Aquarium Volume Calculator: How Many Gallons Is Your Tank?

When you get a new aquarium you will need to know how big it is.

This lets you figure how much water your tank holds.

Once you have calculated your aquarium’s volume you can decide how many fish and decorations you can keep safely in it.

It is important that every aquarist knows how to calculate the volume of their aquarium. Unfortunately, not every fish keeper is good with numbers.

This is where our aquarium volume calculator comes in!

Our handy calculator is here to help you figure out the volume of your new fish tank. Simply measure and input the numbers, and the tool will do all of the hard work for you.

Aquarium Volume Calculator

Rectangle Aquarium Calculator

Cylinder Aquarium Calculator

How To Use The Tool

Follow these step-by-step instructions to calculate your aquarium’s volume and to find out how many gallons it will hold.

  1. You need to start by measuring your aquarium to find out the length, height, and depth.
  2. Now go to our calculator above and choose your shape.
  3. Take your measurements and put them into the volume calculator.
  4. The volume calculator will automatically convert these measurements from inches to gallons, and will tell you the volume of your aquarium.

How To Measure Your Fish Tank

To measure your fish tank you will need a measuring tape and a pen and a paper to mark down your measurements.

Most fish tanks are rectangular, and they are easy to measure.

You just need to measure from one end of the base to the other. To measure the height of the aquarium, measure the distance from the base to the surface. To measure the width, measure the distance across the front of the tank. To measure the depth just measure the distance from the front of the tank to the back.

Other shapes are not so easy to measure though.

There are special ways to obtain the dimensions of a hexagonal, bow-front, or cylindrical fish tank.

For instance, to find the volume of a cylindrical aquarium, you must measure the diameter, radius, and height. The diameter is measured as the distance across the surface of the tank, and the radius is half of the diameter.

With a hexagonal tank you should measure the dimensions at both the widest and narrowest points, and average out these numbers to enter into the calculator.

To measure a bow-front aquarium, you will need measurements for the rear width and the full width. The full width is measured across the front (just like a rectangular tank). The rear width is measured by measuring the width at the narrowest point. You can average out the measurements of the rear and full width to get the actual width.

How To Calculate The Volume Of Your Aquarium

If you want to have a go at working out the volume of your aquarium, then there are several formulas you can use.

A rectangular aquarium is the most straightforward.

The formula for the volume of a rectangular is (l x w x d) / 231.

L is the height, w is the width, and d is the depth. Since one gallon is 231 cubic inches, you divide l x w x d to get the volume in gallons.

Now if your aquarium is a tricky shape you will need to use a different formula.

To calculate the volume of a cylindrical tank, you will need to divide the diameter by 2 to get the radius. Then, use the following formula: π x radius² x height.

Divide this answer by 231 to give you the gallon size.

For the volume of a hexagonal tank you will need to use the following formula: (3√3/2)s² × h.

S is the hexagon base’s edge length, and h is the height. Again, remember to divide your answer by 231 to convert it to US gallons.

How Many Gallons Is Aquarium

How Many Fish Can You Keep In Your Tank?

There is a simple rule to follow when figuring out how many fish you can fit in your tank:

  • One gallon of water per one inch of fish

For example, if you have a shoal of ten fish that each measure one inch in length, you would need a 10 gallon aquarium. You must consider the maximum length that the fish will reach as an adult; do not base your tank size on the size of a juvenile fish.

Here is a general guide to how many fish you can keep in some of the most common aquarium sizes.

Aquarium Size Number Of Fish
10 gallons 5-8
20 gallons 6-12
40 gallons 10-20
50 gallons 15-25
100 gallons 18-40

In a 10 gallon aquarium you can keep schools of Tetras, Rasboras, and other small shoaling species. On average you can keep a total of 5 to 8 small fish in a 10 gallon tank. If your fish are less than an inch long you can keep up to 10 of them.

With a 20 gallon aquarium you can start keeping larger shoals of fish and building small biotopes. A 20 gallon tank can hold between 6 and 12 small fish, or 20 fish that are less than an inch in size.

A 40 gallon tank can hold up to 2 medium sized fish such larger types of Goldfish, Paradise Fish, or Freshwater Angelfish. It can safely house between 10 and 20 small shoaling fish.

50 gallons is a good size for most single-tailed Goldfish like the Common or Comet. It can also hold Freshwater Angels, Yoyo Loaches, and other medium-sized fish in singles or pairs. In total a 50 gallon aquarium can hold up to 25 small fish or between 2-5 medium sized fish.

