A common starting rule is about 5 watts per US gallon when the aquarium needs to run roughly 10°F above room temperature and has a lid, but that is only a guide. Colder rooms, open tops, draughts and large temperature swings require more heating capacity. For larger tanks, two smaller heaters can provide more even heat and some redundancy.
Choosing a heater by tank volume alone misses the biggest variable: the room around the aquarium. A 100-litre tank in a warm living room and the same tank in a cold garage do not have the same heat loss.
Start with actual water volume
Subtract the displacement of substrate and hardscape if you want a closer estimate. Heater calculators work best when the volume reflects the water actually being heated.
Use the heater-size calculator as a planning guide, then verify the result against the heater manufacturer's own chart.
Temperature difference drives demand
The colder the room compared with the target water temperature, the more energy the heater must supply. A tank near a cold exterior wall or air-conditioning vent may also lose heat faster.
Do not assume a small wattage that works in summer will necessarily hold temperature through winter.
Lids reduce heat loss
Evaporation removes heat. A lid can reduce evaporative cooling, although it also changes gas exchange and may not suit every setup.
Open-top tanks often need more heating capacity in cool rooms.
One heater or two
For larger aquariums, splitting the required wattage between two heaters can spread heat and provide partial backup if one fails off. It can also reduce the consequences if one heater sticks on, depending on the total wattage.
Use separate positions with good water movement.
Always verify with a thermometer
Heater dials and thermostats are not a substitute for an independent thermometer. Check the actual aquarium temperature after installation and during seasonal room changes.
Frequently asked questions
Is 5 watts per gallon always correct?
No. It is a rough starting point under specific room-temperature and heat-loss conditions.
Can a heater be too small?
Yes. An undersized heater may run almost continuously and still fail to reach the target.
Should I use two heaters in a large aquarium?
Often it is a sensible redundancy strategy, provided the combined wattage is appropriate.
Sources and further reading
This guide separates general aquarium arithmetic from fish-welfare decisions. For species-specific limits, always check the needs of the fish you intend to keep.