How aquarium heater wattage calculator works
Aquarium heaters are sized with the classic rule of 2.5 to 5 watts per US gallon (roughly 0.7–1.3 W per liter). The low end suits warm rooms and small temperature lifts; the high end suits cold rooms or tropical targets far above ambient. The calculator computes both bounds for your tank and then blends between them based on the actual temperature rise you need.
The rise is what matters, not the absolute temperature: a 78°F (25.5°C) target in a 74°F room is an easy 4°F job, while the same target in a 60°F basement is a demanding 18°F job — the tank size alone cannot tell them apart. That is why the tool asks for room temperature and target temperature, and accepts gallons or liters and Fahrenheit or Celsius.
The recommendation rounds to the nearest 5 W and maps to real stock sizes (25, 50, 75, 100, 150, 200, 250, 300, 400, 500 W). For large tanks needing roughly 200 W or more, two smaller heaters — one at each end — beat a single big unit: they spread heat evenly and add redundancy, since one failing heater is less catastrophic than one failing heater alone.