Aquarium Heater Size Calculator
The right heater wattage depends on two things: how much water you have, and how much warmer than the room the water needs to be. Enter both and we'll give you a wattage — plus a safer two-heater option.
How the maths works
The widely used rule of thumb is 3–5 watts per gallon, and where you land in that range depends on the temperature lift — the gap between your coldest room temperature and your target water temperature:
| Temperature lift | Watts per gallon |
|---|---|
| Up to 5 °F | ~3 W/gal |
| Around 10 °F | ~4 W/gal |
| 15 °F or more | ~5 W/gal (or a little above) |
Use your room's coldest realistic temperature — a winter night with the heating off — not the daytime average. A heater that copes at the coldest point will idle comfortably the rest of the time.
Why two smaller heaters often beat one big one
Heaters fail in two directions. Stuck off, the tank slowly reaches room temperature — stressful, rarely instantly fatal, and you have time to notice. Stuck on, a single oversized heater can overheat a tank overnight. Splitting the wattage across two heaters halves the damage a stuck-on unit can do, and if one fails off, the other still holds most of the temperature. For tanks of 40 gallons and up, we recommend the two-heater setup this calculator suggests.
Choosing fish to go with that heater? Check your plan in the stocking calculator — it will also tell you the single temperature range that suits every species on your list.