FishMaths

Heater size calculator

Tell it how big the tank is and how much warmer than the room you want the water. It suggests a heater size.

Result
0
Enter your numbers

How the math works

The calculator uses about 0.3 watts per gallon for every degree Fahrenheit the water sits above the room. Small tanks lose heat faster, so tanks under 20 gallons get a 25 percent bump. Open tops lose more heat to evaporation, so they get 20 percent more. The result is rounded up to the next common heater size.

Is a bigger heater dangerous?
An oversized heater can cook a tank quickly if its thermostat sticks on. A heater close to the size you need, or two smaller heaters, limits the damage if one fails.
Should I use two heaters?
On bigger tanks, yes. Two heaters spread heat evenly and give you a backup if one dies in winter.
What room temperature should I use?
The coldest the room gets, usually overnight in winter. That is when the heater works hardest.