Aquarium Volume Calculator (Litres & Gallons)

Aquarium Volume Guide

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.

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.

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.