Fireplace Size vs Room Size: How To Tell What You Need

I’m currently shopping for new homes, and I’m thinking about installing a fireplace. However, I couldn’t find out what size fireplace I need. So, after some research, I put together this guide to help measure fireplace sizes to the room size (whether it’s a living room, bedroom, or any other room).

Before you choose a fireplace to install in your home, it’s necessary to measure the dimensions of the room where it will go. These numbers will allow you to calculate the appropriate fireplace size and heat output from there. Fireplace type, your climate, and room design should also be considered. 

So you’ll need to know the dimensions of your room, but how are fireplace size and heat output calculated with these numbers? What are the standard sizes of fireplaces, and what other factors should you keep in mind before you choose one? Let’s take a closer look. 

How to Calculate Fireplace Size to Your Room Size

Width x Length x Height = Volume
Volume x 4 = Minimum BTUs

To calculate fireplace size, measure the width, length, and height of your room in inches. Multiply these three numbers together to determine the room’s volume, then multiply that number by four, which will give you a close minimum heat output. You can select an appropriate size with the original dimensions.

Save your minimum BTU number as we’ll need it in the next section to find the right fireplace size for you. For now, here’s some more information that can help you make your decision.

There are a few simple formulas you can use to calculate fireplace size as it relates to your room size.

One of the easiest ways is the one mentioned above. Start by measuring the room width, height, and length. Multiply these numbers together to get the room’s total volume. Then, multiply the volume by four to get a close estimate of the minimum heat output in British Thermal Units (BTUs).

For example, if your living room is 16 by 20 feet and the walls are 10 feet high, the volume of the room would be 3,200 feet cubed (the result of multiplying those dimensions). After multiplying this number by four, your minimum heat output would be about 12,800 BTU. 

When it comes to fireplace size vs. room size, the size of your fireplace needs to match the size of your room. You should take your room’s dimensions and layout into account as you are thinking about what type of fireplace to install.

While heat output matters, most people no longer rely on fireplaces as their only heat source, so size is the most important factor when it comes to selecting one. You’ll want a fireplace that doesn’t overpower a small room or get swallowed up in a big room. 

In addition to the overall room size, you should also consider the layout of the room and the size of the wall where you plan to mount your fireplace.

If heat output doesn’t concern you, you can size your fireplace even more quickly. To do this, just make sure that the mantel shelf width divided by two is less than the width from the fireplace center to any wall, door, window, or shelf. 

What Sizes Do Fireplaces Normally Come In?

Fireplace Size	Dimensions	Heat Output
Small	24”x24”	10,000-18,000 BTU
Medium	36″x30″	25,000-30,000 BTU
Large	48″x32″	35,000-40,000 BTU

Here is where you can use your minimum BTU number from earlier and determine what size fireplace you need based on your heating requirements. For example, if you got 12,800 BTUs, then you’ll likely need a small fireplace (around 24″x24″).

An average fireplace is about 36”x30”. When it comes to the actual dimensions of a fireplace, these can be found in just about any size larger than 24”x24”, all the way up to 48”x32” and higher. The physical size of a fireplace insert is often listed by the width in inches, and they tend to range from about 20” to 60”.

If you’re getting a fireplace mainly for aesthetics, then simply take your mantel’s shelf width and divide it by two. Then, just make sure it’s less than the width from the fireplace center to any wall, door, window, or shelf. 

For another idea on how much area fireplaces can heat, are some of the most common heat outputs for sizing gas fireplace inserts:

• Small: 10,000-18,000 BTU –  heats up to 800 sq. ft.
• Medium: 25,000-30,000 BTU – heats up to 1,400 sq. ft.
• Large: 35,000-40,000 BTU – heats up to 1,800 sq. ft.

Many homes only need a “small” fireplace in terms of BTU heat output for a single room. This is especially true if you’re planning on supplementing the heat in your home with a central heating unit as well.

With all of the math and numbers out of the way, what other factors should you consider while you’re trying to size your fireplace? 

Other Factors That Determine the Size of a Fireplace

Beyond room size and heat output, you should take the many different types of fireplaces, the climate where you live, and fireplace designs into consideration as you’re trying to determine the proper size for your fireplace.

Type of Fireplace

When it comes to fireplace size as compared to room size, the first thing to decide on is what kind of unit you’re planning on installing.

The type of fireplace you plan on using will play a huge role in how big it ultimately is. In addition to how the fireplace is powered, there are several design elements that you can use to group and categorize fireplace types, too.

Essentially, there are two main types of fireplaces – solid fuel and gas. A solid fuel fireplace will be powered by things like wood. There are also fireplace inserts, which are usually powered by electricity or gas but can also be fueled by pellets and wood. 

These are the eight primary categories of fireplaces:

• Direct-vent
• Natural/B-vent
• Vent-free or ventless (gas)
• Electric
• Alcohol or gel
• Inserts (wood, gas, electric, pellet-burning)
• Masonry (wood or gas)
• Prefab/zero clearance/air-cooled (wood, gas, or electric)

If you’re considering adding a fireplace to an existing home, there’s a good chance that it’s probably not a wood-burning fireplace. 

When you’re adding a gas fireplace, generally, the larger the fireplace, the larger the burner, and the hotter the fireplace will be. 

For electric fireplaces, size has nothing to do with heat output. These units can be operated both with or without heat, as turning the flames on without heat creates a cozy atmosphere without overheating the room.

These don’t heat up the surrounding wall, so you don’t have to worry about size as it relates to the amount of heat the fireplace will push out. 

Climate

For the most part, very few people rely on their fireplaces as primary sources of heat. Instead, they mostly rely on their furnace or central heater.

However, if you are relying on a fireplace for your primary source of heat, remember that the size of the room will translate directly into the minimum number of BTUs you need to receive from your fireplace. 

With a gas fireplace, size matters, since a larger burner will generate more heat. If it’s an electric fireplace, size is irrelevant, as they all have the same heat output. 

Design

Consider the layout and aesthetic of your room as you determine the size of your fireplace. For example, mounting a tv over the fireplace is currently a popular trend. This way, you can enjoy the atmosphere provided by both the fireplace and the television.

If you’re going to mount your television above the fireplace, you should make sure it is at least as wide as the tv, potentially even wider.

Recommended: Should a Fireplace Be Bigger Than the TV? (Solved)

Remember that your television size is measured on the diagonal, while fireplaces are measured straight across.

Taking these measurements into consideration will help you achieve a more balanced look for your living space. 

You should think about the aesthetic you’d like to achieve in your living space, too. This is a question that we can’t answer for you, as everyone has different preferences for fireplace installations.

For instance, do you want a mantel or decorative accessories like louvers or exterior frames? What about the fireplace surround? 

While some of these design features won’t add to the overall size of your fireplace, some will. Even if they don’t, they can add a “busy” look to your fireplace that may make it seem larger or more intricate than it actually is. 

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