Fish Tank Size Calculator

How to Use the Fish Tank Size Calculator

Our free fish tank calculator helps you determine your aquarium's water volume, maximum stocking capacity, and recommended equipment specifications. Enter your tank's dimensions in inches, select the tank shape, and optionally enter your current fish count and average fish size. The calculator instantly displays the usable water volume, stocking percentage, recommended filter flow rate, and heater wattage. All results update in real time as you adjust any input.

Properly sizing your aquarium equipment is one of the most important aspects of successful fishkeeping. Undersized filters lead to poor water quality, which is the number one killer of aquarium fish. An undersized heater cannot maintain stable temperatures, stressing tropical fish and making them vulnerable to disease.

Understanding Stocking Levels

The classic "1 inch of fish per gallon" rule provides a useful starting point for freshwater aquariums. This means a 20-gallon tank can accommodate roughly 20 inches of fish total — for example, ten 2-inch neon tetras. However, this is a simplification. Heavy-bodied fish like goldfish or oscars produce significantly more waste per inch than slim-bodied fish like danios, so they need more water per inch. The calculator accounts for the 10% volume loss from substrate, decorations, and equipment.

Filtration Requirements

Your filter should turn over the entire tank volume at least 4 times per hour. This means a 30-gallon tank needs a filter rated for at least 120 gallons per hour (GPH). For tanks with messy fish, live plants, or heavy stocking, aim for 6-8 times turnover. Canister filters and hang-on-back (HOB) filters are the most popular choices. For tanks over 75 gallons, a canister filter or sump system is usually preferred for adequate biological filtration capacity.

Heater Sizing

The standard recommendation is 5 watts per gallon of aquarium water. For a 20-gallon tank, that means a 100-watt heater. If your room temperature is significantly below your target tank temperature (more than 10 degrees difference), consider sizing up to 10 watts per gallon. For tanks larger than 50 gallons, two smaller heaters placed at opposite ends provide more even heat distribution and serve as redundancy.

Frequently Asked Questions

How many fish can I put in my tank?

The general rule is 1 inch of fish per gallon of water for freshwater aquariums. A 20-gallon tank can hold approximately 20 inches of fish total. Actual capacity depends on fish species, filtration quality, and body type. Heavy-bodied fish need roughly 2 gallons per inch.

What size filter do I need for my aquarium?

Your filter should turn over the entire tank volume at least 4 times per hour. For a 20-gallon tank, you need a filter rated for at least 80 GPH. For heavily stocked tanks, aim for 6-8 times turnover.

How many watts should my aquarium heater be?

A general rule is 5 watts per gallon. A 20-gallon tank needs approximately a 100-watt heater. In cooler rooms, consider 10 watts per gallon. For tanks over 50 gallons, two smaller heaters at opposite ends provide more even heating.

How do I calculate the volume of my fish tank?

For rectangular tanks, multiply length x width x height in inches, then multiply by 0.004329 to convert to gallons. Subtract about 10% for substrate and decorations that displace water.

What does overstocking a fish tank mean?

Overstocking means having more fish than the tank's biological filtration can handle. Signs include cloudy water, fish gasping at the surface, and frequent illness. If your stocking level exceeds 100%, consider upgrading your tank or adding more filtration.