A source-labelled comparison tool instead of an invented universal watts-per-litre formula.
Use this only as a published-range comparison, then check the heater manufacturer’s current instructions and your room-to-water temperature difference.
Heater requirements depend on aquarium volume, the temperature difference between the room and water, heat loss through the tank and lid, airflow and the specific heater design. A single “watts per litre” formula hides too much.
The calculator uses simple volume bands as a comparison example based on common EHEIM thermocontrol product sizing ranges. It is deliberately labelled as a reference, not a recommendation engine.
Use manufacturer sizing tables and account for the temperature rise your setup actually needs.
Check the heater model’s current instructions, the aquarium’s real water volume, the expected coldest room temperature, the target water temperature and whether the manufacturer recommends one or multiple heaters for the setup.
A calculator can multiply volume by an arbitrary constant and look authoritative while being wrong for the room and equipment. Publishing the source of the comparison and exposing the limitation creates a more useful decision aid.
The wattage bands shown by the tool are a labelled reference example based on EHEIM's published thermocontrol range, not a claim that every aquarium of a given volume needs exactly that wattage. Heater choice also depends on the room temperature, target water temperature, aquarium shape, lid, airflow and the heater manufacturer's own instructions.
For the current reference, see the EHEIM UK catalogue. If you buy another brand, use that maker's sizing guidance instead of transferring these bands blindly.