How to size an aquarium heater
Enter your tank's water volume and how many degrees Celsius you need the water to sit above the room it lives in. As a rule of thumb, about one watt per litre maintains a five-degree rise in a normally heated room, and the estimate scales linearly from there. The calculator rounds up to the nearest standard heater size, and for large tanks it recommends splitting the load across two heaters, which gives more even heating and a safety margin if one fails.
A heater is sized for the coldest the room gets, so measure the winter room temperature, not the summer one. A thermometer in the tank is essential regardless of the wattage; the wattage only sets how quickly and reliably the heater can reach the target, not the target itself.
Why "watts per gallon" gets it wrong
The advice you will meet everywhere is three to five watts per gallon. It persists because it is easy to remember, and it is wrong because it leaves out the only variable that actually drives the answer: the temperature difference between the tank and the room. A heater is not warming water from cold. It is replacing heat that leaks out through the glass, and that leak is proportional to the gap it is fighting.
The consequence is that the same tank needs wildly different heaters in different rooms. A 200-litre tank at 25 °C in a heated living room at 22 °C is holding a 3-degree gap and barely needs anything. The identical tank in an unheated garage at 12 °C is holding a 13-degree gap - more than four times the work. Watts per gallon cannot see that difference, which is why a rule that works in one house fails in the next.
Undersized and oversized fail differently
An undersized heater fails gradually and visibly: it runs at 100% duty and still loses ground on the coldest night of the year, and the tank drifts down a degree or two. Unpleasant, rarely fatal, and you can see it coming on a thermometer.
An oversized heater fails suddenly and catastrophically. Every heater eventually sticks on - the thermostat welds closed, and it is the single most common way an aquarium heater kills a tank. When that happens the wattage decides how fast the water climbs and whether you find a warm tank or a boiled one. This is the real argument for splitting the load: two 100-watt heaters hold the same tank as one 200-watt, heat it more evenly, keep the tank survivable if one dies cold, and halve the damage if one sticks on. Above roughly 150 litres, two heaters is simply the better engineering.
Placement and the things wattage cannot fix
Put the heater where water moves - beside the filter outflow or intake - and never in a still corner. A heater in dead water heats a small pocket, satisfies its own thermostat, and leaves the far end of the tank cold while reading perfectly correct. Mount it fully submerged unless it is explicitly rated otherwise, and if you do a large water change, unplug it: a hot heater exposed to air can crack when the water returns.
Finally, wattage does not buy accuracy. Built-in thermostats are commonly off by a degree or more, and the dial is a suggestion rather than a measurement. Keep an independent thermometer in the tank and trust it over the heater. If your livestock is sensitive or the tank is valuable, an external temperature controller is the one upgrade that converts a stuck-on heater from a disaster into an inconvenience.