Reefersdirect is reader-supported. When you purchase through our links, we may earn an affiliate commission. Learn more

Dimensions of a 20 Gallon Fish Tank (Size Chart Included)

dimensions of a 20 gallon fish tank

The standard dimensions of a 20 gallon fish tank are 30 inches in length, 12 inches in width, and 12 inches in height if it has an elongated design.

On the other hand, the 20 gallon high aquarium dimensions are 24 inches long, 12 inches wide, and 16 inches high.

However, it is worth noting that custom-built fish tanks might have a different measurement, although they will still hold 20 gallons.

The Physical Measurements of a 20-gallon Fish Tank



The measurements of a 20 gallon tank vary across types.

For example, a “tall” fish tank is shorter by six inches but is higher by four inches than a “long” version. Its standard measurement is 24” L x 12 “ W x 16” H. A good example of this measurement is a 20 gallon Tetra tank.

So, how big is a horizontally-extended 20-gallon aquarium? For your information, the 20 gallon long tank dimensions are 30” long by 12” wide and 12” high.

It is also worth noting that some aquarists prefer custom-built 20-gallon aquariums. For example, they might want a 20-gallon fish tank with the following measurements – 26” long, 12” wide, and 15” high.

Knowing the 20 gallon tank dimensions is crucial to determine its most suitable placement in the home.

You can ask a fish tank fabricator to create a 20-gallon tank specific to your space requirements. Here is a formula you can use.

Fish tank size = (Length x Width x Height) / 231

For instance, you only have 20 inches by 18 inches of space dedicated for a fish tank at home, and you want a 20-gallon aquarium.

You can substitute the values in the formula to determine the fish tank’s ideal height.

Fish tank size = (Length x Width x Height) / 231

20 gallons = (20 × 18 × X) / 231

20 gallons = (360X) / 231

360X = 20 gallons x 231

360X = 4,620

X = 4620 / 360

X = 12.83 inches

Hence, you need a custom-built 20 gallon fish tank size of 20 inches by 18 inches by 12.83 or 13 inches.

You can also decide how wide you want the 20-gallon fish tank to be, although 12 inches seem the sweet spot.

How Much Does a 20-gallon Aquarium Weigh?


Dimensions are not the only measurements that matter when picking a 20-gallon fish tank. You might also be interested in its weight.

An empty 20-gallon aquarium (without water, substrate, decorations, accessories, and electronics) tips the scale at 25 pounds (both long and high versions).

However, setting up the aquarium can add about 200 pounds to its heft. You can check out the glass aquarium size chart below.

20-gallon Fish Tank Type Dimensions (length x width x height) (in inches) Dimensions (length x width x height) (in centimeters)


Empty Weight (in pounds) Full Setup Weight (in pounds)
Long 30 × 12 × 12 76.2 × 30.5 × 30.5 25 225
High 24 × 12 × 16 61 × 30.5 × 40.6 25 225

Most of a fully-setup fish tank’s weight comes from water, with an average heft of 8.35 pounds for every gallon.

It is safe to assume that a 20 gallon water weight is 167 pounds (8.35 pounds x 20 gallons = 167 pounds), accounting for 74.22 percent of the fully-setup fish tank’s weight.

Add gravel as the substrate to the 20-gallon tank, and you can see its weight shoot up. Its density is 116.43 pounds per cubic foot. You might be interested to know that sand is heftier than gravel, given a similar volume.

Suppose you add two-inch deep gravel to a 30”x12” x12” 20-gallon fish tank. In that case, you must determine the gravel’s density by multiplying 30” by 12” by 2” to get 720 cubic inches (or 0.42 cubic feet).

Multiply 116.43 pounds by 0.42 cubic feet to get 48.9 pounds. Here is what we have (without fish, fish tank decorations, filters, electronics, and accessories).

  • Empty fish tank weight = 25 pounds
  • Water’s weight = 167 pounds
  • Two-inch gravel weight = 49 pounds
  • Total weight = 241 pounds

You can lessen the 20-gallon tank’s weight by filling it with 18 gallons of water (150.3 pounds instead of 167 pounds). You can also decide how many inches of gravel depth to use. For instance, 1.5 inches will save you about 10.6 pounds.

Fish tank weight matters because the cabinet or aquarium stand must be sturdy enough to support the aquarium’s fully-setup weight.

How Many Fish Can Live in a 20 Gallon Tank?


The rule of thumb is to put a one-inch fish for every gallon of aquarium space, regardless of the dimensions or size of 20 gallon aquarium units.

Hence, the number of fish a 20-gallon tank can accommodate depends on the fish specie’s maximum adult size. For instance, goldish might look tiny but can grow to 12 inches or longer.

A 12-inch goldfish needs 12 gallons of water to live in. On the other hand, 20 one-inch goldfish should theoretically live comfortably in a 20-gallon tank.

We recommend researching your fish, including their maximum adult size, to determine how many could live in a 20-gallon aquarium.

You might also want to consider building a fish tank community with varying species. However, you must consider the fish compatibility and its nature to guarantee a harmonious aquaponics ecosystem.

The ideal tank mates have similar nutritional requirements and water quality tolerances. They shouldn’t be too territorial, either.


The dimensions of a 20 gallon fish tank vary across types. Long 20-gallon aquariums measure 30 inches long, 12 inches wide, and 12 inches high.

Meanwhile, high 20-gallon units are shorter by six inches (24 inches) but taller by four inches (16 inches). It is as wide as the long version (12 inches).

Custom-built 20-gallon aquariums might have different measurements depending on the customer’s space requirements or special needs.

A 20-gallon aquarium’s empty weight is 25 pounds, although adding water, substrate, accessories, decorations, and electronics can jack its heft up to at least 225 pounds.

We hope that answers all your questions.

5/5 - (3 votes)