Work out how many gallons your aquarium holds and the filter and heater it needs

How to calculate aquarium volume

Multiply the inside length, depth and water height in inches and divide by 231 to get US gallons (in centimetres, divide by 1000 for litres). If you measure the outside of the tank, take the glass off twice from the length and the depth, and once from the height for the bottom pane. On a framed US tank the plastic trim counts too, so the calculator opens with 1/4 in. The water height is the inside height minus the gap you leave below the rim.

You get two volumes:

• gross, from the bottom of the tank to the water line: this is the tank size that filter and heater charts refer to, and the calculator sizes the equipment on it;
• net, the water above the substrate: use this for water conditioner and medication doses.

The substrate is inside length × depth × bed depth. In the US the calculator weighs it at 93 lb per cubic foot, the density LiveAquaria lists for CaribSea Peace River; in metric it uses 1.6 kg per litre, the low end of sera's figure for quartz sand and gravel.

The filter has to move the gross volume a set number of times an hour: 5 for tropical freshwater, 8 for coldwater goldfish tanks and 10 for saltwater, as in the All Pond Solutions calculator. The heater has to cover the gap between the coldest room temperature and the water: 78 °F for tropical fish (Aquarium Co-op gives 78-80 °F), 77 °F for saltwater (25 °C, the normal reef temperature according to Red Sea), and no heater for coldwater fish. The calculator uses 3.5 W per 10 gallons for each °F of rise, taken from Aquarium Co-op's own example of 100 W for a 29-gallon tank heated 10 °F above the room (3.45 W, rounded). In metric that is 17 W per 100 litres per °C. Then it picks the next heater size up, and above 300 W it splits the load across equal heaters.

A worked example

In inches (US). The calculator opens with an Aqueon 29-gallon tank, 30 1/4 × 12 1/2 × 18 3/4 in outside, 1/4 in of glass and trim, the water 1 in below the rim, 2 in of gravel, tropical fish and a room that drops to 68 °F.

• Inside: 29 3/4 × 12 × 18 1/2 in
• Water from the bottom to the water line: 18 1/2 - 1 = 17 1/2 in
• Gross: 29.75 × 12 × 17.5 ÷ 231 = 27 gal
• Gravel: 29.75 × 12 × 2 = 714 cu in, or 0.41 ft³; at 93 lb/ft³ that is 38.4 lb, so 2 bags of 20 lb
• Net: 29.75 × 12 × 15.5 ÷ 231 = 24 gal
• Filter: 5 × 27 = 135 GPH, so a filter rated for at least 140 GPH
• Heater: 68 to 78 °F is 10 °F, 3.5 × 2.7 × 10 = 95 W, so a 100 W heater

In metres (UK). A 100 × 40 × 50 cm tank measured outside with 6 mm glass, the water 3 cm below the rim and 5 cm of gravel holds 178 L to the water line and 159 L net. It takes 31 kg of gravel (7 bags of 5 kg), a filter of at least 890 L/h and, in a room at 20 °C with the water at 25 °C, 151 W of heat, so a 200 W heater.

Standard tank sizes: gallons, filter and heater

Figures from the calculator with the Aqueon outside dimensions, 1/4 in glass and trim, the water 1 in below the rim, 2 in of gravel, tropical fish at 78 °F and a 68 °F room.

Tank	Net gallons	Filter	Heater
10 gal	8 gal	50 GPH	50 W
20 gal long	14.7 gal	90 GPH	75 W
29 gal	24 gal	140 GPH	100 W
55 gal	45.2 gal	255 GPH	200 W

The 10 gallon takes 21.3 lb of gravel at 2 in, the 20 long and the 29 take 38.4 lb, the 55 takes 63.3 lb. In a room that drops to 60 °F the 29 gallon needs 170 W, so a 200 W heater.

Filters, heaters and substrate in the store

• Filter. The box gives a flow rate in GPH or L/h: compare it with the calculator's number, not only with the tank size printed on the box. LiveAquaria suggests four turnovers an hour on average and All Pond Solutions five for freshwater. In a saltwater tank, powerheads add to the filter, and All Pond Solutions counts them separately.
• Heater. Eheim thermocontrol heaters come in 25, 50, 75, 100, 125, 150, 200, 250 and 300 W, with 75 W for 60-100 L and 100 W for 100-150 L. For a tank that needs 200 W, Aquarium Co-op recommends two 100 W heaters instead of one 200 W: if a thermostat sticks, the damage is smaller and the other heater keeps the water warm. The calculator only splits the load on its own above 300 W.
• Substrate. In the US gravel and sand often come in 20 lb bags, like CaribSea Peace River; LiveAquaria suggests one to two pounds per gallon and 20 lb for tanks up to 20 gallons. In the UK gravel comes in 5 kg bags. Plant soils are often sold by volume: pick "by volume" and the calculator skips the density.

Common mistakes

• Using outside measurements as inside ones. On the 29 gallon the outside numbers taken as inside give 29.1 gal gross instead of 27, 7% more.
• Counting water up to the rim. Without the 1 in gap the 29 gallon holds 28.6 gal gross and the heater load goes from 95 W to just over 100 W, which moves you up to a 125 W heater.
• Forgetting the substrate. Two inches of gravel in a 29 gallon displace 3 gallons of water: medication doses go on the net volume.
• Sizing the heater for the daytime room. JBL notes that some rooms drop to 12-14 °C (54-57 °F) at night: use the coldest temperature the room reaches.
• Heating a goldfish tank. Aquarium Co-op says goldfish, ricefish and white cloud minnows are fine without a heater.