How to Calculate How Big of An Aquarium You’ll Need

If you want to start an aquarium, one of your first questions will be how big of an aquarium you will need. Aquarium tanks come in a large range of sizes, and instinct tells the beginner to reach for the smallest tank. It is not that simple, though, as smaller tankers are more difficult to maintain.

How do you calculate how big of an aquarium you will need? You will take into consideration several factors when selecting the size of your aquarium, including:

  • Your budget
  • Proposed location of the aquarium
  • Maintenance requirements
  • Space requirements of fish
  • Weight of the tank
  • Ability of tank to receive oxygen
  • Whether the tank is made of glass or acrylic
  • Your experience levels

Each of these factors will be discussed in further detail below. There is no need to feel intimidated by the process of selecting an aquarium tank. Once you learn which aspects you need to consider in setting up an aquarium, you will find that there is an aquarium out there for your needs. 

Typical Sizes of Aquarium Tanks

Aquariums can come in many sizes ranging from as little as 2.5 gallons to as large as 180 gallons or even more.

A listing of common fish tank sizes and characteristics shows that most shapes of tanks run in a 10 to 60-gallon range.

Calculating the Size of Aquariums

A close approximation of an aquarium’s volume can be determined by multiplying the tank width x the tank height x the tank depth. This handy online calculator can be used for this and will convert volume to gallons and liters.

It will then direct you to helpful information about how to care for your aquarium. For example, if you put in 10x10x10 for the tank’s width, height, and depth, you will find that tank volume is 4.3 gallons.

The page will then give you the choice of clicking on a link that tells you how to maintain small aquariums. 

If the answer to the tank volume is a higher number such as 35 gallons, the screen will tell you that tanks in the 15-35-gallon range are considered good “starter tanks” for those who are looking into starting their first freshwater aquarium.

Tanks in the range of  29-75 gallons are named as good starter tanks for those looking into starting their first saltwater aquarium.

Calculations like this work best if you are looking at a rectangular tank. However, if you have a bowfront or hexagonal tank, you will be able to use the average of the narrowest dimension and the widest dimension to get a good approximation of total volume.

For example, if your hexagonal tank is 8 inches wide at its narrowest dimension and 12 inches wide at its widest dimension, you can use the average of the two dimensions (10 inches) as the width in the equation: tank width x the tank height x the tank depth. 

How to determine the best aquarium tank size for your needs will be further discussed below.


Before you commit to buying an aquarium tank, you will need to set a budget. Maintaining a healthy environment within an aquarium comes with many costs other than simply just paying upfront for the most affordable equipment you can find.

 A smaller aquarium does not necessarily come with a lower price tag. You will need to be prepared for the possibility that certain species will cost more to keep healthy.


The size of the aquarium you’ll need depends upon the desired location. If you have to place it in the back corner of a room or a basement you or your guests will not be able to enjoy the aquarium to its full potential. Here’s something to consider: once an aquarium is set up it will weigh at least 10 times the tank’s water capacity.

The aquarium will require adequate spacing around it for easy cleaning and maintenance. You will not want to block accessories you will need frequent access to such as filtration devices.

If the aquarium is placed  in a poorly ventilated space, you may encounter issues with mold or other cultures that thrive in humid conditions. Being able to keep a cover on the aquarium will keep humidity levels down and lower the amount of water that will be lost via evaporation.

The aquarium will also need to be put in a location that puts a reasonable distance away from electrical outlets. The recommendation is that the tank is located no more than 3 feet away from the nearest electrical outlet.

You will also want to keep your aquarium as close as possible to the water source in order to make water changes and maintenance easier.

Keep the temperature within the aquarium stable by keeping it a safe distance away from sources of excessively warm or cool air. This means making sure that there are no heating/ air-conditioning vents in the immediate vicinity of the aquarium.

Also, keep the aquarium away from areas of direct sunlight, as sunlight contributes to algal growth in addition to higher temperatures.

Aquariums should also be placed in locations with minimal noise. It should be set as far as possible from noise sources, including TVs and washing machines.

