Aquarium Heater Size Calculator
Calculate the correct wattage aquarium heater needed to maintain your target water temperature above room temperature.
Results
What is it?
The Aquarium Heater Calculator uses the rule of thumb of 0.5W per liter per degree Celsius of temperature difference to determine the minimum heater wattage, then adds a safety buffer to account for heat loss through tank walls, hood gaps, and cold spells.
How to use
Enter tank volume in liters, the minimum expected room temperature in winter, and your target water temperature. Set the safety factor (10-25% is standard). The recommended wattage is rounded up to the nearest commercial heater size.
Example scenario
A 200L tank needs to stay at 26°C when the room drops to 20°C: 200 x 6 x 0.5 = 600W base. With 20% safety: 720W. In practice, use two 300W heaters for redundancy and more even heat distribution.
Pro tip
Using two smaller heaters is better than one large one: if one fails open (heating continuously), a smaller heater is less likely to overheat the tank. If one fails closed (stops heating), the other provides backup. Place the heater near the water circulation pump/return for even distribution. Calibrate your heater thermometer against a reliable reference thermometer before relying on it.