Fish Tank Capacity Calculator
Figure out exactly how much water your aquarium holds and how many fish it can support. This fish tank capacity calculator handles rectangular, cube, and cylindrical tanks, subtracts substrate displacement, and applies the right stocking rule for your water type.
How the fish tank capacity calculator works
Tank volume is geometry. Measure in inches, then convert:
- Rectangular: length x width x height / 231 = gallons
- Cube: side^3 / 231 = gallons
- Cylindrical: pi x (diameter/2)^2 x height / 231 = gallons
- Liters: gallons x 3.785
- Water weight: 8.34 lb per gallon for freshwater, 8.56 lb per gallon for saltwater
Gross volume is only half the story. Substrate, rocks, and decorations typically displace 5 to 15 percent of the water. A planted tank with 3 inches of aquasoil and heavy hardscape can lose 20 to 25 percent. This calculator subtracts whatever displacement you enter.
Stocking: how many fish can the tank hold
The traditional "1 inch of fish per gallon" rule only works for slim-bodied community fish in tanks larger than 10 gallons. Body shape, waste output, and activity matter more than raw length. Here is a more realistic stocking guide:
- Community freshwater (tetras, rasboras, livebearers): 1 inch of adult fish per gallon
- Cichlids and larger freshwater fish: 1 inch per 3 gallons
- Goldfish and koi: 1 inch per 10-20 gallons (they produce 3-4x the waste of similar-sized tropicals)
- Saltwater fish-only: 1 inch per 4-5 gallons
- Reef tank: 1 inch per 5-7 gallons (invertebrates are sensitive to nitrate)
Surface area matters more than depth
Oxygen enters the water at the surface. A long, shallow tank of 55 gallons holds more fish comfortably than a tall narrow 55-gallon hex, because gas exchange depends on surface area. For high-oxygen species like goldfish or planted tank with heavy plant mass, prioritize a footprint over height.
Weight and structural load
Water weighs about 8.34 lb per US gallon, so a 75-gallon tank holds about 625 lb of water alone. Add glass (around 150 lb for a 75), substrate, rock, and stand, and the load reaches 900 lb or more. Anything above 55 gallons should sit on a load-bearing wall or be placed parallel to floor joists. Check with a structural engineer for tanks over 125 gallons on upper floors.
Cycling a new tank
The tank must go through the nitrogen cycle before heavy stocking. Add an ammonia source (fish food or pure ammonium chloride), test weekly, and wait for nitrites and ammonia to fall to zero and nitrates to appear. That usually takes 4 to 6 weeks. Add fish gradually after that, never more than 25 percent of final stocking at a time.