Aquarium Calculator – Tank Volume, Heater Size And More

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How many gallons is my fish tank?

That’s a crucial piece of information for many newbie and experienced aquarists. After all, if you don’t know how much water your tank contains, 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 work out your tank’s volume?

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 work out 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. Simply enter the required details, the calculator will do the rest.

What Dimensions Are Your Aquarium?

Before you can use an aquarium calculator or work out the tank volume manually, you need to know the measurements of the tank. If you have the aquarium packaging, the dimensions of the tank should be indicated in the box. If not, you’ll need to use a tape measure, which you can buy from DIY, crafting, or dressmaker’s supplies stores.

When using an aquarium calculator, remember to choose the unit of measurement that applies to your country. For example, if you live in the U.S., you’ll need to set the calculator metric to U.S. gallons, whereas if you’re in Europe, the liter calculator metric applies to you.

Rectangular Fish Tank Volume Calculator

Vector of rectangular aquarium with dimensional signs.

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

Measure the Height, Width, and Length of your aquarium in inches, and enter those dimensions into the calculator below to find out the volume of the tank.

Cylinder Fish Tank Volume Calculator

Outdoor cylindrical aquarium with koi fish. Dimensional measurements for a cylindrical aquarium.

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

Measure the Height and Diameter of your aquarium in inches, then divide Diameter by 2 to get the Radius  and enter those dimensions into the calculator below to find out the volume of the tank. If the tank is a half-cylinder, simply divide the result of the calculation by two.

Hexagon 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 in my kitchen, where it looks great and isn’t in the way. To work out the volume of a hexagon-shaped tank:

Measure the height and the length of one side of the aquarium in inches, and enter those dimensions into the calculator below to find out the volume of the tank.

Cube Fish Tank Volume Calculator

An unusual aquarium in the form of a cube stands on the surface . Guide on measuring cube glass aquarium.

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

Measure the length of one side in inches, and enter that figure into the calculator below to find out the volume of the tank.

Bowfront Fish Tank Volume Calculator

Bowfront Fish Aquarium. Measuring dimensions of a bow fish aquarium.

Bowfront tanks are fantastic, as they provide a panoramic view of your fish from wherever you are in the room.

The calculation for a bowfront tank is a little more complicated than for other aquarium shapes. 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.

Chiller Size

DIY Aquarium Chiller
Image Source :

If you need a chiller for your aquarium, you can work out the size you need by using the following calculation:

Chiller size is required in horsepower (HP)

Volume in gallons x 8.34 (weight of 1 gallon of water) x temperature pull-down required + 20% padding allowance.

(The padding allowance is added to ensure that the chiller lasts longer and works more efficiently than a unit that is too small.) We recommend the calculator at

Heater Size

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

Hygger Mini Submersible Aquarium Heater inside aquarium.
Image Source : Amazon

Ideally, you want a basic heater wattage of between 2.5 and 5 watts per gallon of water volume. To take into account the ambient room temperature, subtract the average room temperature from the desired aquarium water temperature. The result is the amount of heating that you need, measured in degrees. The larger the degree difference 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
  • Heater wattage required = 50 watts

Filtration System Size

Beautiful planted tropical freshwater aquarium with fishes. Aquascape.

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 aquarium volume. To be sure that all the water in your tank passes through the filter system, you need a unit that circulates the total water volume 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 filter capacity available falls slightly short of 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 do is work out 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 relevant unit of measurements (i.e., gallons or liters).


Coral Saltwater reef aquarium tank

Every hobbyist needs to know the volume of their aquarium from time-to-time, whether that’s to work out how many fish can be kept in the tank, to know what size filtration unit to buy, or to calculate what heater wattage to go for.

You can use the aquarium calculator and the information that we’ve given you in this guide to do that. Or, if you prefer to go old-school, grab a pen and paper, and work out the figure by using the calculations we’ve included.

If you enjoyed this article and found it helpful, please share it with your fellow fish-keepers and let us have your thoughts in the comments section below.

Aquarium Calculator - Tank Volume, Heater Size, And More - Infographic

Alison Page has been an avid fish keeper for over 35 years and has owned many different species of freshwater tropical fish including bettas. Currently Alison has two large freshwater tanks. The first tank has two huge fancy goldfish who are almost ten years old and still looking as good as ever. In the other, she has a happy community of tiger barbs, green tiger barbs, corydoras catfish, platys, and mollies.

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