🌡️ Aquarium Heater Wattage Calculator

The most widely cited rule of thumb for aquarium heater sizing is "3-5 watts per US gallon" (for roughly a 10°F/5.5°C rise above room temperature). This calculator converts that to metric (watts per liter) and applies it in bands by tank size — smaller tanks have a higher surface-area-to-volume ratio and lose heat relatively faster, so they get a higher W/L, while larger tanks get a lower W/L.


If your desired temperature rise differs from the 5.5°C baseline, the calculator scales the result by the ratio of your rise to 5.5°C. This is a starting-point reference for keeping a healthy aquatic pet environment — for your specific setup, also consult an aquarium specialist or species care guide.

The "3-5 watts per gallon" rule of thumb is only a starting-point estimate — actual heater wattage needs depend on several factors. (1) Room temperature — a cold room (basement, unheated space in winter) will need a higher-wattage heater. (2) Tank shape and surface area — a wide, shallow tank loses more heat than a narrow, deep tank of the same volume. (3) Water flow — strong circulation from filters or powerheads changes how heat disperses. (4) Whether the tank is covered — an open-top tank loses more heat to evaporation. So treat the calculated wattage as a starting point, observe actual water temperature with a thermometer over the first few days after installation, and adjust as needed. For advice specific to your aquatic pet, consult an aquarium specialist.