An aquarium heater is sized not by how fast it can warm water but by how well it keeps up with heat loss. That makes the wattage a function of water volume and the gap between your target temperature and the coldest the room gets in winter. Enter those four figures and this tool shows the gap, the raw value and the wattage rounded up to the next watt.
The math is wattage = water volume x temperature gap x watt coefficient, and the formula used is printed below the results. That coefficient has no officially established value. It shifts with tank material, whether the tank is covered, heat from lighting and air movement, which is why product sheets and hobby references disagree. So this tool does not supply a coefficient and asks for yours instead (as of October 2026).
This tool does not recommend heater products or brands, and it does not estimate how long warming up takes. A heater far larger than needed raises the risk of overheating if its thermostat fails, so always verify the water with a separate thermometer. This tool is for reference only and does not replace veterinary care. Contact your veterinarian or an aquatic animal hospital right away if your fish crowd to one side or breathe rapidly.
Frequently Asked Questions
Use the figure in your heater documentation or your own reference. There is no official standard, so this tool does not supply one.
If the target sits at or below the room temperature there is no heat to add, so the formula has nothing to work with. Check both figures.
This tool only gives the total wattage and does not cover split setups. Splitting does mean one failure will not crash the temperature.