Ensure that the aquarium will be placed in a location where the floor is level and will be able to hold the expected weight. If the floor is not completely level, such as an instance in which one side of the aquarium stand is on a carpet and the other half a hardwood floor, you may use a shim to help make up the difference.

Do not use a shim to support large aquariums on uneven flooring. You will need level flooring to properly fill a larger aquarium tank. 

If you are unable to find a good location for a large tank, you will need to settle for a smaller tank and the resulting maintenance requirements of smaller-sized aquariums that will be discussed in further detail below.

Ease of Maintenance

Larger aquariums typically lead to happier fish. This is because larger aquariums are actually easier to maintain than smaller aquariums.

Being able to keep water conditions within a tank chemically stable can be a difficult task, particularly if the aquarium is on the smaller side. It takes a long time for chemical changes in water to affect tanks with higher water capacity. 

This is due to the diluting effects of larger volumes of clean water.

One of the chemicals you will need to keep at low levels within the aquarium is ammonia.

  • Fish release ammonia as a waste product.
  • There are also bacteria within the tank that breakdown extra food and waste. They also release ammonia.
  • In turn, the ammonia is broken down into nitrite before another bacteria breaks down the nitrite to nitrate. 

Cycling is a term used to describe the process a new aquarium will go through as it develops colonies of bacteria to take care of the ammonia and nitrite produced. This puts the health of an aquarium in a vulnerable position.

Ammonia and nitrite can build up into levels toxic for the fish if the aquarium is not properly maintained. Fortunately, larger aquariums go through what is referred to as a “silent cycle.” In such cases the volume of water is large enough to prevent ammonia and nitrite from reaching harmful levels.  

The minimum size tank for this to occur is 55-75 gallons.

In addition to having a large enough tank to control the presence of toxic substances, the amount of fish present in the tank will also affect the concentration of ammonia within the tank. If you introduce too many fish at once, things will go out of whack.

  • It is recommended that you do not fully stock the tank at one time.
  • You should not introduce more than 25% of the total volume of fish you plan on eventually having at one time.

This will allow the helpful bacteria an opportunity to take care of the toxins introduced into the tank.

The key takeaway here is that a larger tank will be more forgiving on beginners than a smaller one. As a word of caution: many stores will try to sell small tanks ( some smaller than 5 gallons) to beginners as starter tanks. 

Experienced aquarium owners will tell you that this is misleading, and that the smallest of tasks can be difficult for even the most experienced individuals to maintain.

Fish Need A Space Bubble Too

The more space that fish have for swimming, the happier they are.  Stressed fish are more likely to fall prey to disease due to weakened immune systems, according to this article.

Too Many Fish

When fish are overcrowded, they are also more likely to become agitated and quarrel with the other fish in the tank. Most of the fish will look for some type of space within the confines of the tank to call their own. They will define their personal areas by physical boundaries.

Give Them Places to Get Personal Space

This is where decorations such as caves, driftwood, and rocks come in. You will want to buy a tank large enough for you to put at least a few decorations inside the tank. Keep in mind that these decorations will decrease the tank’s true water volume.

Different Strokes for Different Fish

  • Some species will be more territorial than others. Angelfish, for example, get somewhat aggressive among other, smaller fish. You will need to allow these species more personal space than others.
  • You will also need to consider that some species, such as Tetras, prefer to be in schools. This type of fish will also increase your need for a larger tank.
  • Marine fish need more space to swim than freshwater fish species do.

Tank Size Requirements of Popular Fish Species

Some species of fish will require differently- sized tanks than others to be happy.

SpeciesMinimum Recommended Tank Size for 1 Fish (Gallons)Minimum Recommended Tank Size for a school of fish (Gallons)
Green Chromis1040
Common Clownfish1060
Royal Gramma30100
Neon Tetra515

A good rule-of-thumb for determining the safe holding capacity of an aquarium is to add one gallon of water per inch of fish for small community  fish like Tetras, Bettas, Rainbowfish, and Platies.

