Aquarium Heater Size Calculator

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Heaters are an essential part of most aquarium setups. Our fishy friends can’t adjust their body temperature, so providing them with the right amount of heat is important for their health. That means you’ll need to have the right amount of heating for your tanks.

So, let’s get into it with our aquarium heater size calculator and some tips to keep things in a nice, tropical heat!



How to Rate Heaters

A heater’s specifications basically boil down to the wattage of the heater. You’ll always find this number somewhere in the documentation or advertising for any given heater, and it gives you the basics you need to figure out if it’s appropriate for your tank.

While choice boils down to wattage, there are a couple of other things that you should keep an eye out for in this case.

The first is to make sure that the heater brand is reliable. Heaters are one place I won’t mess around, heater failure is bad enough but the first time I had a runaway heater was an eye-opener.

A “runaway” heater is one that’s failed by locking on. In my case, the water had reached over 90°F before I realized the problem. From that point on I always made sure of two things: double-check reviews for reports of runaway heaters and using a visible thermometer on all tanks.

Heaters that have a few reports of turning off don’t necessarily need to be avoided. There are a few strategies to mitigate the problems from a failed heater we’ll discuss further along.

Quality-of-life features are also nice to have. Base function is still the most important thing. If the heater’s core function is solid, you can look for things like indicator lights or exact thermostats.

The latter is particularly helpful. Many of the common heater brands just have a knob on top that twists one way for warmer and one way for colder without giving you any idea what temperature the heater is actually set at. Without a thermometer, you’re basically taking a shot in the dark in that case.

But the important thing for our purposes is still the wattage. You need that number to determine if your heater is right. I also strongly recommend making sure that the heater has a functioning thermostat shut off.

Some very old heaters were just made to run constantly in a certain size of tank. These are virtually antiques, but they sometimes pop up when people dig things out of storage to start a new tank. Tech has advanced a lot for everything in our aquariums in the last couple of decades.

If you stumble across one, appreciate it for the curiosity it is and put it back. There’s no excuse to use that kind of outdated equipment, even if you can find it.

A good modern heater will have the following:

  • Proper Wattage
  • Thermostat Shut Off
  • Numbers for Thermostat
  • Fully Submersible

If you’ve got those basics, then you’re good to go!

Determining the Amount of Needed Wattage

The general rule of thumb is that you’ll need 2.5-5 watts per gallon to keep your tank at a consistent temperature. This can change depending on a few different factors since the general guidelines are only applicable with central air controls keeping the temperature at 68°F to 72°F (20°C to 22°C).

In particularly warm tanks, you may need more wattage even within those parameters. The guideline mostly holds when your tank is only 9-°F-18°F above the room’s average temperature.

In colder rooms, you may also require a bit more wattage, up to 7.5W per gallon in extreme cases. For instance, if you need a 27°F above the room’s temperature, you may need a 75-100W heater for a 10-gallon tank.

If you measure tanks in metric, you’ll want to look for about .5-1W/Liter. It’s not an exact cross-over, but heaters just need to be in the rough area of what you need.

You need the following information to accurately judge what size of heaters you want to use:

  • Tank Capacity
  • Average Room Temperature
  • Desired Temperature in Tank

Start your calculations at 2.5W/gallon or .5W/Liter, or double that if you need to raise the temperature of the tank more than 9-10°F. In more extreme cases… you may want to look into changing the temperature of the room, a differential of 20°F will require a lot of energy.

As a general rule, the following common sizes of tank have roughly these heater requirements:

  • 10 Gallon – 50W
  • 20 Gallon- 100W
  • 55 Gallon- 150W
  • 125 Gallon- 300W

If the guidelines look a little bit rough based on the math… they are. Heaters are available mainly in 25W increments, with some skipping around a bit. You’re looking to get the total wattage of heaters in the tank to an acceptable level and a few watts less or more isn’t a big deal.

Of course, once you have a wattage number you’ve still got to decide on how to fill that number in. That may mean multiple heaters, depending on how you want to set things up.

One or Multiple Heaters?

For small tanks, a single heater in the correct wattage is enough but with larger tanks, you may want to rethink things. I recommend buying an extra for these smaller tanks, just in case something fails and you can’t make it to the store immediately.

Once you hit 55 gallons or so, the amount of time needed to re-heat a tank if a heater fails is considerable.

For that reason, you may want to purchase multiple heaters to heat the tank. Putting a heater that fills half the wattage requirement on each end of the tank will use the same amount of energy but gives you a larger buffer in case of tank heater fails.

At 125 gallons or above, you may even want to consider making up the wattage with three heaters.

