About the Aquarium Heater Size Calculator
This aquarium heater size calculator tells you what wattage heater your fish tank needs. Enter the tank volume, the water temperature you want (tropical fish usually 76–80 °F / 24–27 °C) and the coldest the room gets, and it works out the heating power required, the standard heater size to buy and how to split it between two heaters.
The common advice of “5 watts per gallon” only holds for a tank kept about 10 °F above the room. A tank in a heated living room needs far less, while one in a cold basement or garage needs much more — so the calculator scales the wattage with the actual temperature difference, and adds a margin for open-top tanks, which lose extra heat through evaporation.
Size for the coldest night of the year, not the average day. An undersized heater runs constantly and still cannot keep up; a modestly oversized one with a good thermostat is safer. For tanks over about 50 gallons, two smaller heaters at opposite ends spread heat evenly and reduce the risk of cooking or chilling your fish if one fails.
With the default inputs, the heating power needed is 100 W. Change any value above to recalculate instantly.
How to use the aquarium heater size calculator
- 1Enter your tank volume in gallons or liters.
- 2Enter the water temperature your fish need.
- 3Enter the coldest temperature the room reaches.
- 4Choose whether the tank has a lid and whether you will use one or two heaters.
- 5Buy the recommended size — the next standard wattage up.
Formula and method
A heater has to replace the heat the tank loses to the room, and that loss rises in proportion to the temperature difference. Common heater sizing charts work out to roughly 0.5 W per US gallon for every °F the water is kept above the surrounding air — which is where the familiar “5 W per gallon” rule comes from for a 10 °F rise. Liters are converted at 3.785 L per gallon and Celsius differences are multiplied by 1.8.
Open-top tanks lose additional heat through evaporation, so the calculator adds 25% for them. The recommended heater is the next standard size (25, 50, 75, 100, 150, 200, 250, 300, 500, 800 or 1,000 W) at or above the power needed per heater. Very large tanks can need a little less per gallon because they have less surface area relative to volume.
- gallons
- Tank water volume in US gallons
- ΔT
- Desired water temperature minus coldest room temperature, in °F
- cover factor
- 1.0 with a lid, 1.25 for an open-top tank
Worked examples
20-gallon tropical tank in a 68 °F room
Raising 20 gallons from 68 °F to 78 °F is a 10 °F rise: 0.5 × 20 × 10 = 100 W, exactly the classic 5 watts per gallon. A 100 W heater fits.
200 L open-top tank, 25 °C in an 18 °C room
200 L is 52.8 gallons and a 7 °C rise is 12.6 °F. 0.5 × 52.8 × 12.6 = 333 W, plus 25% for the open top gives about 416 W, so a 500 W heater (or two heaters) is needed.
75-gallon tank with two heaters in a 65 °F basement
A 13 °F rise on 75 gallons needs 0.5 × 75 × 13 = 487.5 W. Split between two heaters that is about 244 W each, so two 250 W heaters do the job with a built-in backup.
Frequently asked questions
What size heater do I need for my aquarium?+
Allow about 0.5 W per gallon for every °F you heat above the room. That is 5 W per gallon for a typical 10 °F rise, so a 20-gallon tank needs a 100 W heater and a 55-gallon tank about 275–300 W.
Is the 5 watts per gallon rule accurate?+
It is accurate for a tank kept about 10 °F (5–6 °C) above the room. In a warm room you need less — around 2.5 W per gallon for a 5 °F rise — and in a cold basement or garage you may need 7–10 W per gallon.
Can an aquarium heater be too big?+
A heater with a reliable thermostat simply switches off sooner, so modest oversizing is fine. The risk is a stuck thermostat: a hugely oversized heater can overheat a small tank quickly, which is why two smaller heaters are safer in big tanks.
Should I use one or two heaters?+
For tanks over about 50 gallons, two heaters at opposite ends give more even temperatures and redundancy. If one fails off, the other limits the temperature drop; if one sticks on, each is too small to cook the tank quickly.
What temperature should a tropical tank be?+
Most tropical community fish do well at 76–80 °F (24–27 °C). Discus and some others prefer 82–86 °F, while goldfish and many coldwater species need no heater at all in a heated home.