Aquarium heater size calculator
Get a recommended heater wattage from your tank volume and the gap between your room and your target temperature.
How heater wattage is chosen
The work a heater does depends on two things: how much water it warms and how far above the room it has to hold that water. This tool multiplies your volume by a watts per gallon figure that rises with the temperature gap. A gap under five degrees Fahrenheit uses 2.5 watts per gallon, a gap of five to ten uses 3.5, and a gap over ten uses 5 watts per gallon, which reflects how much harder the heater must work in a cold room.
The raw figure is then rounded up to the next common heater size: 25, 50, 75, 100, 150, 200, 250 or 300 watts. Rounding up gives headroom for the coldest nights and for a heater that ages. An oversized heater is generally safer than one that runs constantly and still cannot hold temperature.
One heater or two
On tanks above about 75 gallons the tool suggests splitting the wattage across two heaters. Two units share the load and spread warmth more evenly along a long tank, and they add a layer of safety. If one heater sticks on, a smaller unit overheats the tank more slowly, and if one fails off, the other keeps the tank from crashing while you react.
Whatever size you choose, use a separate thermometer to check the real water temperature, and consider a controller or a heater guard for sensitive livestock. Place the heater near flow from the filter so warm water is carried around the tank rather than pooling in one corner.
Frequently asked questions
How many watts per gallon does a heater need?
It depends on how far you are raising the temperature. A small gap needs about 2.5 watts per gallon, a moderate gap around 3.5, and a large gap up to 5 watts per gallon.
Should I use one heater or two?
On tanks over about 75 gallons, two smaller heaters are safer. They share the load, spread heat more evenly, and if one fails the other limits how far the tank drifts.
Why round up to a standard size?
Heaters are sold in fixed wattages such as 50, 100 or 200 watts. Rounding up to the next common size makes sure the heater can keep up on the coldest nights.
Does room temperature really matter?
Yes. The heater only has to make up the difference between the room and your target. A warm room needs far less wattage than a cold garage for the same tank.