Aquarium heater size chart
Recommended heater wattage for common tank sizes, by how far the water must be kept above the coldest room temperature. Water at 78 °F; lidded glass tanks.
| Tank | Real water | 73 °F room | 68 °F room | 63 °F room | 58 °F room |
|---|---|---|---|---|---|
| 3 gallon | 2.5 gal (10 L) | 10 W | 10 W | 25 W | 25 W |
| 5 gallon | 4.7 gal (18 L) | 10 W | 25 W | 25 W | 50 W |
| 10 gallon | 9.1 gal (34 L) | 25 W | 50 W | 50 W | 75 W |
| 20 gallon | 17.6 gal (67 L) | 50 W | 75 W | 100 W | 150 W |
| 29 gallon | 25.1 gal (95 L) | 50 W | 100 W | 150 W | 200 W |
| 40 gallon | 39.7 gal (150 L) | 75 W | 150 W | 200 W | 250 W |
| 55 gallon | 50.8 gal (192 L) | 2 × 50 W | 2 × 100 W | 2 × 150 W | 2 × 200 W |
| 75 gallon | 69.2 gal (262 L) | 2 × 75 W | 2 × 150 W | 2 × 200 W | 2 × 250 W |
Columns are the coldest room temperature, a lift of 5, 10, 15 and 20 °F. Sizes use real gallons × lift × 0.3 W, rounded up to a common heater size; tanks over 50 gallons are split across two heaters. Open tops need about 25% more. For any other size or temperature use the heater size calculator; the heater guide explains placement and safety.
- 3 gallon heater size10 W in a 68 °F room · 2.5 gal real water
- 5 gallon heater size25 W in a 68 °F room · 4.7 gal real water
- 10 gallon heater size50 W in a 68 °F room · 9.1 gal real water
- 20 gallon heater size75 W in a 68 °F room · 17.6 gal real water
- 29 gallon heater size100 W in a 68 °F room · 25.1 gal real water
- 40 gallon heater size150 W in a 68 °F room · 39.7 gal real water
- 55 gallon heater size2 × 100 W in a 68 °F room · 50.8 gal real water
- 75 gallon heater size2 × 150 W in a 68 °F room · 69.2 gal real water
FAQ
What size heater do I need for my tank?
About 0.3 W per gallon of real water for each °F the tank is kept above the coldest room temperature, rounded up to a standard size. For a 10 °F lift that is about 3 W per gallon: 50 W for a 10-gallon, 100 W for a 29-gallon and two 100 W heaters for a 55-gallon.
When should I use two heaters?
From about 50 gallons, or when one heater would need more than 300 W. Two heaters of at least half the wattage each heat long tanks evenly and limit the damage if one fails.