Scale & Stem
cm · L · °C
Length
Volume
Temp
Weight
Aquariums

Heater wattage

How many watts of heating you need. Quick rule-of-thumb mode or physics-based detailed mode for non-standard tanks and unusual rooms.

Heater wattage Inputs in L, cm, °C
Most community tropicals like 24–27°C. Discus and rams sit higher, goldfish lower.
Detailed heat-loss math (optional)

Fill these in for a physics-based calculation that accounts for surface area, glass conduction, top losses, and room air movement. Useful for non-standard tanks or unusual rooms.

Substrate to water line. Not full tank height.
Glass is sold by mm worldwide; this stays in mm regardless of unit preference.
quick estimate
Temperature differential --
Recommended heater -- watts total
Advice --
Both modes are estimates. The detailed calc is physics-based and more accurate for unusual tanks or rooms. The basic mode is a hobby rule of thumb. Pick the heater, then watch the thermometer for a week before deciding it's right.

Two modes

The tool starts in quick-estimate mode: volume, room temp, target temp, and you get a watts number. That works for standard rectangular tanks in normal indoor rooms.

For non-standard tanks (very tall, very shallow, open-top rimless) or unusual rooms (basements, sunrooms, drafty corners), open the "detailed heat-loss math" section. That switches the calc to a physics-based estimate using glass thickness, surface area, top type, and room air movement.

What the detailed math does

Steady-state heat loss through each surface is U × area × ΔT, where U combines water-side convection, glass conduction, and air-side convection in series:

1/U = 1/h_water + thickness/k_glass + 1/h_air

Water side is fast (h_water ≈ 500 W/m²K, not the bottleneck). Glass conducts well (k_glass ≈ 0.96 W/m·K). The air side is what actually limits heat loss, and it varies a lot with room conditions:

  • Still air (closed room, tank in a corner): h_air ≈ 5 W/m²K
  • Normal (typical HVAC): h_air ≈ 10 W/m²K
  • Drafty (near a vent or fan): h_air ≈ 15 W/m²K

The top of the tank gets special treatment. Open tops lose heat dominated by evaporation, not conduction, so the model uses an effective 25 W/m²K which is high but matches real-world data. Glass lids drop top loss to roughly a third of the wall U-value because the air gap insulates. Plastic hoods sit in between.

The bottom uses 40% of the wall U-value because most stands are wood and provide some insulation.

Why the recommended wattage is higher than the raw loss

The calc multiplies steady-state loss by 2.5x. That accounts for:

  • Duty cycle. Your heater shouldn't be running 100% of the time. If it is, you have no margin for a cold snap.
  • Recovery. When the room cools at night and the tank cools with it, the heater needs enough output to warm things back up in a reasonable time.
  • Surprises. Lid left off, AC vent suddenly aimed at the tank, big cold-water change.

A 2.5x margin lines up with the rule-of-thumb numbers most hobby sites give but adjusts for the actual tank and room. A heavily insulated setup ends up with less heater than rule of thumb suggests, an open-top in a sunroom ends up with more.

Why two heaters beats one big heater

If the recommendation is above 150W and your tank is over 40 gallons, run two smaller heaters instead. A 300W heater stuck in the ON position will cook a 75 gallon tank in a day. Two 150W heaters on separate outlets give you redundancy: if one fails open the other isn't powerful enough to overheat alone, and if one fails closed you've still got half your heating.

Small tanks don't really fit two heaters and the redundancy matters less anyway because the failure mode is faster to catch.

Further reading