Heater wattage calculator
How many watts does your tank need? 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.
What sets the wattage is the gap between your target and the coldest the room gets, not the volume alone. Size for the coldest night, then a good thermostat idles the rest of the time.
Why wattage is about temperature rise, not volume
The number everyone quotes is watts per gallon, but that only tells half the story. What a heater actually fights is the gap between your target temperature and the coldest the room gets. A 20-gallon tank in a warm 74°F room needs far less heat than the same tank in a 60°F basement. Size for the temperature rise on the coldest night, and the thermostat coasts the rest of the year.
A working guide
- Small rise (up to 5°F): about 2.5 watts per gallon.
- Typical rise (6–10°F): about 3.5 watts per gallon.
- Large rise (11–15°F): about 5 watts per gallon.
- Very large rise or a cold room: 6.5+ watts per gallon, and consider two heaters.
Why two smaller heaters beat one big one on a large tank
On anything over about 55 gallons, two heaters that together hit the target are safer than one oversized unit. If a thermostat sticks on, a half-size heater cannot cook the tank on its own, and the heat spreads more evenly end to end. It also means a single failure never leaves the tank cold. Whatever you buy, run a separate thermometer — a heater dial reads approximately.
The rest of the decision
Wattage gets you in the right size band; reliability and the safety cut-off decide which heater to actually buy. We rank those in the full aquarium heater rankings.
Source
- Aquarium heater sizing and safety — Aquarium Co-Op
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