Aquarium heater size calculator

Estimate heater wattage for your tank using two published rules of thumb, and how long it takes to warm the water.

Two published rules of thumb

• TFH Magazine suggests 5 watts per gallon for small tanks and 3 watts per gallon for larger ones. Its examples, for normal room temperatures, are 100 W for 20 gallons, 175 W for 55 gallons and 300 W for 100 gallons, with more wattage if the room gets quite cold.
• AquariumScience.org argues that 5 W/gal is oversized: a heater that sticks “on” can overheat the tank. It recommends a total of about 3 W/gal or less, split between two smaller heaters (or one heater plus an external controller) for redundancy.

The calculator shows both. The difference between room and target temperature matters most: a tank 5 °F above room temperature needs far less heat than one 20 °F above.

Warm-up physics

Heating water takes about 4.184 kJ per kilogram per °C (1 thermochemical calorie = 4.184 J). Energy = liters × 0.997 kg/L × 4.184 × ΔT(°C). Time = energy ÷ heater watts. Because the tank also loses heat while warming, this is a minimum. Example: 20 US gal (75.7 L) from 68 °F to 78 °F (5.6 °C) needs about 1,750 kJ (0.49 kWh), so at least 4.9 hours with a 100 W heater.

Sources

• Tropical Fish Hobbyist Magazine, “Temperature Control”
• AquariumScience.org, “14.8. Aquarium Heaters”
• NIST Special Publication 811, Appendix B.8 (conversion factors)

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