a treatment not working, and over-dosing carries a real toxicity risk for some medications and for scaleless or invertebrate-sensitive livestock specifically. Calculate your actual water volume properly before dosing, and use the free Medication Dosing Calculator once you have it.
• Reef and planted dosing compound small errors over repeated doses, since you're typically dosing regularly rather than once. The free Reef Dosing Calculator and Planted Aquarium Fertiliser Calculator both start from your tank's actual volume for exactly this reason.
• Water changes sit in between — getting the percentage roughly right matters more than the exact litre figure. The free Water Change Assistant works from your volume either way.

Weight: the other reason volume matters

Once you know your tank's volume, you also know roughly how much it weighs — and that's a separate, genuinely important question before a large tank goes anywhere near an upstairs floor or an unreinforced stand. Freshwater is close enough to 1kg per litre to use directly; saltwater and reef water is measurably denser, at roughly 1.025kg per litre for typical aquarium seawater[2]. That's before adding the tank's own glass or acrylic weight, the stand, substrate and rock — all of which add up quickly on anything above a small tank. The Aquarium Calculator converts your volume to water weight in kilograms and pounds directly, using this same freshwater/saltwater distinction.

Common mistakes

• Mixing up US and UK gallons — a UK (imperial) gallon is about 20% larger than a US gallon, and using the wrong one skews every downstream dosing calculation by that same margin.
• Measuring to the outside of the glass or frame rather than the inside, overstating volume on a large tank with thick panes.
• Using a rectangular formula on a cylinder tank, which will noticeably overstate the true volume — and assuming a bowfront calculation is exact rather than an approximation based on the tank's widest point.
• Dosing medication or reef supplements against the gross (empty-tank) volume on a heavily aquascaped tank, without accounting for how much substrate and hardscape actually displaces.
• Assuming the number on the box is exact when it's a rounded, marketable size rather than a measured figure for your specific tank.

Frequently asked questions

How AquaPulse can help

Your tank's volume doesn't change often, but almost every other calculation you'll make about it starts from that one number — which makes it worth recording somewhere you won't have to recalculate it from memory every time. Once you've worked it out, log your readings and some context about your tank, including its volume, in AquaPulse, so it's already there the next time you need it for a dosing or water-change calculation. AquaPulse doesn't measure your tank for you or verify your working — it just holds onto the number once you've worked it out. Get it on the App Store or Google Play.

Sources and editorial information

Good to know: This guide provides general informational guidance only. For dosing decisions that affect livestock health, calculate your tank's actual water volume carefully and follow your product's own instructions.

• [1] National Institute of Standards and Technology (NIST) — unit conversion reference Used for the US gallon (231 cubic inches) and litre (1,000cm³) conversion constants used throughout this guide.
• [2] Merck Veterinary Manual — "Environmental Diseases of Aquatic Animals in Aquatic Systems" Used for the approximate density distinction between freshwater and typical aquarium seawater referenced in the weight section.

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