Temperature Rise (ΔT)
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Difference between desired and room temp
Aquarium Heater Wattage Calculator
Size your heater by tank volume and temperature rise (ΔT). The tool adjusts for lid style, sump surface area, and room conditions, then suggests one-heater and two-heater (redundant) options.
Recommended Heater Size
Enter tank size and temperatures to see total watts and suggested heater sizes.
Recommended Total Watts
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Adjusted for lid, sump, and room profileOne-Heater Option
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Rounded to a standard wattageTwo-Heater Option
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Split for redundancy and safetyYour Heater Check
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Max ΔT your current heater can likely holdAquarium Heater Wattage: How This Calculator Works
The classic rule of thumb is 3–5 watts per gallon for every 10 °F (≈6 °C) of temperature rise. Small tanks lose heat faster (closer to 5–5.5 W/gal/10 °F), while very large tanks retain heat (≈3 W/gal/10 °F). This tool scales that baseline by tank size, then applies environment multipliers for lids, sumps, and room conditions.
Placement, Flow & Controllers
- Place heaters in strong flow (near filter return or a small powerhead) to avoid hot/cold spots.
- Use two heaters (each ~½ total watts) on a reliable controller for redundancy and temperature stability.
- Keep sensors away from heaters and in the display’s main flow path for accurate readings.
- Cover the tank to reduce evaporation and heat loss; open-top setups need more wattage.
FAQs — Aquarium Heater Size & Safety
Is “5 watts per gallon” always correct?
No. It assumes a ~10 °F rise in a typical room with a covered tank. Open-top, drafty rooms, or big ΔT need more.
One large heater or two smaller?
Two is safer. If one fails, the other can maintain temperature long enough for you to notice. Many hobbyists use two 50–60% units.
What if my room gets much colder at night?
Use the coldest expected room temperature. If the calculator returns a very high wattage, consider insulating the back and sides or raising room temperature.
Where should I set the heater?
Place horizontally near the bottom or vertically near the return flow. Submersible heaters must stay submerged.
