Aquarium heater size calculator
A wattage guideline from tank volume and how far above room temperature you're heating.
Suggested heater size
200 W
A guideline, not a law: lids, drafts, cabinet placement and room swings all matter. Whatever you install, run a separate thermometer and check it.
Heater sizing is quoted as watts per litre, and like every per-litre rule it hides the variable that actually matters: a heater does not warm water to a temperature, it fights the rate at which the room steals heat. What you are sizing for is the lift — the gap between your room at its coldest and the tank at its target. A 100-litre tank at 25 °C in a room that sits at 21 °C needs a fraction of the power the same tank needs in an unheated garage at 12 °C, and no watts-per-litre table knows which one you have. The usual 1 W per litre figure quietly assumes a lift of about 5 degrees, which is why it works in a heated living room and fails everywhere else. Two practical points the arithmetic will not tell you. Undersizing shows up as a heater that runs constantly and still loses ground on the coldest night of the year, which is exactly when a tropical tank can least afford it. Oversizing is the more dangerous failure: thermostats stick on, and a heater with enough power to cook the tank before anyone notices is a genuinely worse outcome than one that cannot quite keep up. On anything over about 200 litres, two heaters of half the wattage at opposite ends beat one big one — better distribution, and neither one alone can boil the tank if it fails closed.
Common questions
- What size heater does my aquarium need?
- Roughly 0.5–1.5 W per litre depending on how far above room temperature you heat: about 0.5 W/L for up to 3 °C, 1 W/L up to 6 °C, 1.5 W/L beyond that. Round up to the next common heater size.
- Is an oversized heater dangerous?
- The thermostat, not the wattage, controls temperature — but if a thermostat sticks on, a big heater overheats the tank faster. That's the argument for two smaller heaters on larger tanks: redundancy in both directions.
- Should I get a bigger heater to be safe?
- No — oversizing is the failure that kills fish. A stuck-on thermostat is the common heater fault, and a heater sized well above the lift you need can push a tank several degrees past safe before you get home. Size for your coldest realistic room temperature with a modest margin, and on larger tanks split the wattage across two units so neither can overheat the tank alone.