FishTankAquarium heater size calculator
What size heater do you need? Enter your tank size and how cold your room gets, and this calculator works out the wattage, then suggests a heater to match.
This is an estimate. A lid, drafts and the shape of the tank all change how fast it loses heat. Use the coldest temperature your room reaches, such as on a winter night.
A heater that fits
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Tanks under 5 gallons are hard to keep at a steady temperature, so this pick starts at 5 gallons.
How heater size works
A heater's wattage is the most heat it can put out. Its thermostat switches it off once the water reaches your set temperature, so a heater that's a little bigger than needed simply runs less. One that's too small runs all the time and still can't keep up on cold nights.
Two things decide the size you need: how much water you're heating, and how much warmer than the room the tank has to stay. Small tanks lose heat faster for their size, so they need more watts per gallon than big ones.
The usual rule of thumb is 3 to 5 watts per gallon. This calculator starts from that rule and adjusts it for your room temperature and tank size.
Aquarium heater size chart
Heater sizes for a tropical tank kept at 78°F, worked out the same way as the calculator above:
| Tank | Room at 72°F | Room at 68°F | Room at 62°F |
|---|---|---|---|
| 5 gallons | 25 W | 25 W | 50 W |
| 10 gallons | 50 W | 50 W | 75 W |
| 20 gallons | 50 W | 75 W | 100 W |
| 29 gallons | 75 W | 100 W | 150 W |
| 40 gallons | 100 W | 125 W | 200 W |
| 55 gallons | 150 W | 200 W | 300 W |
| 75 gallons | 200 W | 250 W | 2 × 200 W |
One heater or two?
For tanks over about 55 gallons, two smaller heaters at opposite ends of the tank are better than one big one. The heat spreads more evenly, and if one heater fails, the other keeps the water from getting dangerously cold. If one sticks on, a smaller heater also takes longer to overheat the tank.
Heater tips
- Put the heater near the filter outflow, so moving water carries the heat around the tank.
- Let a new glass heater sit in the tank water for about 30 minutes before you plug it in, so the glass can adjust to the water temperature.
- Unplug the heater before every water change, so it's never switched on out of the water.
- Always use a separate thermometer. Heater dials are often a few degrees off.
- Not sure how many gallons your tank holds? Use the aquarium volume calculator.
Related: Aquarium volume calculator · Everything you need to start a fish tank · Betta fish tank setup