Aquarium Heater Wattage Calculator
A heater must be large enough to lift and hold the tank at the target temperature against the coolest room conditions. This calculator multiplies the tank volume by a watts-per-gallon-per-degree sizing factor and the temperature rise above room temperature to estimate the heater wattage you need, then suggests rounding up to a typical heater size. The sizing factor is user-editable so you can match a more or less conservative rule.
Heater wattage formula
Wattage = volume (gal) * temperature rise (degrees) * factor
Watts per gallon = wattage / volume
The factor expresses how many watts each gallon needs per degree of temperature rise. Round the result up to the next available heater size, and split across two heaters for large tanks.
Worked example
A 40 gallon tank, 10 degree rise, factor 0.5: wattage = 40 * 10 * 0.5 = 200.00 W. Watts per gallon = 200 / 40 = 5.00. A 200 W heater (or two 100 W units) would suit.
Aquarium heater: frequently asked questions
How many watts does an aquarium heater need?
A common rule of thumb is roughly 3 to 5 watts per gallon of water, with more power needed when the room is cold or the target temperature is high. This calculator multiplies tank volume by a watts-per-gallon-per-degree factor and the temperature rise so you can size for your actual conditions.
What temperature rise should I use?
The temperature rise is the difference between your target tank temperature and the lowest room temperature the tank will face. A larger rise needs more wattage. Enter the gap in degrees for your setup; a 5 to 10 degree Fahrenheit rise is typical indoors.
Should I round up to the next heater size?
Yes. Heaters are sold in fixed wattages, so choose the next size up from the estimate, and for large tanks consider two heaters so a single failure does not chill or overheat the tank. Always follow the heater manufacturer's guidance.
Official sources
- U.S. National Institute of Standards and Technology: Physical Measurement Laboratory (heat and power quantities).
- The wattage estimate follows from heat input scaling with volume and the temperature difference to be maintained.
Reviewed by the CalculatorHub team, edited by James Graham, 19 June 2026. See our methodology.