Aquarium Calculator
Calculate tank volume, fish stocking capacity, and equipment recommendations.
This tool is for informational and educational purposes only. It is not a substitute for professional financial, medical, legal, or engineering advice. See Terms of Service.
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Request a ToolHow to Use the Aquarium Calculator
Select your tank shape, enter the dimensions in inches, and the calculator instantly shows volume plus equipment recommendations.
- Select tank shape. Rectangular tanks are the most common. Bow-front tanks curve outward at the front glass, adding roughly 10% more volume than the same footprint in a flat-front tank. Cylindrical tanks use the diameter and height only.
- Enter dimensions in inches. Measure the inside glass dimensions, not the outer cabinet size, for the most accurate volume. Standard 55-gallon tanks are approximately 48L × 13W × 21H inches.
- Read the results. The calculator shows volume in gallons and liters, the maximum fish stocking by the one-inch-per-gallon rule, heater wattage recommendation, and minimum filter flow rate.
About the Aquarium Calculator
US gallons are calculated by dividing cubic inches by 231. For cylindrical tanks, volume uses the circle area formula (pi × r² × height) before dividing by 231. Bow-front tanks use the rectangular formula multiplied by 1.1 as an approximation for the added volume of the curved front panel.
The one-inch-per-gallon stocking rule is a starting guideline for small, slim freshwater fish. It does not account for fish bioload, aggression, or adult size. Heavily built fish like goldfish and cichlids need more gallons per inch. The heater recommendation of 4 watts per gallon and the filter recommendation of 4x tank volume per hour are standard minimum guidelines for most freshwater setups.
Frequently Asked Questions
How many gallons is a standard 48x13x21 inch aquarium?
A rectangular tank measuring 48 × 13 × 21 inches has a volume of 56.7 gallons. This is close to the nominal "55-gallon" tank sold at most pet stores, which actually holds about 55 gallons when you account for the glass thickness reducing the interior dimensions slightly and the tank typically being filled an inch or two below the rim. The 55-gallon is one of the most popular sizes because it offers a long footprint for schooling fish while being manageable for most stands and floors.
What is the one-inch-per-gallon rule for fish stocking?
The one-inch-per-gallon rule suggests keeping no more than one inch of adult fish body length per gallon of water. A 20-gallon tank could therefore hold 20 inches of fish, such as ten 2-inch tetras. While this rule is a helpful starting point, it oversimplifies stocking. It does not account for fish waste production, swimming behavior, territorial needs, or oxygen consumption. Wide-bodied fish like goldfish and discus produce far more waste than slim fish. A more accurate approach is to research the specific needs of each species you want to keep.
How do I choose the right heater for my aquarium?
Use 3-5 watts per gallon as a guideline, with larger tanks using the lower end of the range per gallon. For a 55-gallon tank, a 200-250 watt heater is appropriate. If your room temperature drops significantly at night or in winter, go to the higher end. Using two smaller heaters rather than one large one provides redundancy and more even heat distribution. Submersible heaters with a thermostat are preferred over hanging heaters for most setups. Always use a thermometer separate from the heater to verify your temperature independently.
What filter flow rate do I need for a fish tank?
A general guideline is to cycle the tank's water volume at least 4 times per hour. A 55-gallon tank would need a filter rated for at least 220 GPH (gallons per hour). For heavily stocked tanks or messy fish like goldfish or cichlids, aim for 6-10x turnover per hour. Keep in mind that filter ratings are measured at no head pressure; actual output decreases with tubing length, vertical lift, and media load. It is generally better to overfilter than underfilter. Canister filters, sump filters, and HOB (hang-on-back) filters are common choices for larger tanks.