Aquarium Heater Size Calculator
Calculate the correct wattage aquarium heater for your tank size and room temperature.
Avoid underpowered heaters that can't maintain fish health.
Why heater sizing matters:
An undersized heater runs constantly but can never reach the target temperature, stressing fish and shortening heater life. An oversized heater can spike temperatures if the thermostat fails. Getting the wattage right means stable, safe temperatures.
The standard rule of thumb:
Watts needed = Tank volume (gallons) × Watts per gallon
The watts-per-gallon factor depends on how much temperature rise you need above your room temperature:
| Room Temp | Target Temp | Watts per Gallon |
|---|---|---|
| 72°F (22°C) | 78°F (26°C) | 2.5–3 W/gal |
| 68°F (20°C) | 78°F (26°C) | 4–5 W/gal |
| 64°F (18°C) | 78°F (26°C) | 5–6 W/gal |
| 60°F (16°C) | 78°F (26°C) | 6–8 W/gal |
For cold rooms or large tanks, always round up and use the next commercial heater size.
Worked example:
55-gallon tank, room temperature 68°F, target 78°F (10°F rise):
- Watts needed = 55 × 4.5 = 247.5 watts
- Round up to nearest standard size: 300-watt heater
For tanks over 75 gallons, it is generally better to use two heaters at half the total wattage each. This provides redundancy — if one fails open (always on), the second heater won’t overheat the tank. If one fails closed (off), the other still maintains temperature.
Common tropical fish temperature targets:
- Bettas: 76–82°F (24–28°C)
- Tetras, rasboras, livebearers: 72–79°F (22–26°C)
- Discus: 82–88°F (28–31°C)
- Goldfish (fancy): 65–72°F (18–22°C) — goldfish do NOT need a heater in most homes
- Axolotls: 60–68°F (16–20°C) — need a chiller, not a heater
Heater placement:
Position the heater near a filter outlet or powerhead so warm water circulates throughout the tank. Never place a heater in a corner with no flow — it creates hot spots that can shock or burn fish.
Titanium vs. glass:
Glass heaters are affordable but fragile — replace immediately if cracked. Titanium heaters cost more but are unbreakable and safe if a fish strikes the heater. For tanks with large cichlids or strong fish, titanium is worth the investment.