Heater wattage calculator
Work out the right heater size for your tank based on its volume and the temperature difference your heater needs to maintain.
Updated 30 July 2026
Don't know your tank's water volume? Use the tank volume calculator first.
Results
These are guideline figures. Heaters are typically sold in standard wattages (50, 100, 150, 200, 300W); choose the nearest standard size at or slightly above the recommended figure. For tanks over 200 litres, two smaller heaters are safer and more reliable than one large one.
How this calculation works
Heater wattage requirements depend on two things: how much water needs to be heated, and how big a temperature increase the heater needs to maintain. A heater isn't heating water from cold; it's maintaining the tank temperature against constant heat loss to the surrounding room.
The general guideline is roughly 1 watt per litre of water for a typical 5°C lift above room temperature. So a 100-litre tank in a 22°C room targeting 25°C needs around 60 watts, and the same tank in a 15°C room targeting the same 25°C needs closer to 200 watts. The bigger the differential between your room and your target tank temperature, the more wattage you need.
This calculator gives a conservative estimate suitable for typical home aquariums. For unusually large temperature differentials or tanks in unheated outbuildings, factor in extra capacity.
Heater wattage by tank size and temperature lift
Expressed as a rate, that works out to 0.2 watts per litre for each degree Celsius of lift (roughly 0.4 watts per US gallon per degree Fahrenheit). The lift is the gap between the coldest your room gets and the temperature you're holding the tank at, not the tank temperature itself: a 100 litre tank held at 25°C faces a 5°C lift in a 20°C room and a 10°C lift in a 15°C one. The table gives the nearest standard heater size at or above the calculated figure:
| Tank volume | 3°C lift | 5°C lift | 8°C lift | 10°C lift |
|---|---|---|---|---|
| 25 L 6.6 US gal | 25 W | 50 W | 50 W | 50 W |
| 50 L 13.2 US gal | 50 W | 50 W | 100 W | 100 W |
| 75 L 19.8 US gal | 50 W | 100 W | 150 W | 150 W |
| 100 L 26.4 US gal | 75 W | 100 W | 200 W | 200 W |
| 150 L 39.6 US gal | 100 W | 150 W | 250 W | 300 W |
The table stops at 150 litres deliberately. Past that point, the wattage needed for the larger lifts is more than any single standard heater provides, so there is no honest one-heater answer to print. For a bigger tank, or for any tank facing a lift beyond about 10°C, use the calculator above and split the result across two heaters, for the reasons in the next section.
Why two smaller heaters often beats one large one
For tanks above 200 litres, splitting the wattage between two heaters offers two real benefits. If one heater fails by switching off, the other prevents the tank cooling dramatically, which is particularly important in cold climates and during overnight failures. And if one heater fails by sticking on, it can only overheat the tank by half as much as a single oversized heater could. Two 150W heaters in a 300L tank is safer than one 300W heater in the same tank.
What this calculator does not account for
This is a wattage estimate, not a heater recommendation. Other factors worth considering when buying a heater: build quality, thermostat accuracy, shatter resistance, in-line vs submersible style, and warranty coverage. A genuinely good 100W heater is a better buy than a cheap 200W one of doubtful quality.
For unusually exposed tanks (those near windows, in unheated rooms, or in cold climates), add 20-50% to the calculated wattage. For tanks in well-insulated cabinets or warm rooms, the calculated figure is often slightly conservative.