Rectangular Tank Volume
length × width × 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 rectangular formula
Volume in US gallons is length × width × 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 glass, not the outside of the trim. On a rimmed tank the plastic adds roughly a quarter inch on each side, and measuring the outside of a 48-inch tank can overstate volume by a gallon or more.
What this shape does to your stocking limit
A rectangle gives the most surface area per gallon of any common shape, which means better gas exchange and a more forgiving stocking limit. It is the shape every standard size is built in.
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
- Cube tanksedge³ ÷ 231
- Cylinder tanksπ × (diameter ÷ 2)² × height ÷ 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 rectangular tank?
length × width × height ÷ 231, with every dimension in inches. Dividing cubic inches by 231 gives US gallons.
How do I measure a rectangular tank correctly?
Measure the inside glass, not the outside of the trim. On a rimmed tank the plastic adds roughly a quarter inch on each side, and measuring the outside of a 48-inch tank can overstate volume by a gallon or more.
Does tank shape change how many fish I can keep?
A rectangle gives the most surface area per gallon of any common shape, which means better gas exchange and a more forgiving stocking limit. It is the shape every standard size is built in.
Keep going with the same pet
Got your gallons? Check how many fish that really supports.
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How much to feed: daily calories and cups by weight, activity and life stage.
Bioload-based stocking guidance, not the oversimplified 1-inch-per-gallon rule.