Aquarium Heater Calculator
Enter your tank size and how much you need to raise the temperature — get the heater wattage, sized to the next standard unit.
Rule of thumb behind the numbers: watts = gallons × W/gal factor, where the factor grows with the temperature rise you need.
How the math works
The classic rule is 5 watts per gallon for the typical tropical aquarium in a normal room — that's roughly a 9–10 °F rise over room temperature. If you keep your home warm and only need a small lift, 2.5 W/gal is plenty. Fish rooms, basements, and winter drafts can double the requirement, which the room adjustment approximates. The calculator then rounds up to the next heater size actually sold (25 / 50 / 75 / 100 / 150 / 200 / 300 / 500 W).
Better than one big heater
For anything over 50 gallons, two heaters of half the total wattage — one at each end — heat more evenly and give you redundancy: if one thermostat sticks off, the other still prevents a temperature crash. Always use a separate thermometer to verify the heater's thermostat; built-in dials can be off by several degrees.
FAQ
- What about tanks in liters?
- Divide liters by 3.785 to get gallons, then calculate as usual. One watt heats about 0.2 gallons per °F over practical aquarium timescales.
- Can a heater be too powerful?
- Yes — an oversized heater in a small tank can cook livestock in minutes if the thermostat sticks on. Match the size to the tank and use a thermometer.
- Do goldfish tanks need heaters?
- Usually not — goldfish prefer 65–72 °F. Heaters are mainly for tropical species like tetras, bettas (78–80 °F), and cichlids.