Heater wattage calculator
Set the volume, the coldest your room gets and your target temperature for the heater size to shop for — and which of our reviewed heaters cover it.
Actual water volume, not the box figure — subtract ~10% for substrate and hardscape.
Use the overnight low in winter, not the daytime average.
76–78°F suits most community freshwater fish.
Rounded up to the nearest real heater size. Undersizing is the risky mistake — a slightly oversized heater on a good thermostat simply cycles off sooner.
The rule everyone quotes is “3 to 5 watts per gallon,” but that ignores the one thing that actually matters: the gap between your target temperature and the coldest the room gets. A 20-gallon tank in a warm room needs far less heat than the same tank in a cold basement.
So we size from that gap, then round up to a real heater size. Undersizing is the risky mistake; a slightly oversized heater on a good thermostat just cycles off sooner.
Common questions
What size heater for a 20-gallon tank?
Roughly 3 to 5 watts per gallon in a normally heated room, so about 75 to 100 watts for a 20-gallon. A colder room or a bigger temperature rise needs more; the calculator sizes it from your actual room low and target temperature.
Is it better to oversize or undersize an aquarium heater?
Slightly oversize. An oversized heater simply cycles off sooner and costs no more to run, while an undersized one runs constantly and still cannot hold temperature on a cold night. Undersizing is the risky mistake.
Should I run two heaters?
On larger tanks, roughly 55 gallons and up, two smaller heaters are a common safety choice: if one sticks off the other covers it. Whatever you run, always pair it with a separate thermometer, since the dangerous failure is a thermostat stuck on.