Guide

How many gallons is a 72x18x24 aquarium?

By the Aquarium Calculator team Updated 9 October 2026

A 72 × 18 × 24 inch aquarium holds 134.6 US gallons (509.7 litres, or 112.1 UK gallons) by its outside dimensions, so it's often sold as a 135-gallon. With a 5 cm (2 in) substrate bed and décor, it holds about 421 litres (111 gallons) of actual water and weighs over 700 kg (1,500 lb) when running.

At a glance

Outside dimensions: 72 × 18 × 24 in · 183 × 46 × 61 cm
US gallons: 134.6
UK gallons: 112.1
Litres: 509.7
Water weight (full): 510 kg / 1,124 lb

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

The calculation

Multiply length × width × height: 72 × 18 × 24 = 31,104 cubic inches. Divide by 231 cubic inches per US gallon and you get 134.6 gallons. In metric terms the tank is about 183 × 46 × 61 cm, which is 509.7 litres, or 112.1 imperial gallons.

This footprint, six feet long and 18 inches front to back, is shared by several popular sizes. The classic 125-gallon is usually 72 × 18 × 21 inches, which works out to 117.8 gallons (446 litres) gross, while a 24-inch-tall version like this one is often sold as a 135-gallon. Always check the height before you buy equipment or a stand, because those 3 inches add about 64 litres of water.

Real water volume and weight

Allowing for a 5 cm (2 in) sand bed and around 10% for rock and wood, the tank holds roughly 421 litres (111 US gallons) of water. The sand bed alone takes about 67 kg (147 lb) of sand across the 72 × 18 inch base, and the empty glass of a rimless tank this size weighs well over 100 kg.

All together, the tank weighs about 712 kg (1,570 lb) when running, before the stand, rockwork and equipment. That concentrates more than half a tonne on roughly 0.84 square metres, so use a stand rated for at least that weight, keep it perfectly level, place it across the floor joists against a load-bearing wall, and have a builder or engineer check the floor if it isn't a concrete slab.

Glass, heating and filtration

Water pressure depends on height, and 24 inches is tall. For a rimless build, the glass thickness calculator recommends 15 mm glass for the front and back and 8 mm for the ends; framed and braced tanks can use thinner glass, so never remove a brace from an existing tank. At this height a euro-brace is strongly recommended.

Heating about 421 litres 5 °C above room temperature needs around 420 W, so use two 300 W heaters at opposite ends for even heat and a safety margin. For filtration, aim for 1,700–2,500 litres per hour (445–668 GPH) of real flow, which in practice means two large canister filters or a sump. Plan your weekly water changes too: 25% of this tank is over 100 litres.

What fish suit a 135-gallon tank?

A tank this size opens up fish that no small tank can hold. Popular options include a group of six to eight discus, which need around 200 litres and do best in groups; a single oscar with a few robust tankmates; an African cichlid community; large schools of rainbowfish, tetras or barbs; or a peaceful community with a big school, several centrepiece fish and a group of corydoras.

Some large fish still outgrow it. Clown loaches grow to 30 cm, live in groups, and need around 450 litres, just above this tank's real volume. Bala sharks reach 35 cm and need even more swimming length. Check each species' adult size and minimum tank size in the species directory, and test your plan with the stocking calculator.

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. The free volume calculator handles every shape and shows the net volume after substrate and décor.

Check your own tank

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

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