Aquarium Heater Size Calculator (Heater Guide)

Aquarium Heater Size Calculator (Heater Guide)


One of the most important pieces of equipment you can purchase for your aquarium is the heater. You obviously want to buy the best submersible heater you can, but that’s not the only factor to consider. Sizing your heater’s wattage is also important. Aquarium heater size will vary depending on several factors. You can’t just spring for the biggest heater on the market because not only will it not be appropriate for your tank but you are potentially wasting your money on something that is that overkill, literally.

How do I calculate what size heater I need?

The general rule that aquarists follow is that you want to have a wattage range between 2.5 and 5 watts per gallon. The watts per gallon is just a general guideline as it can tend to be on the conservative side so you might need more wattage depending on the needs of your fish tank volume and environment.

It takes approximately 1/2 watt to increase the temperature of 1 gallon of water by 1 degree. To get a more accurate estimate for your aquarium heater need you can subtract your room temperature from your target aquarium temperature, multiply by the number of gallons, and divide the total by 2.

As with most things, your situation may be different so please adjust as you see fit. Be sure to account for extra water volume in your sump, if you have one.

If you’re not sure how many gallons your fish tank can hold you should check out our aquarium volume calculator. With it, you can measure your tank and get the precise amount of gallons it will hold.

Look at the aquarium heater size calculator below to help make your final purchasing decision. You want to stick to the results as a minimum in determining the heater wattage for your aquarium as it will keep you in the general vicinity of your aquarium heat needs.

What heater size do I need for my fish tank?

Getting the correct heater size for your fish is incredibly important as it is the only thing standing between your fish and freezing in their home.  The correct size is largely determined by the volume of the tank and the temperature you are trying to stabilize your water at. The volume of the tank makes a big difference as more water means there is more area for the heat to stabilize. The aquarium volume will require higher heater wattages.

You also need to consider the temperature of the room that your fish tank is located. For example, if you live in a colder climate with drafty windows, then you might need to either consider relocating the fish tank or springing for a high-quality heater to keep up with the demand to retain the water’s heat.

A word of warning, however, there is the potential for failure with heaters. You should always do your best to get the correct size heater for your needs as even a heater that is too large can lead to problems in your fish tank. Multiple heaters might be better than one big heater for the winter because if one fails there is another that will give you some time to notice the equipment failure. 

The aquarium volume will be the largest factor in changing the temperature within your tank. Water is an excellent conductor of heat which means that it will absorb heat well but slowly. The more water there is the harder it will be to heat. That is why the specific heater wattage is so important when considering the fish tank volume. If you are trying to heat a 10-gallon aquarium that is going to be completely different compared to if you are trying to heat a 75-gallon tank. The 10-gallon tank might only require a 30-watt heater while the 75-gallon tank might require a 225-watt heater.

What temperature should my fish tank be?

The temperature of your fish tank will largely depend on the species that you are keeping and the tank setup that you are curating. Are you caring for a reef aquarium or a tropical aquarium? Coral will need a temperature range between 72 and 78 degrees Fahrenheit typically while saltwater fish tend to prefer lower temperatures depending on the species. 

The difference in species’ temperature needs will restrict who you can keep with who. Corals can be finicky creatures as they do not like low temperatures but higher temperatures will cause them to bleach and die. It is a delicate balance you need to consider as you plan your aquarium.

Where is the best place to put a heater?

Before I can answer where the best place to put an aquarium heater, we need to talk about the different types of heaters as they will dictate where you can place them. The first type of heater you can consider is a submersible aquarium heater. These heaters lay along the bottom of the fish tank and allow heat to rise through the aquarium water supply. Essentially, it is a great way to evenly distribute the heat throughout the tank and not a single side or point. 

The other type of heater is the older design where a vertical glass tube is set in the aquarium water but rises slightly out of the water so the hobbyist can access the controls. You must remember to switch this heater off when you perform water changes as you risk damaging the heater if it becomes exposed to the air.  The moment a heater becomes exposed to the air it risks cracking as the heat is not being absorbed by the water.

Now, the best place to put your aquarium heater will depend on its distribution method. If you have a vertical model then you should consider placing the aquarium heater near the main water flow of the tank or near a powerhead. That way you are ensuring that heat is being distributed throughout your aquarium as evenly as you can manage.

The aquarium size matters for more than just the heater wattage that you need. If you only have a 20-gallon aquarium one heater should suffice but if your heater is in a 40-gallon aquarium or larger you might need to opt for multiple heaters. You can place one device placed on each side of the tank will ensure even heat distribution. For the most part, you can imagine how the heat might distribute throughout the aquarium.

Is an aquarium heater on all the time?

It depends on the circumstances. For day-to-day use, you should leave your aquarium heater on all the time. There is actually an automatic off switch on aquarium heaters if the device reaches a certain temperature. This helps to maintain the desired heat within the tank. 

