which space heater size do you need

Which Space Heater Size do you need?

Which space heater size do you need? How many Watts do you need? Most guides online, when googling for this question, give you formulas to calculate the needed wattage based on the size of your room.

But that is an over-complication of the approach to solving the question and an oversimplification of the real world. A simple formula can’t consider all boundary conditions in your home: What’s the exact volume of your living space? How well insulated is your home? How moist is the air? What kind of furniture do you have? What’s the weather like outside? Are doors kept open or closed?

I could think of a thousand such questions. Since we can’t consider all these questions in a formula (you’d probably need to hire a research team), we need another way to properly size space heaters.

Luckily, there is a much more practical and stupid simple approach to sizing space heaters and it will always work out perfectly.

No mathematics or formula required. Just real-life knowledge and a little bit of testing.

Quick answer: You need one 1,500W space heater with a built-in thermostat. The thermostat automatically adapts the wattage to the room size and always draws the exact amount of power needed to keep your room at temperature.

Later, I will show you my favorite 1,500W heater with a thermostat, so you don’t have to do the research.

In this article, we’re first at some formulas and some pre-made calculations for you. Since that is too complicated, I am explaining to you later on, why any 1,500W space heater with a thermostat is already a perfect choice.

How to size a space heater by formula

The first approach to properly size a space heater for your room is by formula.

First, you need to measure the size of your room. That’s best done using a folding ruler.

Measure two sides of your room and multiply them with each other to get the area of your room.

If you have a pitched roof within your room, you can decrease the area under the pitched roof by 50%. This is not mathematically exact, since we don’t know the inclination and height of the pitched roof. But for most homes, this will work out.

Once you know the size of your room, you can continue with the following formula.

How many Watts of power does your space heater need?

In this section, we’ll develop a formula based on a mix of an existing formula and personal experience. So, for your calculation use the last formula in this section.

If you are interested in how we adapt existing formulas, then keep reading.

Here’s one existing formula I found:

“Estimate 10 watts per square foot and multiply the square footage of the room by 10. A room with 144 square feet will require 1,440 watts.”


The formula from the quote above is surprisingly simple. Just multiply the square footage by 10.

I will add some value here: If you measured your room size in square meters, then the factor of 10 should be replaced with a factor of 100. The formula for square meters is:

“Estimate 100 watts per square meter and multiply the square meters of the room by 100. A room with 15 square meters will require 1500 watts.”

This formula is a good starting point. However, I think it is too strict. I have a 2000W oil-filled radiator at home, and it can easily heat 300 square feet rooms. According to the formula, it would be best suited for 200 square feet rooms.

So, I’d suggest to adapt the calculation. My personal formula is:

“Measure the square footage of your room and multiply it by 6.67. The result is the amount of Watts a space heater needs to heat your room. If you measured your room in square meters, multiply the area by 66.7.”

Space heater sizes for different room sizes pre-calculated

Here I have already calculated some wattages for you, so you don’t need to. If you are looking for an electric space heater, the power is rated in Watts. The formula is a simple division by 6.67, as we saw in the previous section.

Watt table

Heater power in WattsRoom size in square feet
Different wattages and the room sizes they are able to heat

If your room is bigger than 300 square feet, then you should just get multiple space heaters. It is very hard to find home space heaters rated over 2000W in the US. In Europe, there are some, but it is generally not common.

Space heaters stronger than 2000W are usually industrial heaters.

If you plan to use a non-electric heater such as a propane heater, the following table is for you. Fuel and gas-based heaters don’t specify their power in Watts but in BTU.

Here’s how many BTUs you need to heat various room sizes. 

Heater power in BTURoom size in square feet
Different BTU ratings and the room sizes they are able to heat (source: pickhvac.com)

Which space heater sizes are available?

If you went for the calculation approach, you should by now know how many Watts your space heater needs.

But still, what should you do now? For example, if you have a 200 square feet room, you know by multiplying the area by 6.67 that you need a 1,334W space heater.

However, 1,334W space heaters don’t exist. So, another question comes up: What should you do now? Do you pick a bigger or a smaller space heater? And will you waste energy by going for the bigger space heater?

The answer is absurdly simple. It doesn’t really matter which space heater size you need. Almost all common space heaters are rated at 1,500W. That’s the size you get.

