Softonic

Aquarium Size Calculator

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Enter the inner dimensions of a rectangular aquarium and the calculator returns gross volume, net volume (accounting for substrate and displacement), filled weight for load planning, and a rough stocking guide using the classic 1-inch-of-fish-per-gallon heuristic. Essential before drilling into a floor or spec’ing a stand.

How to plan an aquarium size

  1. 1

    Enter interior dimensions

    Length, width, height in cm or inches. Use the glass inner face measurements, not outer, for accurate volume.

  2. 2

    Set fill level

    Most tanks are filled 2-5 cm below the rim. The tool subtracts the airspace from volume.

  3. 3

    Add substrate depth

    Gravel or sand occupies 10-15% of tank depth typically. Net usable water volume is lower than the geometric calculation.

  4. 4

    Read volume, weight and stocking

    Volume in US gallons, UK gallons and litres; weight filled (water plus substrate) for floor-load checks; rough stocking range in inches of fish.

Volume formula

A rectangular tank:

Volume (litres) = L × W × H / 1000   (where L, W, H are in cm)
Volume (US gallons) = L × W × H / 231   (where L, W, H are in inches)

1 US gallon = 3.785 litres. 1 UK gallon = 4.546 litres. Tank specs in the US are almost always US gallons; UK hobbyist specs can be either — check.

Standard aquarium sizes

Nominal size Dimensions (in) Volume (gal) Weight filled (lb)
10 gal 20 x 10 x 12 10 ~110
20 gal high 24 x 12 x 16 20 ~220
29 gal 30 x 12 x 18 29 ~325
40 gal breeder 36 x 18 x 16 40 ~450
55 gal 48 x 13 x 21 55 ~625
75 gal 48 x 18 x 21 75 ~850
90 gal 48 x 18 x 24 90 ~1,050
125 gal 72 x 18 x 21 125 ~1,400

Water weighs 1 kg per litre (8.34 lb per US gallon). Add 10-15% for substrate, rock and glass weight.

The 1-inch-per-gallon rule

The classic beginner heuristic: 1 inch of adult fish length per US gallon of net water. A 30-gallon tank supports about 30 inches of fish total. Caveats:

  • Only valid for slim-bodied community fish (tetras, rasboras, small barbs).
  • Not valid for large-bodied cichlids, goldfish (which produce enormous bioload) or schooling species that need shoals.
  • Plecos and large catfish can outgrow any reasonable rule — always check adult size, not shop size.

Floor loading

A 75-gallon setup weighs around 850 lb concentrated over a 48 x 18 inch footprint, which is about 8 lb per square inch. Wooden residential floors can usually handle it over a joist, but always place the stand perpendicular to floor joists near a load-bearing wall. For tanks over 150 gallons, consult a structural engineer.

Frequently Asked Questions

Break the shape into geometric primitives (rectangle + segment of a circle for bow-fronts) or use the volume printed on the manufacturer’s spec sheet. Rough approximation: for a bow-front, add 5-8% to the rectangular equivalent.

Nameplate capacity is usually the geometric volume. Actual water capacity after accounting for the airspace at the top (usually 1-2 inches) and any internal bracing is typically 10-15% less.

As a rough ceiling for beginner community tanks, yes. For anything complex (large predators, planted biotopes, reef tanks) it is meaningless — stocking is driven by bioload, territorial space, and filtration capacity, not linear length.

Plants displace very little water (under 2% typically). Hardscape (rocks, driftwood) can displace 10-20% of gross volume in a heavily-decorated tank. Subtract hardscape volume from net water for dosing and filter sizing.

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