Aquarium Heater Wattage Calculator

A common guideline is about 2.5 to 5 watts per gallon, depending on how far above room temperature you heat. This tool uses 2.5 watts per gallon for a tiny rise up to 5 watts per gallon for a 10 F rise, and more for larger gaps, clamped to a sane 1.5 to 8 watts per gallon band.

Use the gap between the coldest your room gets and your target tank temperature. A 68 F room and a 78 F tropical tank is a 10 F rise. If the room drops at night or in winter, base the rise on that low so the heater is not undersized.

Heaters are sold in fixed sizes such as 50, 100, 150, 200 and 300 watts. The tool rounds your raw figure up to the next 25 watt step so it lines up with a real product you can actually buy, while also showing the exact raw wattage.

On tanks above roughly 40 gallons many keepers use two smaller heaters that together meet the target wattage. If one fails off the tank cools slowly, and if one sticks on the other is too small to overheat the tank, which adds a margin of safety.

It is only a starting point. Real needs depend on room temperature swings, tank insulation, lid coverage, airflow and how fast you want heat to recover. Size up for cold or drafty rooms, and always use a separate thermometer to confirm the actual water temperature.