Aquarium Heater Wattage Calculator
Cold tank, stressed fish. Enter your aquarium's size and how far the water needs to rise above room temperature, and this calculator recommends the right heater wattage — with a nudge toward two smaller heaters for big tanks.
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How to use this calculator
Enter your tank's gallon capacity, the room's usual temperature, and the water temperature your fish need. The calculator picks a watts-per-gallon rate from the temperature gap and rounds up to the nearest standard heater size.
For 55 gallons and up, strongly consider two heaters at half power each — it is the standard safety setup.
How it works
The rule of thumb is 3–5 watts per gallon: 3 W/gal for a lift of 8°F or less, 4 W/gal up to 14°F, and 5 W/gal beyond that. Required watts = gallons × rate, rounded up to the nearest standard size (25, 50, 75, 100, 150, 200, 250, 300 W).
Heavily planted or open-top tanks lose more heat; in a cold room, size up one step.
Frequently asked questions
How many watts per gallon does a fish tank need?
The rule of thumb is 3-5 watts per gallon: 3 for a small lift above room temperature, 5 when the tank must run much warmer than the room. This calculator picks the rate from your temperature gap.
Should I use one big heater or two smaller ones?
Two smaller heaters (each ~50% of the total) are safer: if one sticks on, it cannot cook the tank alone, and if one fails, the other holds temperature. Recommended for tanks of 55 gallons and up.
Where should I put the heater in the tank?
Near the filter outflow or a powerhead so warmed water circulates. Keep it fully submerged and away from the substrate it could get buried in.
Do I need to unplug the heater during water changes?
Yes — unplug it 15 minutes before the water level drops near the heater. Exposed heating elements can crack or overheat in air.
What temperature do tropical fish need?
Most tropical community fish do best at 76-80°F. Bettas like 78-80°F, discus 82-86°F. Match the heater setting to your specific stock.