Tool
Heater wattage calculator
Heater size depends on how far the water must be raised above room temperature — not just on tank size.
Your result
Around 115–175 W
A rise of 7°C. Round up to the nearest heater size you can buy.
What this calculator does
A heater has to replace the heat the tank loses to the room. The bigger the gap between the room and your target temperature, the more watts you need. This tool estimates a range from the water volume and the temperature rise, then rounds to a sizeable figure. It is a guide for choosing a heater, not a promise that one wattage will hold a temperature in every room.
When to use it
- Choosing a heater for a new tank.
- Deciding whether to upgrade in a cold room.
- Judging how much a heater must work through winter.
How to use it
- Use the real water volume, not the nominal tank size.
- Note your coldest room temperature — the winter low, not today's reading.
- Enter your target for the species you keep, and read the range.
Worked example
A 100-litre tank intended to run at 25°C in a room that drops to 18°C needs to lift the water 7°C, pointing to roughly 100–175 W. In a cold shed dropping to 12°C the same tank needs a larger heater, because the gap is wider.
Limitations
- Insulation and lids matter. A covered, insulated tank loses less heat than an open one.
- Two smaller heaters beat one big one for safety — if one fails on, it is less likely to cook the tank, and heat is spread more evenly.
- Always use a separate thermometer. Never trust the dial on the heater alone.
Frequently asked questions
Can a heater be too powerful?
Yes. A very large heater can overheat a small tank quickly if it sticks on. Size sensibly, and consider two smaller units.
Do I need a heater in a warm room?
Tropical fish usually need a stable temperature well above typical room temperature, so a heater is normal. Coldwater species may not need one.
Why does my heater run constantly?
It is undersized, the room is too cold, or the tank is losing heat. A bigger heater or better insulation is the fix.
Related guides
Sources and research notes
Heater sizing rules of thumb vary; a common guide is roughly 1 watt per litre for a modest temperature rise, rising with the gap to room temperature.