Space Heater Sizing Calculator – How To Find The Best Size Home Heater

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Did you know that heating accounts for 42% of your home utility bill on average? That is a huge sum of money that you can save on if you are able to use your energy wisely and efficiently to heat up your house.

An easy way to save on heating costs is to use space heaters instead of the central heating system when you just intend to heat up part of the house. However, space heaters can become an inefficient waste of energy if not used properly. 

In order to maximize your space heater, the first step is to size your space heater. This means getting the correct wattage that corresponds to the size of the room. Besides the room size, other room conditions such as room insulation and outdoor temperature also needs to be considered.

Our guide will tell you exactly how to size your space heater in 5 easy steps so that you do not waste energy and money getting a large space heater that is too powerful or one that is too weak for your room.

If Math is not your strong suit or you are in a hurry, you can use our Space Heating Sizing Calculator below to calculate the exact wattage you need!

How Do I Calculate The Size Of Heater I Need?

Space Heating Sizing Guide

Step 1: Measure Your Room

Measure the length and breadth of your room (in ft) and multiply these two figures together to find out the area of the room in sq. ft.. For example, if the length of the room is 10 ft. and the breadth of the room is 8 ft., then the room area is 10 ft. x 8 ft. = 80 sq. ft.

calculate area

Step 2: Calculate The Wattage

As a general rule of thumb, the ratio to consider is 10 watts to 1 sq. ft. of space. For example, a 10 ft. x 12 ft. room, for example, requires a 1200 watt space heater (120 sq. ft. x 10 = 1200 watts). 

However, if you just stop here and get your space heater based on that, you will likely be disappointed when the heater does not perform to your expectations. This is because there are still a few factors to consider that might throw us off


Step 3: Measure Your Ceiling Height

The ratio of 10 watts to 1 sq. ft. of space applies only to rooms with a standard ceiling height of 8 ft. However, if the ceiling height is greater than 8 ft. then you will need increased power and wattage to ensure the heater can heat up the increased size of the room.

Generally speaking, you should increase the wattage of your space heater an extra 25 percent for every 2 feet of extra space. For example, a 10 x 12 room with 10-foot ceilings would require a 1500 watt space heater. 

Room area – 10 ft. x 12 ft/ = 120 sq. ft.

Wattage estimation – 120 sq. ft. x 10x 1.25 = 1500 watts


Step 4: Determine Your Room Insulation

If your room is poorly insulated or has a faulty window that lets in cold air, you will also need to factor in for greater wattage. Typically, an increase of 20% wattage would be a good place to start depending on how poor the insulation is. 

Conversely, if your room is heavily insulated, then you can reduce the wattage by 20%. If you think that the insulation is moderate and decent, you can skip this step.

Pro Tip: Improve your room insulation with easy DIY methods like weatherstripping and caulking. Visit our guide here to find out more!


Step 5: Determine The Outside Temperature

The rate at which heat is lost from your room depends on the external environment as much as the construction scheme of the house. In particularly cold regions (below 32ºF), homes will experience way more heat loss than a house in a warm region. Therefore if you happen to be living in these extreme cold situations, you will need to increase the wattage ratio from 10 watts per square foot to 15 watts per square foot when you’re estimating space heater size.

In temperate climate where the external temperature ranges from 32ºF to 68ºF, the wattage ratio can be increased from 10 watt per square foot to 12.5 watts per square foot. For temperatures above 68ºF, there is no need to tweak the ratio.

infographics on how to size a space heater

Frequently Asked Questions (FAQs)

Q: HOw many square feet will a 1500W heater heat?

A: Assuming a ceiling height of 8 ft. and the room is properly insulated in a climate above 68ºF, a 1500W heater will heat a room of 150 sq. ft. comfortably.

Q: What is BTU?

A: A BTU (British Thermal Unit) is the amount of energy needed to heat 1 pound of water 1 degree Fahrenheit. To convert watt to BTU, it can be obtained by multiplying the wattage by 3.41. All space heaters clearly identify this number in the product description or on the packaging, so you don’t have to worry about doing any additional calculations.

Q: How many square feet will 5000 BTU heat?

A: 5000 divided by 3.41 = 1467. Therefore, it roughly translate to a 1500W heater. A 1500W heater will heat a 150 sq. ft. room comfortably if the ceiling is of normal height and the room is properly insulated in a climate above 68ºF.

Q: Is it safe to have a heater on all night?

A: The National Fire Protection Association (NFPA) recommends that a heater should never be left unattended so it should be turned off when going to sleep. However, we know that totally defeats the purpose of the heater then! Therefore, we advise that homeowners apply some safety precautions if intending to leave it turned on overnight.

  • The heater should be kept 3 ft. clear of any flammable objects
  • Do not hang any cloth over the heater
  • Ensure heater comes with overheating and tip-over protection
  • Plug portable space heaters directly into an outlet; do not use an extension cord

Wrapping it up

With this guide, you would have understood the main considerations behind sizing a space heater which should help you in your own purchase later on. The math may be slightly bothersome to calculate so do feel free to use our sizing calculator to your convenience!

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