How Do You Measure a Fireplace Opening: Everything you need to know

Going through a fireplace renovation sounds easy because you only need to get a new fireplace and install it, right? Not so because fireplaces have very different sizes that may or may not fit the space you have now. So, how can you be sure that what you’re buying will actually fit the fireplace opening?

You can measure any indoor or outdoor fireplace opening by taking the height and width of the firebox. Then use it as the measurement guide for the fireplace that you’re going to buy. However, if you want to have a more accurate measurement, it would be best to measure multiple sections of the fireplace opening.

This process may seem straightforward, and it’s easy to think that nothing could go wrong. But this article will show you how easy it is for many people to mismeasure their fireplace opening. Stick around because we’ll also show you what you can do to make your fireplace opening bigger. This could also be applied to get the dimensions of the inside of your fireplace. You may need to remove your grates to do this measurement.

How To Properly Measure a Fireplace Opening

Fireplace openings or a fireplace insert are also referred to as a firebox or cavity. Unfortunately, this variance in the terms we use is one of the most common reasons people mismeasure a fireplace opening, as people confuse the terms to mean something else. The other most common cause for inaccuracy when measuring a fireplace opening is simply human error.

It seems like a straightforward task because you only need to measure the height and width of the fireplace insert, but it could be one of the worst ways for anyone to measure a fireplace opening.

A fireplace opening could have different heights and widths in different areas, making it crucial for you to measure it carefully. For example, the opening could measure 26″ (66.04 cm) wide and 18” (45.72 cm) high, but it could be arched, leading to a smaller mid- or rear-section, which may not fit the fireplace you’re going to buy.

So the best way to measure a fireplace opening is to use a measuring tape to measure its height and width from the opening, mid-section, and rear. It’ll allow you to find out if there are smaller areas in your fireplace opening that may cause issues when you’re trying to install your new fireplace.

When measuring a fireplace opening, make sure you’re taking the smallest height and width in all the sections you measured of your firebox. Here’s an example of what your measurements may look like:

Front18.25 inches (46.36 cm)Top26 inches (66.04 cm)
Mid18.5 inches (46.99 cm)Mid26.5 inches (67.31 cm)
Rear18 inches (45.72 cm)Bottom26.75 inches (67.95 cm)

This example is an oversimplified measurement for a firebox. As you can see, measuring only one section may not give you the right size because the smallest areas are the rear and top sections. By measuring different sections of your fireplace insert, you’ll be able to determine the smallest areas and use them as the actual measurement for your firebox; in this example, it’s 18″x 26″ (45.72 cm x 66.04 cm).  

What Is the Standard Size of a Fireplace Opening?

The size of a firebox or fireplace insert will depend on the space that you allot for them. Obviously, you can’t have a large fireplace in a small to medium-sized room. So to help you get an idea of how big — or small — a fireplace opening should be, here are the standard sizes that you’ll find:

Width24 (60.96 cm)28 (71.12 cm)32 (81.28 cm)36 (91.44 cm)40 (101.6 cm)42 (106.68 cm)48 (121.92 cm)
Height24 (60.96 cm)24 (60.96 cm)29 (73.66 cm)29 (73.66 cm)29 (73.66 cm)32 (81.28 cm)32 (81.28 cm)
Depth16 (40.64 cm)16 (40.64 cm)16 (40.64 cm)16 (40.64 cm)16 (40.64 cm)16 (40.64 cm)16 (40.64 cm)

These are the usual fireplace sizes you’ll find in most stores, but you can also find smaller or larger ones. What’s important is that you can get the actual measurement of the fireplace opening to be sure of what you’re getting, or at least do something to make your fireplace opening bigger.

How To Make a Fireplace Opening Bigger

It’s easy to make a fireplace opening bigger, but if you want to move the lintel, you may have to contact a professional to do it for you. You also need to ensure that the renovation you’re going to do with your fireplace opening complies with any regulations mandated by your city.

To make a firebox bigger, all you have to do is use a hammer to knock down the bricks behind the lintel. The lintel is a bar-like structure made of bricks that protects the foundation of your fireplace.

If you want to make your fireplace opening even bigger, avoid taking down more bricks. This process will require the removal of the lintel, which can cause issues with the integrity of the fireplace. If this is something you want to do to your fireplace, it would be best to have a professional do it for you because there are several factors that you need to consider, including the foundation.


Measuring a firebox is easy because you only have to measure the height and width. The depth remains the same, regardless of the size of your fireplace. However, if you want to be sure that you’re getting the right measurement for your fireplace, it would be best to measure it from different sections. Especially if you want the proper

It’ll be a lot easier if you know that your fireplace opening is straight without arches. But fireplaces aren’t the most symmetrical areas in our house. So be sure to take the measurements from different sections and take the smallest measurements to avoid issues with installation.

Our blog has more content like this if you have any other questions or concerns with your fireplace. Check out our article on how far a fireplace mantle should stick out.

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