Aquarium Heater Size Calculator
Heater wattage depends on both your tank size and how far you need to heat above room temperature. Enter both for a recommendation, or use the watts-per-gallon chart below.
Aquarium heater watts-per-gallon chart
The classic rule of thumb is 3–5 watts per gallon, but the honest answer depends on the temperature rise, the gap between your room temperature and your target tank temperature (76–80°F for most tropical fish). Small tanks lose heat faster and need proportionally more watts per gallon; large tanks heat more efficiently but benefit from splitting the load across two heaters.
| Tank size | Warm room (raise ~9°F / 5°C) | Average room (raise ~13°F / 7°C) | Cold room (raise ~18°F / 10°C) |
|---|---|---|---|
| 5 gallon | 25 W | 25 W | 50 W |
| 10 gallon | 50 W | 50 W | 75 W |
| 20 gallon | 50 W | 75 W | 150 W |
| 29 gallon | 75 W | 100 W | 200 W |
| 40 gallon | 100 W | 150 W | 2 × 150 W |
| 55 gallon | 150 W | 200 W | 2 × 200 W |
| 75 gallon | 200 W | 300 W | 2 × 300 W |
Room temperature swings matter more than averages: if your house drops to 62°F overnight in winter, size for that low point, not the daytime reading. When a tank falls between sizes, round up. And once you know your wattage, see our tested aquarium heater picks for reliable units at each size.
Frequently Asked Questions
How many watts per gallon for an aquarium heater?
The standard guideline is about 3–5 watts per gallon. Use the lower end if your room stays warm and the higher end (or more) if the room is cold or the tank is large. Small tanks need proportionally more watts per gallon because they lose heat faster.
What size heater do I need for a 10 gallon tank?
A 50-watt heater covers a 10-gallon tank in a normally heated room (about a 9–13°F rise). If the room runs cold, a basement or a house kept below 65°F, step up to 75 watts.
What size heater do I need for a 20 gallon tank?
A 75–100 watt heater suits a 20-gallon tank in a typical room. Use 150 watts (or two 75s) if the room is cold.
Should I use one heater or two?
For tanks over about 40 gallons, two smaller heaters at opposite ends give more even heating and a safety margin, if one sticks off you have backup, and if one sticks on it's less likely to cook the tank.
Is a bigger heater always better?
No. An oversized heater heats faster but cycles harder, and if the thermostat ever sticks on it can overheat the tank quickly. Match wattage to the chart and prefer two mid-size heaters over one giant unit on large tanks.