Multiple heaters are great insurance for larger tanks, a heater wattage drop can often be handled without the water getting all the way back to room temperature.

However you choose to do it is up to you, but I advise using more than one heater for any tanks over 48” or so.

One other thing to keep in mind: try to use heaters with equal wattage throughout the tank if you use multiple heaters. It makes them interchangeable and prevents forming “hot spots” in larger tanks.

Does Heater Type Affect the Required Wattage?

There are three main types of heater that you’ll find sold for use in aquaria: submersible, clip-on, and under gravel cables.

Under gravel heating cables aren’t of much use for keeping the water column clean. They’re more to promote microcirculation at the level of the substrate. At best they’re horribly inefficient since they’ll be in contact with the glass on the bottom of the tank which acts as a heat sink.

They’re just the wrong tool for the job in most cases, although they have plenty of other uses.

Clip-on heaters are an older type, but still viable. The problem is that they often hang part of the element above the water level, which means that the energy isn’t going into your water. That’s bad, and there’s not a lot of reason to use them if you can avoid it.

Err on the higher side of wattage if you’re using these kinds of heater.

Submersibles are what most people mean when they’re talking about aquarium heaters. They consist of a tube, a heater element, and controls for temperature. Most will have an automatic cut-off when water is below their critical line.

These tubes can be planted vertically or horizontally with suction cups. Most of us keep them horizontal to keep them as hidden as possible on the back of the tank. Horizontal placement also avoids any trouble with the heater shutting off due to low water levels.

There are also in-line heaters, which heat the water outside of the tank. This can be done in a sump or other attached water source. They’re a little bit more advanced than a lot of equipment, but can be estimated at about the same wattage as a submersible heater.

Checking Your Heater’s Calibration

The truth is that a difference of two or three degrees won’t affect most fish one way or the other, but you should still have a better aim than “warm enough.”

A thermometer is essential for a fish tank. It can warn of heater failure and it’s the only way to be sure that your heater is calibrated properly.

If you’ve purchased a heater with a numbered thermostat, then you should make sure that the temperature is what’s displayed on the piece of equipment. In my experience, most are 2-3°F off in one direction or the other.

If your heater doesn’t have a numerical indicator, a thermometer is the only way to get an accurate reading.

Can I Use an Oversized Heater?

You can almost always use a heater that’s rated for higher wattage than your tank needs without issues. The only caveat is that it has to be adjustable, with an internal thermostat.

I’ve never seen an aquarium heater that lacked an internal thermostat, but you do occasionally stumble across heaters that aren’t adjustable. Instead, they’ll be set to one temperature and that’s what you’ll get.

As long as it fits, very few modern aquarium heaters will cause an issue if they’re higher wattage than required.

That said, make sure it’s a trusted brand. Small tanks will heat more rapidly, and an oversized heater can cause serious problems much more quickly. An internal thermometer is your best insurance, and it should be visible from the front of the tank.

You can also use a water-safe probe thermometer and leave the digital display somewhere near the tank as well.

Can I Use an Undersize Heater?

If you’re just a few watts short of what you need, the heater will be fine but run for longer.

On the other hand, as a heater’s wattage gets smaller the device will have to run more. After a certain point, the heater may not be able to keep up at all, which means that it will be running 100% of the time for no real benefit.

And the cost of that adds up quickly.

With the advent of LED lighting on fish tanks, your heater is probably the most energy-intensive device in your equipment. Even a 50W heater running constantly will add up quickly.

If you’re worried that your heater is undersized then watch the indicator light. If it rarely or never goes off, you should invest in a larger heater. Otherwise, you’re just burning energy for no real advantage!

Heater Troubleshooting

I’ve “revived” a lot of heaters for people over the years. Almost all of them had the same simple problem.

If you have a heater that shuts off when the water level drops… it may not turn back on when the water level rises. I’ve found it’s best to just disconnect the heater during water changes to avoid any trouble.

The problem is that the automatic shut-off may not always turn back on. All that needs to be done afterward is to unplug the heater and then plug it back in.

If that doesn’t work?

The majority of aquarium heaters are one-trick ponies. If the heating coil, or any part of the electronics, has failed then the heater is toast. In my experience, most submersible heaters last between 2-3 years before they need to be replaced.

Their simple nature is what makes heaters prone to total failure. The wiring inside of the element is very simple, and if any part of it fails the whole thing belongs in the recycling can.
Just try removing the plug and putting it back in if your heater doesn’t seem to be running. If that doesn’t work, put in the backup and make sure you get around to picking up another backup in case a heater fails in the future!