Cylinder Tank Volume
π × (diameter ÷ 2)² × height ÷ 231. Enter your measurements for gallons and liters.
Volume
28.05 gal
Liters
106.2 L
This is the full water volume; actual capacity is a bit lower once you add substrate, rocks and the fill line.
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The cylinder formula
Volume in US gallons is π × (diameter ÷ 2)² × height ÷ 231, with every measurement in inches. The 231 is fixed: a US gallon is exactly 231 cubic inches, so cubic inches divided by 231 is gallons, and gallons × 3.785 is liters.
Measuring it without inflating the number
Measure the inside diameter across the widest point, and the height from the inside bottom to the intended water line, not the rim.
What this shape does to your stocking limit
A cylinder is tall for its footprint, so surface area per gallon is low. Oxygen exchange suffers, and the same gallon count supports fewer fish than a rectangle would. Curved glass also magnifies, so the tank looks fuller than it is.
Standard sizes in this shape family
Nominal gallons and the real footprint they ship with.
- 5 gallon tank16" × 8" × 10.5"
- 10 gallon tank20" × 10" × 12"
- 20 gallon tank24" × 12" × 16"
- 29 gallon tank30" × 12" × 18"
- 40 gallon tank36" × 18" × 16"
- 55 gallon tank48" × 13" × 21"
- 75 gallon tank48" × 18" × 21"
- 90 gallon tank48" × 18" × 24"
- 125 gallon tank72" × 18" × 21"
Volume by tank shape
- Rectangular tankslength × width × height ÷ 231
- Cube tanksedge³ ÷ 231
- Bowfront tankslength × width × height × 1.06 ÷ 231
- Hexagonal tanks(3√3 ÷ 2) × edge² × height ÷ 231
What to do with the number
- What bioload actually isGallons is the denominator, not the answer
- Cycle the tank firstFishless cycling, step by step
Frequently asked questions
How do you calculate the volume of a cylinder tank?
π × (diameter ÷ 2)² × height ÷ 231, with every dimension in inches. Dividing cubic inches by 231 gives US gallons.
How do I measure a cylinder tank correctly?
Measure the inside diameter across the widest point, and the height from the inside bottom to the intended water line, not the rim.
Does tank shape change how many fish I can keep?
A cylinder is tall for its footprint, so surface area per gallon is low. Oxygen exchange suffers, and the same gallon count supports fewer fish than a rectangle would. Curved glass also magnifies, so the tank looks fuller than it is.
Keep going with the same pet
Got your gallons? Check how many fish that really supports.
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Bioload-based stocking guidance, not the oversimplified 1-inch-per-gallon rule.