heaters
Aquarium Heater Wattage by Tank Size — the Actual Formula
How many watts your aquarium heater needs by tank size and room temperature — the real formula, not a lookup guess.
By Tank & Tech · updated 2026-08-131 min read
The working formula
Aim for roughly 3-5 watts per gallon, then adjust for how cold the room gets. A tank in a 70F room needs less than the same tank in a 60F basement.
| Tank size | Warm room | Cold room |
|---|---|---|
| 10 gallon | 50 W | 75 W |
| 20 gallon | 75 W | 100 W |
| 40 gallon | 150 W | 200 W |
| 55 gallon | 200 W | 300 W |
Why not just buy the biggest
An oversized heater swings the temperature further if the thermostat sticks on. Match the wattage to the tank rather than over-provisioning, and let a separate thermometer be your check.
The verdict
Match wattage to tank size and room temperature rather than over-provisioning.
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