Aquarium heater size calculator
Most heater guides assume a warm American house. Size heaters for the coldest your room really gets, split across two, and check how hot the tank could get if one sticks on.
Formulas and sources last checked . How we check
Calculator
Result
2 heaters, each150 watts
- Total needed
- 300 watts
- Room band
- cool room, 15 to 18 C: 1.5 W per litre
- One heater stuck on, warmest day
- about 31.5 C
- One heater failed, coldest day
- the rest hold about 23.5 C
Danger: A single 150 W heater stuck on could take the tank to about 31.5 C. Put each heater on its own controller, and consider more, smaller heaters.
What size heater does a reef tank need?
We take the larger of two estimates. UK and EU retailer guidance (seen in search summaries) gives about 1 watt per litre in a heated room near 19 C, 1.5 W per litre at 15 to 18 C, and up to 2.5 W per litre at 13 to 14 C. A cruder rule from another retailer allows 1 W per litre for every 10 C between the room and the tank. The total is split across your heaters and rounded up to a common size. For the stuck-heater check we use the same crude rule the other way round: a heater left on holds the tank about watts / (0.1 x litres) degrees above the room.
total watts = litres x watts per litre for the coldest room (or litres x 0.1 x degrees of lift, if larger)each heater = total / number of heaters, rounded upstuck on = warmest room + heater watts / (0.1 x litres)
Worked example: 200 litres in a cool room
- The room drops to 16 C on winter nights: 1.5 W per litre, so 200 x 1.5 = 300 W in total.
- Two heaters: 150 W each.
- On a 24 C summer day, one 150 W heater stuck on could hold the tank 150 / 20 = 7.5 C above the room: about 31.5 C. That is dangerous, so each heater needs its own controller that cuts the power.
- If one fails on a cold night, the other can hold about 7.5 C above 16 C: 23.5 C. Cool, but survivable while you replace it.
Where do sources disagree?
| Topic | What sources say | Our reading |
|---|---|---|
| Watts per litre |
| Size for the coldest the room gets. A UK spare room with the heating off in January needs far more than a US living room. |
| How to split heaters |
| Both cannot always be met. Where they clash, a controller on each heater matters more than the split. |
What are the limits?
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