Aquarium Heater Wattage Calculator
Size the aquarium heater you need from your tank volume and how far you have to raise the water above room temperature — with the next standard heater size to buy.
How it works
Watts ≈ tank gallons × 0.5 × (target °F − room °F) · round up to the next standard heater size
A heater only has to replace the heat the tank loses to the room, so the wattage you need scales with both the water volume and how far above ambient you want to hold it. The hobby rule of thumb is about 0.5 watt per gallon for every degree Fahrenheit of rise, which lands close to real heat-loss figures for a typical glass tank in a heated home. Colder rooms, larger surface areas and open-top tanks lose heat faster, so size up — an oversized heater simply cycles off sooner, while an undersized one runs constantly and still falls short.
Worked example
A 20-gallon tank in a 68 °F room targeting 78 °F needs a 10 °F rise: 20 × 0.5 × 10 = 100 W. The next standard heater size is 100 W, which will comfortably maintain the tank.
Frequently asked questions
Should I use one big heater or two smaller ones?
Two heaters that together meet the wattage are safer for tanks over about 40 gallons. If one sticks off the other keeps the tank warm, and if one sticks on it can only overheat the tank half as fast, giving you time to react.
Does a bigger heater overheat the tank?
No. A thermostatic heater only runs until it reaches its set point, then switches off. A larger heater just reaches temperature faster and cycles more often; the risk with oversizing is only if the thermostat fails on, which is why two smaller units are preferred on big tanks.
Why does room temperature matter so much?
The heater fights the temperature difference between the tank and the room. A tank in a 60 °F basement needs far more wattage to hit 78 °F than the same tank in a 74 °F living room, because heat escapes faster the bigger that gap is.
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This calculator is for educational purposes. Double-check important results before acting on them.