Aquarium Heater Wattage Calculator
Calculate the right aquarium heater wattage from tank size, room temperature, and target water temp.
Get a recommended watts-per-gallon for your setup.
Aquarium heater sizing depends on three main factors: tank volume, the difference between room temperature and desired water temperature, and whether the tank is in a drafty location.
General rule:
Watts needed = Tank Volume (gallons) × Temperature Difference (°F) × 0.5
For metric: Watts needed = Tank Volume (liters) × Temperature Difference (°C) × 1.0
Standard heater sizing chart (imperial):
| Tank Size | 5°F Diff | 10°F Diff | 15°F Diff |
|---|---|---|---|
| 5 gal (19 L) | 25 W | 50 W | 75 W |
| 10 gal (38 L) | 50 W | 75 W | 100 W |
| 20 gal (76 L) | 50 W | 100 W | 150 W |
| 40 gal (151 L) | 100 W | 200 W | 300 W |
| 55 gal (208 L) | 150 W | 275 W | 400 W |
| 75 gal (284 L) | 200 W | 375 W | 500 W |
Common fish temperature requirements:
- Tropical freshwater (tetras, guppies, angelfish): 75–80°F (24–27°C)
- Goldfish: 65–72°F (18–22°C) — may not need a heater
- Bettas: 76–82°F (24–28°C)
- Discus: 82–86°F (28–30°C)
- Marine/reef: 75–80°F (24–27°C)
Important considerations:
- For tanks over 40 gallons, consider using two smaller heaters instead of one large one. This provides backup if one fails.
- Submersible heaters are more efficient than hang-on heaters.
- Place the heater near a water flow source (filter output) for even heat distribution.
- Always use a separate thermometer to verify temperature, not just the heater’s built-in dial.
Tip: If your room temperature fluctuates widely (e.g., drops at night), size up to the next wattage level for consistent water temperature. A slightly oversized heater with a good thermostat is better than an undersized one running constantly.