Aquarium Heater Watts
Calculator

The calculator multiplies the tank volume by the temperature rise needed above ambient room temperature, using a standard wattage-per-liter (or per-gallon) rule of thumb, since larger volumes and bigger temperature gaps both demand more heating power.

The heater only needs to make up the difference between the room's ambient temperature and your target tank temperature. A tank in a cold basement needs significantly more wattage than the same tank in a warm, climate-controlled room.

Round up. Undersized heaters struggle to maintain temperature during cold snaps and run continuously, which shortens their lifespan, while a slightly oversized heater cycles on and off normally and holds temperature more reliably.

For tanks larger than roughly 40 gallons (150 liters), many aquarists split the wattage across two heaters placed at opposite ends. This gives more even heat distribution and means a single heater failure won't leave the tank completely unheated.

The calculator assumes a standard covered tank; an open top or a shallow, wide tank loses more heat to evaporation and air contact, so you may want to size up slightly if your setup lacks a lid or canopy.