How to use the aquarium heater calculator
- Enter your tank size in US gallons, or switch to metric and enter litres.
- Read the standard heater size and the two-heater option.
Not sure of the size? The aquarium volume calculator works it out from the tank’s measurements.
The formula
Standard sizing: watts = gallons × 5. This is the rule of thumb most heater makers use. The calculator rounds up to the next common size: 10, 25, 50, 75, 100, 150, 200, 250 or 300 W.
Safer sizing: total watts ≤ gallons × 3, split over two heaters. Aquarium Science (aquariumscience.org) points out that 5 W per gallon can raise the water by 30–50 °F (17–28 °C) if a heater sticks on, and recommends at most 3 W per gallon in total with two heaters for redundancy. The calculator picks the largest common size for each of the two heaters that keeps the total within that limit.
For tanks over 60 gallons, a single standard heater would be over 300 W, so the calculator tells you to use two or more.
Tips
- A thermometer is not optional. Check the water every day with a separate thermometer, because heater dials are not always accurate.
- Use a heater controller on big tanks as an extra safety cut-off.
- Mind the room. The colder the room and the warmer your fish like it, the harder the heater works. A lid reduces heat loss.
- Unplug before water changes if your heater is not rated to run out of water.
Sources
- Aquarium Science (aquariumscience.org): “Aquarium Heaters”, on manufacturers’ 5 W per gallon sizing and the 3 W per gallon redundant setup.
- Manufacturer sizing guidance as commonly published on heater packaging (about 5 W per US gallon).
Frequently asked questions
What size heater do I need for my aquarium?
Heater makers commonly size heaters at about 5 watts per US gallon (1.3 W per litre). Aquarium Science, a fishkeeping reference site, argues that 5 W per gallon is more than most tanks need and recommends a total of no more than 3 W per gallon, split across two heaters, so a heater stuck on cannot overheat the tank as fast.
What size heater for a 20 gallon tank?
A 100 W heater by the standard 5 W per gallon rule. The more cautious approach is two 25 W heaters, which keeps the total at or below 60 W (3 W per gallon) and gives you a backup if one fails.
How many watts per gallon for an aquarium heater?
The common rule is 5 W per US gallon, and many charts give a range of about 3 to 5 W depending on how much warmer than the room you keep the tank. A warm room or a lid that keeps heat in means you need less.
Should I use two heaters in a large tank?
Yes, it is a good idea. Two smaller heaters at each end heat the water more evenly, and if one fails stuck on, it cannot cook the fish as quickly as one big heater could. If one fails off, the other keeps the tank from getting cold.
Where should I put my aquarium heater?
Near the filter outlet or another spot with good water flow, so warm water spreads through the tank, and fully under water if the heater is the submersible type. Follow the maker’s instructions for the minimum water line, and check the temperature with a separate thermometer.