With a 100 gallon tank you can start keeping fish that grow beyond 10 inches in size. Oscars, Bala Sharks, Bichirs, and Discus are all good options. In total 100 gallons can hold between 18 and 40 fish.

Which Size Aquarium Should You Get?

There are many factors to take into account when considering the right aquarium size for you. The first thing that you should consider is your skill level.

A large and diverse aquarium can be very fun, but it takes a lot of work to maintain. Larger aquariums also require more equipment, time, and money.

If you are looking for your very first aquarium then you should consider a 10 to 20 gallon tank. This size is very common in starter kits. A 50 gallon beginner Goldfish tank is also a good idea if you are just starting out, but consider keeping just one Goldfish without trying to create a community just yet.

Larger community aquariums over 50 gallons are better for hobbyists who have had some experience caring for a smaller, easier setup. Consider sizing up to a large tank after you have a year or two of experience.

You will also need to consider the amount of space that you can dedicate to your new fish tank. Measure the area thoroughly and pick out the tank dimensions that will fit right in.

Make sure you include enough space for all the equipment that you will need to run your setup, including a heater, filter, water pump, aerator, and lighting system.

10 Gallon

A 10 gallon aquarium is the best size for a beginner.

This size can hold miniature tabletop replicas of freshwater ecosystems, and keep popular nano fish like Bettas, Neon Tetras, and Harlequin Rasboras.

You can even keep an African Dwarf Frog in this tank.

These aquariums are cheap to run and ideal for kids to start out with.

20 Gallon

A 20 gallon aquarium gives you more room than 10 gallons and allows you to start planning small scale aquascapes.

You can use this tank size to create a brackish biotope for Livebearers and Bumblebee Gobies. In a saltwater tank you can keep Gobies and small Damselfish.

This is also suitable for beginners who want a bit more space to work with.

40 Gallon

The 40 gallon tank is an excellent mid-sized tank.

You can keep lots of popular Cichlids, such as Rams and Freshwater Angelfish, in this tank. It also provides plenty of space for helpful invertebrates like Cherry Shrimp and Nerite Snails, and offers plenty of aquascaping room for Java Ferns, Crypts, and Java Moss.

With saltwater you can use a 40 gallon tank to create a live rock reef with Royal Grammas, Damselfish, small Wrasses, and dwarf Angelfish.

50 Gallon

The 50 gallon aquarium is the ideal size for most single tailed Goldfish species.

You can design entire setups around these Goldfish, including their favorite plants and friendly algae eaters for company. This is also a good size for lots of Cichlid species, such as the Peacock and the Jewelfish.

With saltwater 50 gallons is ideal for creating a small reef full of Clownfish, Royal Grammas, Wrasses, and Damselfish.

100 Gallon

The 100 gallon tank is not suitable for beginners.

Hobbyists should have at least 2 years of experience before trying a tank this big.

You can keep territorial Cichlids like the Oscar and the Jack Dempsey. It is also an excellent size for large schooling fish like the Discus.

In a 100 gallon tank you can also create some truly colorful coral reefs.

Aquarium Dimensions Chart

Aquarium Size Width Depth Height
5 gallons 16″ 8″ 10″
10 gallons 20″ 10″ 12″
20 gallons 30″ 12″ 12″
30 gallons 36″ 18″ 12″
40 gallons 48″ 12″ 16″
50 gallons 48″ 12″ 20″
60 gallons 48″ 13″ 22″
70 gallons 48″ 13″ 26″
80 gallons 48″ 13″ 30″
90 gallons 48″ 18″ 24″
100 gallons 72″ 16″ 20″
150 gallons 72″ 17″ 28″
200 gallons 72″ 25″ 26″


While surface area is important for external measurements, the volume of your aquarium determines what can go inside. Calculating aquarium volume by hand can be tricky, especially if your aquarium is an unconventional shape.

Hopefully now that you have used our aquarium gallon calculator, you have figured out how many gallons your tank is.

When setting up an aquarium, it is important to know exactly how much volume you have to work with.

Your aquarium size determines what types of fish you can keep and what kind of setup you can design, as well as the equipment needed to keep the aquarium running.

What setup are you going to design for your tank? Let us know in the comments section below…

David Thomas Author Bio Picture
David Thomas leads the team at Everything Fishkeeping as the Editor-in-Chief. David has been keeping fish since he was a child. In his first tank he kept goldfish and since then he has kept over 30 different species. Now he has 4 separate tanks and his favorite is a 100 gallon freshwater tank with a school of Rasboras, Tetras and Loaches.