A common rule is about 1 watt per litre of water in a heated room, and 1.5–2 watts per litre in a cold garage, conservatory or draughty room. For tanks of about 150 litres or more, split the total across two smaller heaters, so a heater stuck on can’t overheat the tank and one failing doesn’t leave it cold.
Enter your water volume to see the heater size.
The rule of thumb
AllPondSolutions, a UK retailer, gives about 1 W per litre in a normally heated room, rising to 1.5–2 W per litre in a cold or draughty space. Top Shelf Aquatics gives 3–5 W per US gallon (about 0.8–1.3 W per litre), using more where the room is much colder than the tank.
| Water volume | Heated room | Cool room | Cold room |
|---|---|---|---|
| 60 L | 60 W | 90 W | 120 W |
| 100 L | 100 W | 150 W | 200 W |
| 200 L | 200 W (2 × 100 W) | 300 W (2 × 150 W) | 400 W (2 × 200 W) |
| 300 L | 300 W (2 × 150 W) | 450 W (2 × 250 W) | 600 W (2 × 300 W) |
Why two heaters
Top Shelf Aquatics suggests that for tanks above about 150–190 L, two smaller heaters are safer than one large one. If one sticks on, it can’t overheat the tank on its own. If one fails, the other keeps the water warm. AllPondSolutions suggests placing them at opposite ends for even heating.
Use a separate thermometer
Top Shelf Aquatics notes that heater dials are often inaccurate. Check the real temperature with a separate thermometer or a controller, and use the controller to switch the heater off if it overshoots. See reef tank temperature.
Common questions
What size heater for a 200 litre tank?
About 200 W in a heated room, split across two 100 W heaters.
Can a heater be too big?
A heater that sticks on does more damage the bigger it is. That’s why two smaller ones are safer.
Do I need a heater in summer?
Keep it in. A controller stops it adding heat on hot days.
Sources and limits
Last checked 13 October 2026. A rule of thumb, not a heat-loss calculation. Lids, room temperature, lights and pumps all change the real figure.
- AllPondSolutions: aquarium heater buying guide
- Top Shelf Aquatics: heater size calculator
- Charterhouse Aquatics: heatwave survival guide
Keep going
Heater size calculator · Free Reef Companion · All calculators