Aquarium Heater Size Calculator
Tropical fish need stable warm water. Tell us your tank size and temperatures — we'll recommend the right heater wattage.
Calculate heater wattage
How heater sizing works
The key number isn't just tank size — it's the gap between room temperature and target temperature. Heating a 20-gallon tank from 68°F to 78°F is easy; heating it from 55°F to 78°F in a cold basement needs far more power.
This calculator uses the widely followed guideline:
- 3 watts per gallon — when the room is within 10°F of the target
- 5 watts per gallon — when the gap is 10–20°F
- 7 watts per gallon — for cold rooms with a gap over 20°F
It then rounds up to the nearest standard heater size (25, 50, 75, 100, 150, 200, 250, 300W).
Heater safety tips
- Two smaller heaters beat one big one on tanks over 75 gallons — they heat more evenly and provide a backup if one fails.
- Place the heater near the filter outflow so warm water circulates through the whole tank.
- Always use a separate thermometer to verify — never trust the heater's dial alone.
- Unplug the heater during water changes if the water level drops below its minimum line.
Frequently asked questions
What size heater do I need for a 20 gallon tank?
Usually 100W. The rule of 5 watts per gallon covers most rooms: 20 × 5 = 100W. In a warm room you can drop to 75W; in a cold room go up to 150W.
What size heater for a 10 gallon tank?
A 50W heater is the standard choice for a 10-gallon tropical tank in a normal room.
Can a heater be too powerful?
Yes — an oversized heater with a stuck thermostat can overheat a tank fast. Slightly oversizing is fine, but massively oversizing adds risk. Two medium heaters are safer than one giant one.
Do I need a heater for goldfish?
No. Goldfish are cold-water fish and do best at 65–72°F without a heater, as long as the room doesn't freeze.