Estimate heater wattage from net water volume and room temperature.
Open live calculator →A 180 litre tank with a 5°C lift returns a practical starting estimate around 150 watts.
Use tank volume and the required temperature lift to find a conservative starting wattage, then compare it with the heater maker’s chart.
The live tool updates immediately and shows the result in the units commonly used by aquarium keepers around the world.
Follow the manufacturer’s sizing chart.
Use a reliable thermometer.
Large tanks may be safer with two smaller heaters for redundancy.
A heater must replace the heat the aquarium loses to the room, so the two numbers that matter are water volume and the temperature difference between the room at its coldest and your target water temperature. A common rule of thumb is roughly 1 watt per litre for a moderate lift, less for a small lift, more when the room gets cold at night or in winter. The calculator applies a conservative version of that rule and then rounds up to the nearest wattage actually sold.
For tanks above roughly 200 litres, many experienced keepers split the required wattage across two smaller heaters at opposite ends of the tank. Two heaters spread warmth more evenly, and if one fails stuck-off the other keeps the tank from crashing overnight; if one fails stuck-on, half the wattage cooks the tank far more slowly than a single oversized unit would. A separate thermometer—never the heater's own dial—should be your reference for the true temperature.
Heaters only work where water moves. Place the heater near the filter outlet or in the sump so heated water circulates instead of layering. An inexpensive external thermostat controller adds a second cut-off that protects livestock from the most common heater failure mode, a stuck-on element. Always unplug heaters before water changes—running one exposed to air for even a minute can crack the tube.
Size for the worst realistic case, not the average. A living room that sits at 22°C in the evening may fall to 16°C on winter nights with the heating off, and that 6-degree gap is what the heater must bridge continuously. If your home is warm and stable year-round you can size lighter; an unheated basement or a drafty window wall justifies the next wattage up.
With a typical 4–6°C lift above room temperature, a 75–100 W heater is the usual starting point for 100 litres. Cold rooms or big night-time drops justify 150 W. Always confirm with the manufacturer's sizing chart.
A correctly working oversized heater just cycles less often, but if it fails stuck-on it overheats the tank faster. That is why conservative sizing—or two smaller units—is safer than one much larger heater.
If the room never drops below your target water temperature, the heater rarely runs—but nights, winters and air conditioning cause dips, so most tropical tanks still benefit from a modest thermostatic heater as insurance.
Constant running means the heater cannot keep up: it is undersized for the temperature gap, placed in dead water with no flow, or the tank loses heat quickly (open top, thin glass, cold room). Fix flow and evaporation first, then consider more wattage.