Aquarium Volume, Weight & Fish Stocking Calculator

Understanding the aquarium volume calculator — concept and principle

This calculator finds how much water a rectangular aquarium holds, how much that water weighs and roughly how many fish of a given adult length it can hold under a simple stocking rule. People use it to dose medicines and water conditioners correctly, choose a filter and heater, and check whether a stand or floor can carry the weight.

Filled to 42 cm, the tank holds 151.2 L — room for about fifteen 5 cm fish.

How it is calculated

Water volume (L) = length × width × (height − gap below rim) ÷ 1,000, with all dimensions in cm. 1,000 cm³ is one litre.
Water weight (kg) = volume in litres, because 1 L of fresh water has a mass of about 1 kg.
Fish = volume ÷ 2 ÷ adult fish length, rounded down. This applies a rule of about 1 cm of adult fish per 2 L of water.

The principle

A rectangular tank is a box, so its volume is the base area times the water depth. Water is rarely filled to the rim, so the gap is subtracted from the height. Fresh water near room temperature has a density close to 1 kg per litre, which makes litres and kilograms interchangeable; seawater is about 2–3% heavier. The stocking rule is a crude proxy for how much waste the water can dilute and how much oxygen it can supply. It works only for small, slim fish; a deep-bodied fish produces far more waste than its length suggests.

Worked example

Inputs: 90 × 40 × 45 cm tank, water 3 cm below the rim, adult fish length 5 cm.
Water depth: 45 − 3 = 42 cm.
Volume: 90 × 40 × 42 = 151,200 cm³ = 151.2 L.
Weight of water: 151.2 kg.
Fish: 151.2 ÷ 2 = 75.6 cm of fish; 75.6 ÷ 5 = 15.1, rounded down to 15 fish.

Each point is the calculator re-run with only “Height (cm)” changed; the gold point is the default example. Change the inputs above and this chart follows your numbers.

Height (cm)	Water volume (L)
11.2	29.7
22.5	70.2
33.8	110.7
45	151.2
56.2	191.7
67.5	232.2
90	313.2

How to read the result

The volume uses outer dimensions, so the true water volume is slightly lower because of glass thickness, and gravel, rocks and decorations displace more. For dosing, many keepers reduce the figure by around 10–20%. For load, add roughly 20–30% for glass, substrate and stand: this tank would put about 180–200 kg on the floor, concentrated on the stand's feet. Treat the fish count as an upper limit for small schooling fish and stock gradually so the filter's bacteria can keep up.

Common mistakes

Using total height instead of water depth.
Applying the length rule to large or bulky fish such as goldfish or cichlids.
Ignoring adult size and buying juveniles that will outgrow the tank.

Keep in mind: Stocking rules are rough guides; filtration, aquascape, species temperament and maintenance matter more. Check that your floor and furniture are designed for the total load, and consult a qualified professional for very large tanks.

Frequently asked questions

How do I convert litres to US gallons?

Divide by about 3.785; 151.2 L is roughly 40 US gallons.

Does saltwater change the weight?

Slightly: seawater is about 1.025 kg/L, so the same tank holds about 155 kg of water.

How often should I change water?

Many keepers change around 10–25% weekly, adjusted to test results for nitrate.

Sources: CRC Handbook of Chemistry and Physics (density of water)

Checked 8 Nov 2026. Typical coverage rates and standard formulas. Always check the product label and local building rules; buy 5–10% extra for waste.

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