Aquarium Volume Calculator (Litres & Gallons)

Aquarium Volume Guide

What should I know about Calculating Tank Volume?

Aquarium volume is calculated from the internal dimensions. For a rectangular tank: volume = length × width × height. Working in centimetres gives a result in cubic centimetres, and since 1 litre = 1,000 cm³, you divide by 1,000 to get litres. For example, a 60 × 30 × 36 cm tank is 64,800 cm³ = 64.8 litres gross. For a cylinder: volume = π × radius² × height. Bow-front tanks have a curved front that adds a little volume over a plain rectangle — this calculator approximates it. Gross versus actual water volume: the gross figure assumes the tank is filled completely to the brim with nothing inside. In reality, you fill to a few centimetres below the rim, and substrate (gravel or sand), rocks, and decor displace water. So the actual water volume — the figure that matters for dosing, stocking, and filtration — is meaningfully less than the gross tank size, often by 10-20%. This calculator works out both, so you know the real volume your fish actually live in.

Why Real Volume Matters?

Knowing the actual water volume, not the advertised tank size, matters for several practical reasons. Stocking: fish stocking guidelines are based on water volume. Using the gross tank size overestimates how many fish the tank supports, risking overstocking — a leading cause of poor water quality and fish stress. Medication and treatments: doses are specified per litre or per gallon of water. Dosing to the gross volume overdoses (potentially harming fish or invertebrates); dosing to a guessed figure is unreliable. Knowing the true water volume lets you dose accurately. Water conditioner and fertilisers: same principle — accurate volume means accurate dosing. Water changes: a 25% water change means 25% of the actual water volume, not the tank's nameplate size. Filtration: filter flow rate is recommended as a multiple of tank volume per hour (often 4-6× for tropical freshwater), so you need the real figure to choose an adequately-sized filter. In short, almost every aquarium decision depends on the actual water volume, which is why estimating it properly — accounting for substrate and the unfilled rim gap — is worthwhile.

What should I know about Weight and Placement?

A filled aquarium is extremely heavy, and weight is a critical safety consideration. Water alone weighs 1 kg per litre, so a 100-litre tank holds 100 kg of water — plus the glass (which can be 20-40+ kg for that size), substrate, rocks, and equipment. A modest 120-litre setup can easily exceed 150 kg fully assembled. Placement implications: the stand or cabinet must be designed for aquarium loads and perfectly level — an unlevel tank stresses the glass and silicone seals and can lead to leaks or, rarely, failure. The floor must support the load. Large tanks (200+ litres) concentrate substantial weight on a small footprint; on upper floors or older buildings, consider where the weight bears (ideally over a load-bearing wall or joists, not the middle of a span). Never site a tank on furniture not designed for the weight. Filling: fill a new tank in its final position — you cannot safely move a filled tank even a short distance. This calculator estimates the approximate filled weight so you can check your stand and floor are adequate before setting up. When in doubt about floor loading for a very large tank, seek advice.

What should I know about Using the Volume for Stocking?

Once you know the real water volume, you can stock and equip sensibly. Stocking guidelines: old 'rules' like '1 inch of fish per gallon' are crude and don't account for fish behaviour, adult size, bioload, or territory — use them only as a very rough starting point and research each species' actual needs. Many fish sold small grow large or need shoals/space far beyond what a small tank provides. Understocking is almost always safer than overstocking. Cycling: a new tank must be 'cycled' to establish beneficial bacteria before adding fish — the nitrogen cycle typically takes several weeks. Filtration and flow: match filter capacity to volume (commonly 4-6× turnover per hour for tropical freshwater, more for messy or high-oxygen-need species). Heater wattage: roughly 1-2 watts per litre for tropical tanks, depending on room temperature. Water changes: regular partial water changes (often ~25% weekly) based on the real volume keep parameters stable. Maintenance dosing: conditioners and treatments dosed to the actual volume. This calculator gives you that essential foundation figure — the true water volume — so every downstream decision is based on reality rather than the box the tank came in.

What should I know about Accounting for Substrate, Decor, and Water Displacement?

The simple length × width × height formula calculates a tank's gross volume, but the actual water volume available to fish is always somewhat less, since substrate, rocks, driftwood, and other decor all displace water. A typical allowance is subtracting roughly 10% from the gross volume calculation for average substrate depth and moderate decoration, though a heavily decorated tank with thick substrate and large rockwork can displace meaningfully more than that estimate. This matters most for stocking and dosing calculations — medication, water conditioners, and fertilizer dosages are all based on actual water volume, and consistently overestimating volume by ignoring displacement leads to systematic underdosing, which can be a particular problem when treating disease, where an underdosed medication may fail to clear an infection effectively. Measuring actual water volume directly (noting water level before and after adding substrate and decor) gives the most accurate figure when precision genuinely matters.

Worked Example: Calculating Volume for an Irregular (Bow-Front) Tank

A bow-front aquarium has a flat back panel 90cm wide, a maximum front bow depth of 35cm (compared to a flat-sided equivalent depth of 30cm), and a height of 45cm. Treating it first as a simple rectangular box using the flat-equivalent depth: 90 × 30 × 45 = 121,500 cm³ = 121.5 litres gross. The curved bow front adds extra volume beyond the simple rectangular calculation — manufacturers typically specify this directly, but a reasonable approximation is adding roughly 5-8% for a moderate bow curve.

For an exact figure on a bow-front or other irregularly shaped tank, the manufacturer's stated volume (if available) is more reliable than any formula-based estimate, since curved glass shapes vary too much between manufacturers for one simple adjustment factor to cover every design.

Common Tank Sizes: Gross vs Net Litres

Advertised tank sizes are gross (external glass) measurements, and the real usable water volume is always somewhat lower once substrate, decor and equipment are accounted for:

The roughly 10-13% reduction shown here reflects typical substrate depth and moderate decoration; a heavily planted tank with thick substrate and large rockwork can reduce usable volume by considerably more, which matters directly for stocking calculations based on volume.

Why Filtration Turnover Rate Is Calculated From Volume, Not Tank Size

Filter manufacturers rate their products by flow rate (litres per hour) and recommend a target turnover — commonly 4-6 times the tank's water volume per hour for a typical freshwater community tank, sometimes higher for heavily stocked or messy fish like goldfish or cichlids — and this calculation is based on actual water volume, not the tank's advertised gross size. Using gross volume rather than real net volume (after substrate and decor) when selecting a filter means slightly overestimating the turnover actually being achieved, though the margin involved (typically a 10-15% difference) rarely changes the practical filter choice much given the ranges filters are usually rated in. A more significant error is using the advertised tank size in litres for a tank that's actually a US gallon measurement (1 US gallon ≈ 3.785 litres) or UK/imperial gallon (≈4.546 litres) rather than a true litre figure — a common source of real-world filtration mismatches, since a filter sized for '55 gallons' assuming litres would be dramatically undersized for an actual 55-US-gallon (208-litre) tank.