How to Calculate Furnace Size for a House

How to Calculate Furnace Size for a House

Buying a new furnace is a major decision these days since there is so much choice on the market, including different brands, models and variants, technical capabilities, furnace types, and efficiency ratings. But one of the most essential factors to consider before purchasing a new furnace is the size that is ideal for your home.

Installing a correctly sized furnace is more than just estimating the square footage of your home. When sizing a furnace, there are several factors you need to consider. Now, we’ll review what those aspects are, why it’s essential to properly size a furnace, and how you can buy a furnace that’s the perfect size for your location, environment, and comfort goals.

The goal of this article is to guide you in choosing the required size of furnace for your heating needs. It is calculated by taking the required heating BTU per square foot for most homes and your climate zone. Our heating BTU calculator takes these critical things into consideration, allowing you to give an answer for common questions as “how many BTUs to heat a home?” or “What size furnace do I need?”

The BTU per square foot heating rule of thumb varies greatly depending on your climate and the area of your house or room you wish to heat up.

Why choosing the best Furnace Size Is So Important?

Although it may be tempting to choose a smaller furnace to save money on the purchase price or a larger furnace to boost space-heating, but with furnaces just as with so many things in our lives we should follow the rule of Golden mean: you need one that is just perfect, for your home.

A furnace which is far too powerful will cycle irregularly. This is problematic because it will generate hot and cold zones around the home, decrease efficiency, require additional maintenance, and shorten the furnace’s lifespan.

A furnace which is undersized will operate continuously and won’t be able to maintain a comfortable temperature in your home. The additional workload will also increase your energy costs, cause maintenance issues, and shorten the life of your furnace.

Another drawback of having a furnace that is the incorrect size is that it will reduce your heating system’s overall efficiency.
The AFUE (Annual Fuel Utilization Efficiency) is the thermal efficiency measurement of a furnace based on normal operation. The AFUE tries to represent the equipment’s actual, season-long average efficiency, taking into account all operating factors.

You won’t be able to get the efficiency rating you paid for if your furnace runs continuously or irregularly because it is the wrong size. That means more energy use and higher heating expenses.

Your furnace must be correctly sized for your home in order to perform at the efficiency level it is rated for.

How many BTU do I need for my house?

That is a crucial question that must be answered correctly. On the coldest winter days, a furnace that is way too undersized will not heat your home properly. Areas further away from the furnace may become uncomfortably cold in your home.

An oversized furnace will waste large amounts of energy, and create hot patches around vents close to the furnace before the heating cycle stops.

When you talk about furnace size, you are not talking about the actual physical dimensions of the unit. Rather, the size of a furnace is determined by its heating output and the amount of heat it can provide.

The output of a furnace is often measured in British thermal units (BTU) per hour. A BTU is a unit of measurement for energy, particularly the amount of energy necessary to raise the temperature of one pound of water by one degree Fahrenheit. Heat is now understood to be the same as energy. The joule (J) is the current SI unit for heat and energy; one BTU equals about 1055 J.

Furnace BTU

You need to remember, that “BTU/h” is sometimes abbreviated to just “BTU”. The modern unit of power for heating and cooling systems is the watt. 1000 BTU/h is approximately 293.1 W.

Some manufacturers have started to calculate furnace size in MBHs rather than BTUs. One MBH is 1000 BTU per hour, then you’re still using the same unit: 60 MBH is equal to 60000 BTU.

The general BTU per square foot heating rule of thumb varies from 30 BTU per square foot to 60 BTU per square foot based on your climate zone. That is a significant variation, with one furnace being twice as powerful as the other, so the size factors are explained here to help you in how to size a new furnace for a house.

A bigger furnace, such as one with an output of 120000 BTU per hour, will provide enough heat for a larger space. A smaller furnace will have a smaller BTU capacity, such as 40000 BTU per hour, and would be suitable for only a smaller pace.

Calculating BTU requirements for a house

Let’s start with this very useful map. The climate becomes progressively colder from zone 1 to 3.

Find your location on the map. If you’re on or close the border between two zones, choose the higher number, the cooler zone.

Canada climate zones

Take the square footage of your home. If you don’t know it, you can look up the blueprint of the house, or measure rectangles within your home. Multiply the width by length for each rectangle and add them together.

It is not quite accurate, but nevertheless very useful for our purposes. While you shouldn’t use this calculation to make a final decision about furnace size, you can use it to get an idea of the size range to look at when you buy a new furnace. You can get a basic estimate using the square footage of your house.

The general rule of thumb is that you need between 30 and 60 BTU per square foot. Depending on your climate zone, you can determine how many BTU per square foot you need.

Furnace BTU sizing chart

Square footageFurnace BTU for
Zone 1
Furnace BTU for
Zone 2
Furnace BTU for
Zone 3
100030000 BTU45000 BTU60000 BTU
125037500 BTU56250 BTU75000 BTU
150045000 BTU67500 BTU90000 BTU
200060000 BTU90000 BTU120000 BTU
250075000 BTU112500 BTU150000 BTU
300090000 BTU135000 BTU180000 BTU

Let’s say you live in the Greater Toronto Area (Zone 2), in a home which is 2000 square feet. The chart would tell that you need approximately a 90000 BTU furnace.

These are only examples with approximated values to show how these factors would affect the furnace size you need to buy, how many BTUs needed to heat a house.

Professional furnace sizing

The best way to ensure your furnace is the right size for your home is to get an evaluation from a professional HVAC expert at Toronto Furnaces.

Toronto Furnaces Inc. has been proudly providing professional Furnace Installation services to accurately calculate the square footage of your home, factor in all the other elements that can impact furnace size, and give you a precise BTU capacity for your furnace.