Aquarium heater wattage calculator

How the calculator works

The starting point is a practical rule: about 3-5 watts per gallon (roughly 1 W per liter). The calculator then adjusts that figure with the difference between the coldest room temperature and the temperature you want in the tank. The bigger the gap, the more power you need.

• Warm room or small gap (under about 5 °F / 3 °C): around 2-3 W per gallon (0.5 W per liter) is usually enough, especially in large tanks.
• Typical room: about 3-5 W per gallon (1 W per liter).
• Cold room or small tank: up to 5-6 W per gallon (1.5 W per liter).

Why tank size matters

Small tanks lose heat faster relative to their volume, because they have more surface area per gallon. That is why nano tanks need proportionally more watts. Large tanks are more stable, but it is wise to split the power between two heaters: if one fails stuck on, it is less likely to overheat the tank, and if it fails off, the other keeps the water from cooling too quickly.

A lid makes a big difference: open-top tanks lose a lot of heat through evaporation. Tanks near windows, drafts or on cold floors need more power.

How to read the results

The result is a recommended wattage. Choose the nearest heater size available, ideally rounding up slightly. An oversized heater with a reliable thermostat is not a problem by itself, but a very oversized one can cause big swings if the thermostat fails, so do not exaggerate. Always check the actual temperature with a separate thermometer.

Most tropical fish do best at 75-80 °F (24-27 °C). Place the heater near the filter outlet or in an area with flow so heat spreads evenly, and unplug it during water changes before the water level drops.