Aquarium Calculator: Tank Volume, Heater Size & More

How many liters does my fish tank have? That is crucial information for many novice and experienced aquarists. After all, if you don’t know how much water your tank holds, you won’t know what size filtration system or heater to buy. And you could accidentally overstock a small nano aquarium with too many fish!

But don’t you need a fancy aquarium calculator to calculate the volume of your tank?

Well, in this essential guide, we show you how to measure a fish tank, convert dimensions to gallons, and provide you with a super simple tank capacity calculator.

Aquarium Calculator Guide

Home fish tanks come in many different shapes and sizes, which can make it extremely confusing when you want to calculate the volume of your aquarium.

In this section of our guide, you can find a calculator for each of the most common fish tank shapes. Just enter the required details, the calculator will do the rest.

How big is your aquarium?

Before you can use an aquarium calculator or calculate tank volume manually, you need to know the measurements of the tank. If you have the packaging of the aquarium, the dimensions of the tank must be indicated in the box. Otherwise, you’ll need to use a tape measure, which you can buy at DIY supply stores, craft stores, or dressmakers.

When using an aquarium calculator, remember to choose the unit of measure that applies to your country. For example, if you live in the US, you’ll need to set the calculator’s metric to US gallons, while if you’re in Europe, the calculator’s metric to liters applies to you.

Rectangular Fish Tank Volume Calculator

A rectangular tank is the most popular shape for home aquariums. Rectangular fish tanks can be small nano or betta tanks all the way up to massive 400 gallon setups. However, you can use the same calculator and equation to calculate the volume of the aquarium, regardless of its size.

Measure the Height (H), Width (W), and Length (L) of your aquarium in inches to use those dimensions in the following formula to find out the volume of the tank.

Height x Width x Length = Volume

Cylinder Fish Tank Volume Calculator

Cylindrical fish tanks are quite unusual, but if you have one, you can calculate the volume quite easily.

Measure the Height and Diameter of your aquarium in inches and then use those dimensions in the following formula to find the volume of the tank. If the tank is half-cylinder, then simply divide the result of the calculation by two.

Height x Diameter = Volume

Hexagonal Fish Tank Volume Calculator

Hexagonal aquariums are popular and fit nicely into corners without taking up too much space in the room. I have mine in an alcove off my kitchen, where it looks great and out of the way. To calculate the volume of a hexagonal-shaped tank:

Measure the height and length of one side of the aquarium and use those dimensions in the formula below to find the volume of the tank.

Height x Length of one of the sides = Volume

Cube Fish Tank Volume Calculator

Cube tanks can add a nice modernist vibe to your room. To calculate the volume of a cube-shaped tank:

Measure the length of one side to use in the following formula to find the volume of the tank.

Side x Side x Side = Volume

Curved Fish Tank Volume Calculator

Bow tanks are fantastic as they provide a panoramic view of your fish from anywhere in the room.

The calculation for a bow tank is a bit more complicated than for other forms of aquarium. You need to measure four dimensions: Length, End Width, Center Width, and Height of the aquarium in inches, and enter that figure into the calculator below to find out the volume of the tank.

Height x Width x Length = Volume

(height pi r²) / 2 = elliptical area

Square Area + Elliptical Area = Volume

cooler size

If you need a cooler for your aquarium, you can calculate the size you need using the following calculation:

Cooler size is required in horsepower (HP)

Volume in gallons x 8.34 (weight of 1 gallon of water) x required setback temperature + 20% fill allowance.

(The fill allowance is added to ensure the cooler will last longer and run more efficiently than a unit that is too small.) We recommend the calculator at

heater size

If you have tropical fish, you will need an aquarium heater to keep the water at the preferred temperature of the species. The size of heater you need is based on the volume of water in the tank, the average room temperature, and the temperature of the water you want.

Ideally, you want a base heater wattage of between 2.5 and 5 watts per gallon of water volume. To account for ambient room temperature, subtract the average ambient temperature from the desired aquarium water temperature. The result is the amount of heating you need, measured in degrees. The greater the difference in degrees, the more watts per gallon you’ll want.

For example:

  • Average room temperature = 68° Fahrenheit
  • Desired water temperature = 77° Fahrenheit
  • Heat required = 9° Fahrenheit (77-68 = 9)
  • Aquarium volume = 20 gallons
  • Required heater power = 50 watts

Filtration system size

Every fish tank should have a filtration system, regardless of its size. But a filter will only be effective if it is the right size for the volume of the aquarium. To ensure that all of the water in your tank passes through the filtration system, you need a unit that circulates the full volume of water at a rate of at least four times per hour.

So, calculate the total volume in your tank, as shown above, and then use the following example:

For a 20-gallon tank, you want a flow rate of at least 80 gallons per hour (GPH). If the available filter capacity is slightly less than that figure, choose a unit that has a slightly higher GPH flow rate.

How to use the calculator

Using an aquarium volume calculator is simple. All you need to do is calculate the relevant tank dimensions as described above, enter the figures into the calculator, and let the calculator do the hard work of producing the figures you want in the relevant unit of measure (i.e. gallons or gallons). liters).


Every hobbyist needs to know the volume of their aquarium from time to time, whether it’s calculating how many fish can be kept in the tank, knowing what size filtration unit to buy, or calculating heater wattage..

You can use the aquarium calculator and the information provided in this guide to do so. Or, if you prefer to go old school, grab a pen and paper and calculate the figure using the calculations we’ve included.

If you enjoyed this article and found it useful, please share it with your fellow aquarists and let us have your say in the comments section below.

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