Aquarium Heater Size Calculator
Heater wattage depends on how far above room temperature you need to hold the water, not just tank size. Enter your volume and both temperatures to get a recommended wattage in standard heater sizes.
Uses standard hobby guidance: about 2.5 W/gal for a rise up to 9°F, 4 W/gal up to 18°F, 5 W/gal up to 27°F, and 6 W/gal beyond that. Base the rise on the coldest your room actually gets — winter nights, not the daytime average.
How heater sizing actually works
The watts-per-gallon rules of thumb are all about the temperature gap. A tank sitting in a 74°F living room only needs to add a few degrees, so 2.5 W per gallon is plenty. The same tank in a 62°F basement is fighting a 16°F gap around the clock and needs roughly 4 W per gallon. Undersized heaters run constantly, never quite reach the set point on cold nights, and wear out their thermostats faster. Size for the coldest your room realistically gets, not its average.
On larger tanks — roughly 75 gallons and up — two smaller heaters beat one big one. Placing them at opposite ends evens out warm and cold spots, and the failure modes are far more forgiving: a stuck-on 150 W heater overheats a big tank slowly enough to catch, where a stuck-on 300 W unit can cook it overnight. If one fails off, the other keeps the tank out of dangerous territory until you notice. The AquaLens app logs your temperatures over time, so drift from a failing heater shows up before your fish tell you about it.