How to use the aquarium heater size calculator
- Enter the tank volume and choose gallons or liters.
- Enter the watts per gallon you plan to use. The 3 shown is an example rule of thumb.
- Open More options to list the heater sizes you can buy and the supply voltage.
- Read the power needed, the smallest single heater that covers it and the size of each heater if you use two.
Formula
gallons = liters / 3.785411784
power needed = gallons x watts per gallon
one heater = the smallest size at or above the power needed
two heaters = the smallest size at or above half the power needed, for each
current = power / voltage
Watts per gallon is a rule of thumb, not a physical measurement of your room. It changes with the room temperature, the lid and the fish you keep.
A 40 gallon tank
At 3 watts per gallon the tank needs 120 watts. From the sizes 25, 50, 75, 100, 150, 200, 250 and 300, a single 150 watt heater covers it, or two 75 watt heaters (150 watts in total), which draw 1 amp at 120 volts.
A 151.4 liter tank is 40 gallons and gives the same answer. A 125 gallon tank at 5 watts per gallon needs 625 watts, which none of those sizes covers alone. Two 300 watt heaters give only 600 watts, so a larger size or three heaters would be needed.
Why two heaters can help
Two smaller heaters share the load, so if one fails the tank is not left without heat, and a stuck heater is less likely to overheat the whole tank. Place them at opposite ends so the water warms evenly. The page checks the size of each heater when it takes half the load.
Choosing the watts per gallon
A cool room, a draft or an open top calls for more power, while a warm room needs less. Fish keepers often use a range of values depending on how much warming is needed. Ask the maker of your heater and a source on your species, because the safe temperature depends on the animals. The page does not set a temperature.
Related aquarium tools
Find the volume with the Aquarium Volume Calculator, mix salt with the Aquarium Salt Calculator, size the floor with the Aquarium Substrate Calculator, and estimate running cost with the Electricity Cost Calculator.
Assumptions and limits
- The tank holds the volume you enter, with no allowance for decorations or substrate.
- The watts per gallon is a number you choose, not a measured need.
- The sizes list is what you can buy where you shop.
- The page does not know the temperature your fish need or the temperature of your room.
Frequently asked questions
What size heater do I need for a 40 gallon aquarium?
At 3 watts per gallon the tank needs 120 watts, so a 150 watt heater is the smallest common size that covers it, or two 75 watt heaters.
How many watts per gallon should I use?
The page starts with 3 as an example. The right number depends on your room, the tank and the animals. Ask the heater maker or a source on your species.
Is it better to use two smaller heaters?
Two heaters share the load and give some protection if one fails. The page shows the size of each heater when you split the power in half.
How many amps does the heater draw?
Divide the watts by the voltage. A 120 watt load on a 120 volt supply draws 1 amp. Change the voltage under More options if yours differs.