What wattage aquarium heater do I need for my tank?

The rough rule cited across reviewers is roughly 3 to 5 watts per gallon, so a 50W unit suits 5 to 15 gallon tanks, a 300W unit covers around 40 to 75 gallons, and 500W and up is for 75 to 120+ gallons. Several r/Aquariums threads recommend sizing slightly higher in cold rooms and splitting the load across two heaters in big tanks so a single failure can't cook or chill everything.