Guide

How many gallons does a 20x10x12 aquarium hold?

By the Aquarium Calculator team Updated 9 October 2026

A 20 × 10 × 12 inch aquarium holds 10.4 US gallons (39.3 litres, or 8.7 UK gallons) by its outside dimensions. It's the standard 10-gallon tank. With a 5 cm (2 in) substrate bed and some décor, it holds about 30 litres (7.8 gallons) of actual water.

At a glance

Outside dimensions: 20 × 10 × 12 in · 51 × 25 × 30 cm
US gallons: 10.4
UK gallons: 8.7
Litres: 39.3
Water weight (full): 39 kg / 87 lb

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

The calculation

Multiply length × width × height: 20 × 10 × 12 = 2,400 cubic inches. Divide by 231 cubic inches per US gallon and you get 10.4 gallons. In metric terms the tank is about 51 × 25 × 30 cm, which is 39.3 litres, or 8.7 imperial gallons. It's one of the most common aquariums in the world, often sold as a starter kit.

Because the tank is small, the substrate and décor take up a large share of it. A 5 cm (2 in) gravel or sand bed alone uses about 6.5 litres, and with some rocks and plants the real water volume drops to around 30 litres, or 7.8 US gallons, a quarter less than the label. That matters for dosing: treating a 10-gallon as if it held 10 gallons of water overdoses by about 30%.

Why a 10-gallon needs extra care

Small volumes change fast. In 30 litres, a missed water change, a dead snail or a heater that sticks can push ammonia or temperature to dangerous levels within a day, while the same mistake in a 75-litre tank takes much longer to cause harm. That's why experienced keepers recommend 20 gallons or more for a first tank, and why a 10-gallon should be stocked lightly and maintained on a strict weekly schedule.

A weekly 25–30% water change, about 8–9 litres (2–2.5 gallons) of real water, keeps a 10-gallon stable. Test ammonia and nitrite whenever you add fish, and feed sparingly. The water change calculator shows how much to replace and how fast it lowers nitrate.

Weight and equipment

Filled with water and a 5 cm sand bed, a 10-gallon weighs about 57 kg (126 lb) according to the tank weight calculator, light enough for a sturdy desk or cabinet rated for the load, but always check. A 5 cm bed across its 20 × 10 inch base takes about 10 kg (23 lb) of sand.

A 50 W heater holds a tropical temperature in this size comfortably. For filtration, aim for roughly 120–180 litres per hour (31–47 GPH) of real flow. Gentle filters such as sponge filters or small hang-on-back filters with adjustable flow suit the small fish usually kept in a 10-gallon. If you build a rimless version, 4 mm glass is enough for all panes.

What fish suit a 10-gallon tank?

Think nano. A 10-gallon suits one male betta with a few snails or shrimp, a school of 6–10 tiny fish such as chili rasboras or ember tetras, or a colony of cherry shrimp. Neon tetras are borderline: our neon page works out 6–7 as the only fish, because once substrate is in, a 10-gallon holds less water than they ideally need.

Avoid goldfish, plecos, angelfish, most cichlids and anything that grows beyond about 5 cm (2 in). Plan around adult size rather than the size in the shop, and use the stocking calculator to check your list against the tank's real volume.

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 does this for every shape and also shows the real 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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