It is highly recommended that you buy schooling fish in multiples. If you add only one or two schooling fish to a tank, the fish will become stressed, which in turn will shorten their lifespan. As a result, you will need a larger tank for schooling species, even if the individual fish themselves are small.

You will also need to consider the age of the fish you are buying. If you buy young fish, they will most certainly grow in size. For this reason, you will need to calculate size requirements based on the size of full-grown adults of the species, as is done with the chart and the methods listed below. 

Larger fish should have at least 3 to 4 gallons of water for every inch of fish. Species considered large in this case include Oscars, Piranhas, and Blue Damsels.

Other methods include:

  • Calculating Fish Weight Per Volume of Water ( 1 gram of fish for every 4 liters of water)
  • Comparing Fish Length to Filter Capacity 
  • Calculating Fish Length Per Surface Area( 1 inch of fish for every 12 square inches of surface area)

The results of each of these rules may contribute to different required tank sizes, depending upon the method used. These rules should be interpreted as guidelines and not approximate ranges. 

The author of the website describing the different methods recommends the “add x gallons of water per inch of fish” method because it is the simplest and is reported to have given reliable results in the past.


One gallon of water weighs 8.34 pounds. Once an aquarium is set up with all the necessary accessories, it will weigh at least 10 times the tank’s water capacity.

This is the standard for glass aquariums; acrylic aquariums will weigh slightly less.

If you have no choice but to put your aquarium on an upper floor of a building or live in an apartment, this is the recommendation:

  • Place the aquarium somewhere held up by several parallel studs in the floor beneath it rather than just one.

Too much weight incorrectly placed on just one stud can push down on flooring and subflooring and cause damage.

If the only way to place a tank is through incorrect weight placement, there is a way that a crisis can be averted:

  • Place a sheet of wood underneath the aquarium that will help distribute the weight of the aquarium more evenly

Still, the best option will be to avoid buying a tank that may have even the slightest possibility of being too heavy for your location. It is easy to underestimate the weight of a tank in the store when it is still empty. 

The significant majority of the aquarium’s weight once complete will be the water and decorations inside the tank.

Oxygen Availability

A larger tank is easier to oxygenate than a smaller one.  A larger surface area of water contributes to a more efficient gas exchange of Oxygen and Carbon Dioxide within an aquarium.

Fish breathe in oxygen and give off carbon dioxide. In this way, the water within an aquarium swaps carbon dioxide for the oxygen in the air. The oxygen that enters the water is referred to as dissolved oxygen, often abbreviated as DO.

Factors influencing the dissolved oxygen need of your aquarium include:

  • Water temperature– the amount of oxygen that water can hold decreases as the temperature of the water increases. As a result, tropical fish species are used to having less oxygen available to them than cold fish species are
  • Salinity- saltier water holds less oxygen
  • Atmospheric Pressure- the lower the air pressure is, the less oxygen the water can hold
  • Size of fish- generally larger species of fish will require more oxygen than smaller fish
  • Slow-moving fish species need less oxygen than fast swimming species

Also influencing the amount of dissolved oxygen that is able to make its way into the tank is  the shape of the tank. Some species prefer tanks that are not rectangular in shape. The challenges of these types of tanks will be further discussed in the next section.

Shape of Tank

Different fish species prefer different shapes and swimming spaces:

Active Fish Species Prefer Longer Aquariums

Species such as Danzios and Barbs, prefer longer aquariums. This means that a rectangular design that provides more horizontal space will suit them well. Territorial species of fish such as Cichlids will thrive in tanks with lots of bottom space. Narrower tanks will not suit territorial fish well.  

The tall, narrow shape of the tank can be less suitable for active species such as angelfish. One possible concern with the tall and narrow variety of tank is that it will be more difficult to maintain a healthy amount of dissolved oxygen for the fish since there will be less surface area available for gas exchange processes to occur. 

Taller tanks will also be more difficult to clean than rectangular-shaped tanks. The plants at the bottom of taller tanks will also receive less sunlight than the plants in rectangular tanks.

