Finding the Right Aquarium Heater Size

The basic rule of thumb for wattage is to use between 2.5 and 5 watts per gallon of actual water volume in the aquarium. You can use a single unit of the correct size or multiple units that add up to the needed wattage. But then you need to adjust for the temperature of the room and the temperature desired in the tank.

First, subtract the average temperature of the room the aquarium is located in from the temperature at which you wish to maintain the aquarium water.

Using the Aquarium Heater Size Guide below, find the size of your aquarium in the left-hand column and move to the column that shows the number of degrees the aquarium needs to be heated. If the heating requirement is between levels, move up to the next larger size.

In larger tanks, or a situation where the room temperature is significantly below the desired water temperature, two heaters may be required.

Heaters should be installed at opposite ends of the aquarium to heat it more evenly.


Average Room Temp = 68 degrees F

Desired Water Temp = 77 degrees F


Heating required = 9 degrees F

Tank Size = 20 gallon

Heater size needed = 50 watts


Aquarium Heater Size Guide

Tank Size Heat
5 Degrees C, 9 Degrees F
10 Degrees C, 18 Degrees F
15 Degrees C, 27 Degrees F
5 Gallon/25 Liter 25 watt 50 watt 75 watt
10 Gallon/50 Liter 50 watt 75 watt 75 watt
20 Gallon/75 Liter 50 watt 75 watt 150 watt
25 Gallon/100 Liter 75 watt 100 watt 200 watt
40 Gallon/150 Liter 100 watt 150 watt 300 watt
50 Gallon/200 Liter 150 watt 200 watt two 200 watt
65 Gallon/250 Liter 200 watt 250 watt two 250 watt
75 Gallon/300 Liter 250 watt 300 watt two 300 watt