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.
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.
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 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
The advice line under the result follows a ladder. At 250 W and up it tells you to split the load into two heaters, no question: a 300 W heater stuck in the ON position will cook a 75 gallon tank in a day. Between 150 and 250 W on a tank of 40 gallons or more, one heater works and pairing is optional but worth it if you have the outlet. Under that, one heater. And if the differential is 20 °F (11 °C) or more and the answer is 200 W or above, it asks for a pair plus some insulation on the back and sides, because a heater fighting a cold room that hard is running near full duty anyway.
Two heaters on separate outlets give you redundancy in both directions: if one fails open the other isn't powerful enough to overheat the tank 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.
Related tools
- Tank volume - Calculate your tank volume first.