About this tool
The Aquarium Heater Wattage Calculator is a free, in-browser tool that sizes an aquarium heater from three inputs: tank volume, the temperature you want to hold, and the coldest the room gets. Because a heater only has to make up the gap between the water and the room, the required power scales with the temperature rise, not just the tank size. Everything is computed locally in your browser.
The method finds the required rise dT = target − coldest room temperature (in °C; °F differences are converted). It then applies the classic 3–5 watts-per-gallon rule of thumb, scaled by the rise: about 2.5 W/gal for a small rise (dT ≤ 3 °C), rising to about 5.5 W/gal for a large rise (dT ≥ 7 °C), interpolated in between (so a 5 °C rise is about 4 W/gal). Wattage = volume in US gallons × rate. The result is rounded up to the nearest standard heater size (25, 50, 75, 100, 150, 200 or 300 W), and if the need exceeds 250 W it recommends splitting across two heaters for redundancy and even heating. It also shows watts per gallon (typically 3–5) as a sanity check.
As a worked example, a 100-liter tank (about 26.4 US gallons) held 5 °C above the room needs about 4 W/gal, or roughly 106 W — a single 150 W heater covers it. A well-insulated room, a tight lid, or a warmer house reduces the requirement; a chilly, drafty room raises it. Remember a heater can only warm water: cooling a tank in hot weather needs a fan or a chiller, not a bigger heater.