Fish Tank Volume Calculator — get your real water capacity in litres and gallons.

How the calculator works

From tank dimensions to real water volume, in under 30 seconds.

The Fishhear aquarium volume calculator handles the three tank shapes you'll encounter in 95% of fishkeeping setups: rectangular (the standard L × W × H glass box), cylindrical (round columns and pillar tanks), and spherical (bowls — included for completeness, though we don't recommend them for long-term fishkeeping). It accepts measurements in centimetres, millimetres, or inches, and returns capacity in litres, UK gallons, and US gallons.

What makes our fish tank capacity calculator different

Three things, all rooted in actual aquarium experience rather than pure geometry:

• Glass thickness compensation. On a 200-litre tank with 12 mm glass, the walls occupy 4–6 litres of "missing" volume. Our calculator subtracts this from rectangular tanks when you enter the thickness — important when you're dosing copper-based treatments or sensitive medications where 2% precision matters. Worked example: a 100 × 40 × 40 cm tank with 10 mm glass loses roughly 2.8 litres to wall thickness alone — most "fish tank volume calculators" online ignore this entirely.
• Realistic 90% water-fill estimate. Listed tank capacity is the empty box. Real water volume is roughly 90% of that, because substrate eats 5–10% of volume, decor and equipment displace another few percent, and you need a 1.5–3 cm gap below the rim for filter intakes. We show both figures.
• Stocking guidance with caveats. The classic "one inch of fish per US gallon" rule appears in our output — but flagged as a beginner heuristic, not an absolute. For goldfish, large cichlids, and active swimmers, real stocking math looks different. Read our full stocking guide for the four variables that actually matter.

How to calculate aquarium volume manually (the formula)

If you'd rather work it out by hand, the formulas behind our calculator are simple:

• Rectangular tank: Length × Width × Height (in cm) ÷ 1000 = litres
• Cylinder: π × (Diameter ÷ 2)² × Height (in cm) ÷ 1000 = litres
• Sphere: (4/3) × π × (Diameter ÷ 2)³ (in cm) ÷ 1000 = litres

Worked example, in litres: a 91 × 38 × 45 cm tank is 91 × 38 × 45 = 155,610 cm³ → 155.6 litres gross. Fishhear's calculator defaults to litres because the metric formula above is the one most of the world actually measures with — set your unit toggle to cm or mm and the litres figure comes out directly, no conversion step.

To convert to gallons: multiply litres by 0.219969 for UK gallons, or by 0.264172 for US gallons — they are not the same unit, and mixing them up is the single most common aquarium-calculator mistake we see (a US gallon is about 17% smaller than a UK gallon). Our calculator outputs both automatically, labelled separately, so you never have to guess which "gallon" a spec sheet meant.

The maths is straightforward; the part most beginners miss is that the answer is always the gross volume, not the water you'll actually fill. Use the 90% rule for that.

Common fish tank sizes and their real water capacity

If you've bought a tank advertised at a round number, here's what it actually holds after the 90% fill rule:

For your specific tank, use the calculator above — these table values assume standard rectangular proportions. A tall, narrow tank or a bow-front will give different real-water figures even at the same advertised capacity.

If you shop by gallon size rather than litres, here's the same logic applied to the sizes most commonly sold in the US and UK:

Why accurate water volume matters for fishkeeping

Four reasons, in rough order of how often they catch beginners out:

1. Medication dosing. Most aquarium treatments are dosed per litre or per gallon. Overdose copper-based parasiticides by even 20% and you'll kill your fish faster than the disease would have. Underdose anti-fungals and you'll select for a resistant strain. The 10% gap between gross and real volume sits inside both error margins.
2. Filter sizing. A healthy community tank wants 5–8× water turnover per hour. A "200-litre rated" filter on a tank that actually holds 180 litres of water gives you 5.5× — fine. But a filter rated for 100 L/h on a real 180 L tank gives you 0.55× turnover — pollution accumulates faster than the bacterial colony can process it.
3. Heater wattage. Rule of thumb: 1 watt per litre to maintain temperature 8°C above ambient. Underestimate your real volume and your heater runs constantly and burns out early. Overestimate, and your tank cooks during a hot July.
4. Stocking decisions. The "inch per gallon" rule (or its metric equivalent) breaks down on small tanks, where the difference between 50 litres and 45 litres of actual water means another two cardinal tetras can — or absolutely cannot — be added.

If you're new to fishkeeping, the second use is the most expensive lesson to learn the hard way. Read our buyer's checklist for first aquariums before committing to a filter that's wrong for your real water volume.

Fish tank shape comparison: which holds more water?

Two tanks of identical advertised volume can be very different aquariums. The shape matters for both water capacity and fish welfare:

• Long & shallow (e.g. 100 × 40 × 25 cm): 100 litres gross, ~90 litres real fill, 4,000 cm² surface area. Excellent for community species. Best gas exchange. Easiest to aquascape.
• Tall & narrow column (e.g. 40 × 40 × 62 cm): Identical 100 litres gross, ~90 litres real fill — but only 1,600 cm² surface area. Limits oxygen exchange. Stocks more conservatively than a long-shallow tank of the same volume.
• Cylinder (40 cm diameter × 80 cm): 100 litres gross, similar surface limit to the column. Striking visually but harder to aquascape and maintain.
• Bow-front (90 × 35 × 45 cm with curved front): Roughly 145 litres gross. The curve adds 10–15% volume over a rectangular tank with the same back-panel dimensions.

Bow-front aquarium volume calculator: working it out by hand

Our calculator handles rectangular, cylinder, and sphere shapes directly — bow-fronts don't have a clean single formula because the curve radius varies by manufacturer, so here's the working aquarist's approximation instead of a wrong "exact" one:

1. Treat the tank as rectangular first. Use the back-panel length, the full depth (front to back at its widest point), and the height, in our rectangular calculator above.
2. Add 10–15% to the result. The curved front bulges outward beyond a flat panel, adding that much extra volume. Use 10% for a gentle bow, 15% for a pronounced one.
3. Worked example: a bow-front with a 90 cm back panel, 35 cm depth, and 45 cm height gives 90 × 35 × 45 ÷ 1000 = 141.75 litres as a rectangular base figure. Add 10–15% → roughly 156–163 litres gross, which matches manufacturer specs for this size class closely.

For dosing and stocking purposes, run that adjusted gross figure through the same 90% real-fill rule used elsewhere on this page.

Real water volume vs. stated tank capacity: why they're never the same number

Every tank you can buy is advertised by its stated capacity — the empty internal box, calculated from external dimensions with a rough allowance for glass. Nobody fills a tank to the literal brim and puts nothing inside it, so that number is never what you actually work with day to day.

Real water volume is what's left after three things eat into stated capacity: substrate (5–10%, depending on depth), decor and equipment (a few more percent), and the rim gap you need for filter intakes and gas exchange (1.5–3 cm). Together, that's the basis of the 90% rule our calculator applies by default — a 200-litre stated tank holds roughly 180 litres of actual water once it's set up and running.

The gap matters most for medication dosing and filter/heater sizing (see below) — it rarely matters for simply comparing two tanks before you buy one.

Pair the volume calculator with the feeding calculator

Once you know your real water volume, the next question is: what do I feed the fish that live in it? Our fish feeding calculator covers 28 freshwater and marine species — bettas, tetras, guppies, goldfish, cichlids, clownfish, tangs, and more — with feeding schedules, recommended food types, and species-specific notes. Free, like everything on Fishhear.