On the flip side, you should always turn off the heater if you are doing anything where the device might become exposed to the air. Whether you are performing a water change or other tank maintenance (like replacing the heater) you should make sure the device is off before proceeding. Aquarium heaters can break if not properly handled outside their normal heating parameters.

How accurate are aquarium heaters?

Aquarium heaters are actually pretty accurate but like all products, there are differences in performance. The one you initially choose might be accurate up to only 2 degrees Fahrenheit while the better models on the market might be as accurate as .5 degrees Fahrenheit. That might not seem like a lot but the degree difference can mean the difference in life or death in some instances.

What is the best aquarium heater?

Though it is a little subjective what heaters are the best, when you are considering accuracy related to measurements there is certainly a degree of objectivity. However, you need to take into consideration the level of control and access you have with your heater preferences as well. Ultimately, the choice is yours but remember that the heater is one of the most crucial pieces of equipment for your fish’s survival so it might not be a great idea to just throw the first heater you find on the market in your reef tank.

Best Aquarium Heater: Cobalt Aquatics Neo-Therm

The Cobalt Aquatic Neo-Therm Heater might be one of the pricier heaters on the market but the level of control and accuracy that you receive in exchange can’t be beaten. The device is accurate up to .5 degrees Fahrenheit and is an incredibly manageable size for you to fit in most spaces in your tank. It does however have lower wattages than other models on the market so it alone will not be enough for large fish tanks. If it is within your budget you can always pair this heater with another of the same model or another heater entirely. 

Best budget aquarium heater: Eheim Jager TruTemp

The Eheim Jager TruTemp heater has a fine pedigree behind it as it is manufactured by one of the oldest companies in the hobby. This heater is accurate within .5 degrees Fahrenheit as well and can be recalibrated as needed. However, this old design is immediately apparent as it is large and made of that glass design that can shatter when not properly heating water. However, it can be extremely affordable so as long as you are careful with the device, there should be limited issues. 

Best Aquarium Heater
Best Budget Heater
Temperature Controller

Temperature Controller

If you have it in your budget, one of the nicest pieces of equipment that you can pair with your heater is a temperature controller. These devices allow you to more accurately monitor your tank’s temperature and control any changes you need to make to it. There are two types of controllers like the prewired temperature controllers where everything comes packaged together and DIY controllers where you have to buy everything separately. The device comes in a few parts: the controller, socket, and temperature sensor probe.

  • The controller is the brain of the operation. It has a screen that you can monitor the temperature with and buttons to make any variations that you might need.
  • The socket is used to plug everything into it. The heater can be plugged in here and a chiller if you happen to have one.
  • Temperature Sensor Probe actually goes inside your aquarium to monitor the temperature of the water. Note that some probes are not necessarily graded for saltwater aquariums. You need to make sure yours will resist the corrosion of the salt.

Aquarium Temperature Controller: Inkbird ITC-306T

This easy-to-use controller comes with everything you need to set up your heat control for your aquarium within moments of arriving home. The Inkbird is able to display the temperature and set the desired temperature simultaneously. It can even change the temperature setting depending on the time of day. The Inkbird is a very convenient tool to have in conjunction with your aquarium heater.

Always have a backup plan

Equipment failure is something that can happen in this hobby so it is important to plan for any issues that might arise with your tank. Of course, if you are just starting out, it might be hard to justify having a backup for every device that you currently own just in case something happens. You can work up to that point and until then you can incorporate methods that allow you to have some control in case of emergency or measurements that are off.

For heaters, the simplest device you can purchase for yourself is a floating thermometer. A  floating analog thermometer is a great option for your system. It can even help measure your salinity and you wouldn’t even need it to be in the display tank. You can keep the device in your sump as a handy sanity-check. The thermometer can help you especially if your equipment is off by a few degrees.

Do fish tanks need heaters?

If you are keeping a reef tank or any saltwater tank, you should have a heater. For instance, tropical fish need a consistent and accurate aquarium temperature to thrive and if the temperature drops suddenly in the night, your fish will have a rough time trying to make it to the morning. 

Fish, corals, crustaceans, and other creatures are all made for particular temperatures. If the temperature of your fish tank does not match their needs, your tank’s inhabitants will start to die.

Final thoughts

Whether you are keeping tropical fish, corals, or some other underwater creatures, any aquarium needs to be heated. Your aquarium heater size will be determined by the needs of your tank. What gallon aquarium do you have and what is the average temperature of the room your tank is located are two of the most important questions you need to ask yourself. 

The tank heater is one of the most important pieces of equipment that you can purchase for your aquarium but you need to take the time to find the one that will adequately heat your aquarium without risking being too large. The heater wattage is the final number you are looking for but you need to take into account the aquarium volume and fish type.

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