ScapeMath

Aquarium heater wattage calculator

Heater charts assume a room at about 20 °C (68 °F) and a closed lid. If your room is colder, your tank warmer or the top open, you need more. This calculator scales a manufacturer's rating to your conditions.

L
°C
°C
L/day
Suggested heater
–
Estimated heat loss
–
Evaporation loss
–
With 25% headroom
–

How it works

A heater only has to replace the heat the tank loses. Heat loss through the glass grows in proportion to the temperature difference between water and room, and to the tank's surface area (Q = U·A·ΔT). For tanks of similar shape, surface area grows as volume2/3: a tank with 8 times the water has only 4 times the glass. We anchor the formula to a published rating, the Fluval E100 (100 W for up to 120 L at 26 °C water in a 20 °C room), and scale from there:

Loss (W) = 100 × (V ÷ 120 L)^(2/3) × (ΔT ÷ 6 °C) Evaporation (W) = litres per day × 0.997 kg/L × 2,441,700 J/kg ÷ 86,400 s ≈ 28 W per L/day Suggested = (loss + evaporation) × 1.25, rounded up to a common heater size

As a cross-check, the same formula gives 97 W for 150 L at a 5 °C lift, matching EHEIM's 100 W rating (up to about 150 L, 21 °C room, 26 °C water). Every litre that evaporates carries away 2.44 MJ of heat. That's why open-top tanks need noticeably more wattage, and why your top-off volume is a good way to measure it.

Published ratings we checked against
HeaterRated forConditions
Fluval E50 / E100 / E200 / E30060 / 120 / 250 / 375 L26 °C water, 20 °C room
EHEIM thermocontrol 50 / 100 / 150 W25–60 / 100–150 / 200–300 Lclosed tank, 21 °C room, 26 °C water
Aqueon 50 / 100 / 150 / 200 / 300 W5–20 / 15–40 / 30–55 / 50–75 / 65–90 US galnot stated

Worked examples

60 L nano, 20 °C room, 26 °C water

Loss = 100 × (60/120)^(2/3) × 6/6 = 63 W. With 25% headroom that's 79 W, so a 100 W heater. (Fluval rates its 50 W for 60 L at these exact conditions with no headroom, so our figure is deliberately cautious.)

75 US gal in a 65 °F room, kept at 78 °F

75 US gal = 283.9 L, and the lift is 13 °F = 7.2 °C. Loss = 100 × (283.9/120)^(2/3) × 7.2/6 = 214 W, or 267 W with headroom: one 300 W or two 150 W heaters. If it's open-top and you top off a quarter gallon a day, add 27 W, which tips it to two 200 W heaters.

Why two heaters?

Two smaller heaters give the same total but fail more safely. If one sticks on, it's too small to cook the tank alone. If one dies, the other keeps the tank from crashing. For tanks needing more than 300 W, the calculator splits the total automatically. A separate temperature controller adds another layer of protection.

Limits

References

  1. Fluval E-series heater instruction manual (ratings at 26 °C water, 20 °C room)
  2. Aqueon heater wattage guidelines (PDF)
  3. EHEIM thermocontrol ratings as compiled by Aquarium Stocking (maker specs)
  4. Engineering ToolBox: Water heat of vaporization vs temperature (2441.7 kJ/kg at 25 °C)
  5. Incropera, F.P. et al. Fundamentals of Heat and Mass Transfer, Wiley (overall heat transfer, Q = U·A·ΔT)

Formulas on this page are checked by automated unit tests against the reference values above. Last reviewed October 29, 2026. Found an error? Tell us.

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