equipment
Heater sizing calculator
Estimate the heater wattage needed to hold a target temperature above the coldest room temperature the tank will see.
Overnight in winter, not the daytime average.
25 W
Approximately 25 W, rounded up to the nearest common size.
Working — every step, so you can check it
- Temperature lift
- 25 − 18 = 7 °C
- Coefficient
- 0.03 W per litre per °C (covered tank)
- Wattage
- 100 × 7 × 0.03 = 21 W → 25 W
Formula
- Temperature lift (°C) = target water temperature − minimum room temperature
- Estimated wattage = tank water volume (L) × lift (°C) × k
- where k is a heat-loss coefficient in W per litre per °C
- AquariumHQ uses k = 0.03 for a covered tank and k = 0.045 for an open-top tank.
Assumptions
An estimate whose assumptions are invisible is indistinguishable from a guess.
- The heat-loss coefficients are AquariumHQ editorial estimates for a typical glass aquarium in a domestic room, not measurements of a specific tank.
- Evaporative loss from an open-top tank is treated as an increase in the coefficient rather than modelled separately.
- The heater is expected to run intermittently, not continuously, at the calculated size.
Limitations
Where this calculation stops being reliable. Read these before acting on a result.
- Glass thickness, cabinet enclosure, draughts, room humidity and surface area all affect real heat loss and are not modelled.
- In tanks above about 150 L, two heaters of half the calculated wattage each are safer than one — a single stuck-on failure then cannot overheat the tank as fast.
- A heater that is far too large is a hazard, not a margin of safety.