Guide

How many gallons is a 24x12x16 tank?

By the Aquarium Calculator team Updated 9 October 2026

A 24 × 12 × 16 inch tank holds 19.9 US gallons (75.5 litres, or 16.6 UK gallons) by its outside dimensions. It's the standard 20-gallon high. With a 5 cm (2 in) substrate bed and some décor, it holds about 60 litres (15.7 gallons) of actual water.

At a glance

Outside dimensions: 24 × 12 × 16 in · 61 × 30 × 41 cm
US gallons: 19.9
UK gallons: 16.6
Litres: 75.5
Water weight (full): 76 kg / 166 lb

Gross volume of the glass box, calculated from the outside dimensions.

The calculation

Multiply length × width × height: 24 × 12 × 16 = 4,608 cubic inches. One US gallon is exactly 231 cubic inches, so 4,608 ÷ 231 = 19.9 gallons. In metric terms the tank is about 61 × 30 × 41 cm, which is 75.5 litres, or 16.6 imperial gallons. Manufacturers round this up and sell it as a 20-gallon tank, usually labelled "20 high" to distinguish it from the longer, shallower 20 long.

The outside measurement includes the glass and the gap you leave below the rim, and the substrate and décor take up more space. With a 5 cm (2 in) sand bed and about 10% for rocks and wood, the real water volume is close to 60 litres, or 15.7 US gallons. Use that figure for medication, fertiliser and water conditioner doses. The volume calculator works it out for your exact setup.

20 high vs 20 long

Both tanks hold about 20 gallons, but the shapes suit different setups. The 20 long measures 30 × 12 × 12 inches and holds 18.7 gallons, almost a gallon less, yet its 360-square-inch footprint is 25% larger than the 20 high's 288 square inches. Fish swim horizontally and oxygen enters through the surface, so the long tank supports a little more life and gives schooling fish more swimming room.

The 20 high's extra 4 inches of height work well for tall plants such as vallisneria and Amazon swords, a dramatic aquascape, or a display where the tank is viewed from a desk or shelf. It also takes up less shelf width. For active schooling fish or bottom-dwellers, the 20 long is the better choice; for a planted showpiece with calm fish, the 20 high works well.

Weight and equipment

Filled with water and a 5 cm sand bed, a 20 high weighs about 109 kg (240 lb), using the tank weight calculator with typical glass. That's well within the rating of any purpose-built 20-gallon stand, but too much for many shelves and small furniture. A 5 cm bed across its 24 × 12 inch base takes about 15 kg (33 lb) of sand.

For about 60 litres of water held 5 °C above room temperature, a 75 W heater is the right size. Aim for a filter delivering roughly 240–360 litres per hour (63–94 GPH) of real flow, four to six turnovers an hour, which in practice means a filter rated for 30–40 gallon tanks. If you build a rimless version, the glass thickness calculator recommends 6 mm glass for the front and back and 5 mm for the ends.

What fish suit a 20-gallon high?

A 20 high suits small, peaceful fish that use the whole water column. Good options include a school of 8–12 neon or ember tetras, a group of cherry barbs, a pair or trio of honey gouramis, or a single betta with a few tankmates such as pygmy corydoras and nerite snails. The extra height also suits fish that like to hover among tall plants.

Despite the height, it is too small for angelfish, which grow tall and need around 150 litres, and for most cichlids, goldfish and plecos. Plan around each fish's adult size and minimum tank size in the species directory, and check your full list with the stocking calculator before you buy.

Calculate any tank

For any rectangular tank, multiply length × width × height in inches and divide by 231 for US gallons, or multiply in centimetres and divide by 1,000 for litres. For other shapes, or to find the real net volume with your substrate and décor, use the free volume calculator.

Check your own tank

Enter your real dimensions to get exact figures for your setup in a few seconds.

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