The shape of the tank will also affect your ability to observe the fish as they are moving around. Some types of tanks, such as cylindrical tanks alter the shape of the fish when viewed from certain angles.

Glass vs. Acrylic

You may think that calculating the size of an acrylic aquarium tank needed can be a slightly different process than calculating the size of glass tank needed. Glass is a denser material and will often weigh 4-10 times more than an acrylic tank of the same volume. 

However, most of the weight of an aquarium tank comes from the water and decorations stored within the tank. 

As a result, it is not necessarily a viable option to buy an acrylic tank just because you think your stand will be able to a larger tank if you go with an acrylic tank rather than a glass tank. Many of the same guidelines regarding the location and the ability of that location to support a tank of that size will still apply.

One significant difference with acrylic tanks is the ease with which the acrylic tank can be molded into almost any shape. Differing species of fish prefer different shaped tanks. It may be easier to find an appropriately-sized acrylic tank in some cases.

Acrylic tanks don’t share the same tendency to distort things behind a curve that glass aquarium tanks do. If your location requires an unusual-shaped tank, going with an acrylic tank may be a more viable option for your aquarium.

Experience Level of Owner

Small Aquariums are not recommended for beginners in the aquarium-owning experience. This is because:

  • Changes in temperature and the chemical quality of water can occur rapidly
  • It is easier to overpopulate a tank that is small
  • Smaller tanks require more careful filtration

If this is going to be your first aquarium, it is recommended that you buy a 5-gallon tank at the minimum. However, you will be better off buying a tank that is at least 20 gallons.

Size of Tank vs. Water Capacity

One common source of error when calculating the size of the tank that will be needed is assuming that the size of the tank is exactly equivalent to the total gallons of water it holds.

A 10-gallon tank filled with substrate such as gravel, sand, or rocks and plants as well as decorations will not hold ten gallons of water. The water volume is typically 10-15% less than the total volume of the tank. This means that a 10-gallon tank may actually hold 8.5 gallons of water or even less in the case that the tank is heavily decorated.

You will especially need to consider this difference if you are looking into buying a smaller tank. As has been discussed, it is recommended that you buy as large of a tank for your aquarium as you can. This is because there is less room for error with smaller aquarium tanks.

If you add a lot of decorations to a smaller tank you will probably have to add fewer fish than is recommended. With a large tank you will be able to add more decorations.

I’m new to aquariums, what size tank should I buy?

  1. Select based on whether your aquarium will be saltwater or freshwater:
  1. Freshwater aquariums: look for tanks in the 15-35-gallon range
  2. Saltwater aquariums: look for tanks in the 29-75-gallon range
  1. Consider the proposed location to ensure it will be able to handle weight:
  2. Once an aquarium is set up, it will weigh 10 times its water capacity.
  1. Consider how much effort you will need to exert to ensure that ammonia/nitrite/nitrate cycle doesn’t go out of whack. A tank that is at least 55 gallons should be able to dilute the effects of this potentially harmful cycle
  1. Consider the species you want:
    1. Smaller species should be in tanks where there is at least 1 gallon of water for each inch of fish
    1. Larger species need at least 3-4 gallons of water for every inch of fish
    1. Schooling species will need to be kept in schools; they will not be happy alone
  1. Consider the surface area of water exposed to air
    1. Rectangular tanks will be best at maintaining a healthy supply of dissolved oxygen
  2. Don’t think that buying acrylic tanks will allow you to buy a tank that is significantly larger than the glass tank your location will be able to support.
    1. Although glass tanks themselves weigh 4-10 times more than acrylic tanks of the same size, the majority of the weight of an aquarium comes from the water and decorations inside the tank,almost negating the initial weight difference

In Conclusion

Rather than choose a tank without research, take the time to do a little thinking and calculating. Once you know which factors to consider, you’ll find it isn’t difficult at all. When you get a tank that works well for the fish you have, the budget you can spend, and the place you’ll put it in your home,  you and your fish will be much happier.