Fish Tank Heater Calculator
Calculate aquarium heater wattage from tank volume and the difference between room and target water temperature.
Covers freshwater and marine tropical tanks.
Aquarium heaters must be sized correctly to maintain stable water temperature for your fish. An undersized heater will struggle in a cold room; an oversized heater risks temperature spikes if the thermostat malfunctions.
Standard heater sizing formula: Heater Wattage = Tank Volume (gallons) × Temperature Rise Needed (°F) × 1.0 Or more conservatively: Heater Wattage = Tank Volume (liters) × 1 watt per liter (for a 10°F/5°C rise) For larger rises (cold rooms): ×1.5 to ×2 watts per liter
What each variable means:
- Tank Volume: the total water volume in gallons or liters. Note: actual water volume is less than tank capacity (subtract ~10% for substrate, rocks, decorations, air space).
- Temperature Rise Needed (ΔT): the difference between ambient room temperature and your target water temperature.
- 1 watt per liter: a common rule of thumb for rooms that stay above 65°F (18°C).
Target temperatures by fish type:
- Tropical freshwater (tetras, guppies, angelfish), 76–82°F (24–28°C)
- Discus: 82–88°F (28–31°C), warmest tropical
- Goldfish: 65–72°F (18–22°C), cooler water; often no heater needed
- Betta fish: 78–82°F (25–28°C)
- Reef/saltwater coral: 75–80°F (24–27°C)
- African cichlids: 76–82°F (24–28°C)
Worked example: 55-gallon (208-liter) freshwater tropical tank. Room temperature: 65°F. Target: 78°F. ΔT = 13°F ≈ 7°C. Water volume ≈ 208 × 0.90 = 187 liters (after displacement). Wattage needed = 187 × 1.5 (cold room multiplier) = 281 watts Round up to a 300W heater — standard size available.
Pro tip: Use two heaters at half the total wattage each. If one fails open (stuck on), the second provides redundancy. If one fails closed (stuck off), one heater maintains temperature while you find a replacement. Two 150W heaters beat one 300W heater for reliability in a tank with expensive fish.