The result of your calculation does not matter.

Some space heaters are 750W. But we are not talking about those. They are too small for most use cases. When you buy a 750W heater, you restrict yourself to small room usage. Let’s ignore the 750W heaters for now.

The practical approach to space heater sizing

There is a much better approach to finding the perfect-sized space heater. And it is the following:

Buy a 1,500W heater. Here’s my personal favorite 1,500W heater with thermostat (click to see the review).

Since 1,500W are your only real option, buying 1,500W heaters is what you should do. You can just as well use a 1,500W heater for a small room, even if you calculated you only need, say, 700W of power.

Let me explain in the following section why a 1,500W heater is perfectly suited for almost any room size.

The magic of thermostats: Wattage doesn’t matter

The power (wattage) of a space heater describes how much heat the space heater can output at most.

But this maximum amount of power is not always used. 

Lots of space heaters come with a built-in thermostat. A thermostat controls the amount of heat your space heater outputs.

On the thermostat, you set the desired room temperature. Your space heater will then heat your room to that temperature.

Now comes the magic: Once the desired temperature is reached, the space heater turns off the heat. This means that a 1,500W space heater with a built-in thermostat will never overheat your room, regardless of the size.

When you have a small room and the 1,500W heater is supposedly (according to the mathematical formula in the previous sections) too big for you, the 1,500W heater, thanks to the thermostat, automatically reduces the power it uses.

A space heater with a thermostat automatically adapts its wattage to the room size.

The power rating (1,500W) of a space heater with a thermostat does not describe how much power the space heater uses at all times. It describes the maximum power usage of the space heater.

How to size a space heater without formula

There is a simple rule you can follow to properly size a space heater without doing any calculations.

  1. Buy a 1,500W space heater with a built-in thermostat
  2. Spend one day heating your room with it. Is it warm enough? If yes, you have properly sized your space heater.
  3. If the space heater does not provide enough heat, that means 1,500W is too little power for you. Supplement with central heating.

It’s that easy.

When you have two heat sources (electric space heater + central heating or a stove) in one room, the space heater will not overheat your room thanks to its thermostat.

The space heater will adapt itself to the other heat source and only produce the necessary heat to keep your room at the desired temperature.

The space heater lowers its heat output and generates the exact amount of wattage that your room needs. You don’t need to pick the space heater size. A space heater picks its own size by automatically adapting to its surroundings.

How to choose space heaters for big rooms

Now, what if your room needs 2,500W of power? Well, it doesn’t matter that you need a 2,500W space heater since you won’t find space heaters rated this high anyways. Personally, I have never seen a regular household space heater rated at 2,500W.

Additionally, most American wall outlets and electric home circuits are limited to 1,800W and 15A. A space heater using 1,500W draws about 12.5A already maxes out what your home circuit can handle.

For big rooms, you can try to use two 1,500W space heaters, if the room is big enough to have multiple separate electric circuits.

However, generally, if a 1,500W heater is not enough for your room, then you should use central heating.

Space heaters will save you money only if you use them in smaller spaces. When your room requires this much heating power, you may as well use central heating. Normally, the price for central heating is cheaper than electricity.

Space heaters only have an advantage in smaller spaces, where you can decrease the central heating temperature and use a space heater to supplement heat where you need it.

Also, using central heating with an additional space heater is a much safer route to go, since the central heating does not demand your electric circuit.


To properly size a space heater, you can either use one of the formulas given or just look up the pre-calculated results from the table above.

Both of these approaches require you to know the size of your room. Since this is not always the case, I suggest another approach.

Buy a 1,500W space heater with a built-in thermostat. The thermostat ensures that your space heater only produces the required amount of heat. By keeping your room at the target temperature, your space heater automatically adapts its wattage to your room size and insulation.

If you notice that 1,500W is not enough for your room, you should supplement the space heater with your central heating or vice versa. If you don’t have central heating available, you can also install a stove.

Operating more than one space heater in a single room is not safe since the space heaters will overload your home circuit’s amperage and wattage limitations. This can cause fuses to trip and in the worst case, a fire.

About the Author

Daniel Hirsch

Daniel is an electrical engineer, blogger, and author. He studied electrical engineering and information technology and decided to blog about heaters after working in the temperature sensing industry.