Aquarium Heater Calculator
Find the right heater wattage for your aquarium based on tank size, room temperature, and desired water temperature. Includes dual-heater recommendations for large tanks.
Recommended Heater
Recommended Wattage
75W
Temperature Differential
10°F
Tank Size
20 gal
Heater Wattage Reference
| Tank Size | 5°F diff | 10°F diff | 15°F diff |
|---|---|---|---|
| 5 gallons | 25W | 25W | 25W |
| 10 gallons | 25W | 50W | 50W |
| 20 gallons | 50W | 75W | 100W |
| 30 gallons | 75W | 125W | 150W |
| 40 gallons | 100W | 150W | 200W |
| 55 gallons | 150W | 200W | 275W |
| 75 gallons | 200W | 275W | 375W |
| 100 gallons | 250W | 350W | 500W |
Recommended Heaters
How It Works
The calculator determines heater wattage based on the temperature differential between your room and desired water temperature, using industry-standard watts-per-gallon guidelines:
- 5°F or less: 2.5 watts per gallon
- 6-10°F: 3.5 watts per gallon
- 11-15°F: 5 watts per gallon
- 15°F+: 6 watts per gallon
The result is rounded up to the nearest 25W to match standard heater sizes available on the market. For tanks over 40 gallons, the calculator recommends splitting the wattage across two heaters for safety and even heat distribution.
Frequently Asked Questions
How many watts per gallon do I need for an aquarium heater?
It depends on the temperature differential between your room and desired water temperature. For a 5 degree F difference, use about 2.5 watts per gallon. For 10 degrees, use 3.5 watts per gallon. For 15+ degrees, use 5 watts per gallon. Our calculator handles this math automatically.
Should I use two heaters instead of one?
For tanks over 40 gallons, using two smaller heaters is strongly recommended. If one heater fails stuck-on, a smaller wattage unit is less likely to overheat the tank. If one fails off, the other keeps the temperature from crashing. Place them at opposite ends for even heat distribution.
What happens if my aquarium heater is too powerful?
An oversized heater will cycle on and off more frequently and can overheat the water dangerously fast if the thermostat fails in the on position. Always match the heater wattage to your tank size and temperature needs. A heater that is slightly underpowered is safer than one that is overpowered.
Do I need a heater if my room stays at 78 degrees F?
If your room temperature consistently matches your desired water temperature, you may not need a heater. However, nighttime temperature drops, air conditioning, and seasonal changes can cause fluctuations. Many fishkeepers use a heater as insurance even in warm rooms to prevent sudden temperature swings.