Calculator · About the heater number · Standard sizes · References

Aquarium volume calculator

Pick a standard tank or measure yours in inches. I’ll give you the gallons and liters, about how much water it really holds, and what size heater it would need.

Calculator

About the heater number

Most fancy goldfish don’t need a heater. They do fine at room temperature in a heated house. The heater number is for a hospital or quarantine tank you need to warm up, or for a tropical tank.

It assumes the room is about 10°F colder than you want the water, and that the tank has a lid. For that, I use the usual 5 watts per gallon and round up to a common heater size. If the room is colder or there’s no lid, go bigger: a 20°F rise needs about twice as much. The heat treatments on this site run 84–86°F, which is more than a 10°F rise in most houses.

Over 40 gallons, I split it across two heaters, one at each end. If one sticks on, it can’t cook the tank as fast, and if one dies, the other keeps going. Heater dials are often off, so check with a separate thermometer.

Standard sizes

The list uses Aqueon’s outside dimensions for its standard glass tanks. Outside numbers include the glass and the rim, so the math comes out a bit over the size on the box, and gravel and decorations take up more. That’s why I also show the 90% figure. One US gallon is 231 cubic inches, and one gallon is 3.785 liters.