Retaining wall blocks are an easy way to make a beautiful circular fire pit for your backyard. They come in different sizes, shapes, and colours, so you can create the perfect design for your outdoor space. But how many do you need? This post walks through the steps of designing a retaining wall block fire pit and gives an estimate on how many blocks you’ll need based on size.
Table of Contents
How Many Retaining Blocks Do I Need for My Fire Pit?
There are many variables that go into planning your fire pit design, so it’s impossible to pin down an exact number for how many retaining wall blocks you will need. You can narrow the field by defining what your needs and preferences are, then calculating how much stone is required to fill those needs. For example: do you want a tall fire pit or more of an accent feature? How wide do you want it to be? Will your design include stairs, seating walls, etc.?
The following are factors that affect how many retaining wall blocks you will need:
- Size of the Fire Pit: How deep and wide is the stone base for your fire pit? You need to know this in order to calculate how much stone you will need.
- Type of Retaining Wall Blocks: There are three types of retaining wall blocks: standard, split face, and interlocking (sometimes called “stackable”).
- Standard blocks have a front and backside that is relatively flat; they also come with two smooth sides.
- Split face blocks have a flat front and back, but the sides are angled. They look like standard blocks with one side split in half.
- Interlocking blocks can be stacked on top of each other to build retaining walls without using mortar; they come in different heights and widths for building both straight or curved rows.
- The Shape of the Fire Pit: Does your fire pit have a rounded edge, or will it be squared off? Your design can affect how many blocks you need as well as their shape. For example, if you want to build an angled seating wall that juts into the fire pit area then more blocks are needed than for a straight wall. A rounded edge will also require more blocks than a squared-off edge.
- Height of Retaining Wall Blocks: How high do you want your retaining wall blocks to be? A taller fire pit will require more blocks than a shorter one.
- Length of Fire Pit Row: Does your desired design include multiple rows or just one straight row? Multi-row designs need more stone than single-row designs because of the lengths
How Many Retaining Wall Blocks Needed For a Circular Pit?
First, you need to calculate the diameter of your pit size, which is the measurement of the widest part of your fire pit. This may be predetermined if you’re planning on placing a fire ring in the middle of your stone base. If you’re not placing a fire ring in the centre, then outline the circle with spray paint or chalk and use a measuring tape to determine the diameter of this shape.
Next, you’ll want to choose a size of trapezoidal block that you will use. The trapezoidal block is the best option for circular fire pits because they are slightly smaller on one end, which allows for a well-fitted curvature as you place each stone.
The standardized size of trapezoidal blocks are:
- 8″ x 3″ x 4″ (20.32 x 7.62 x 10.16cm)
- 10.25″ x 3.5″ x 7″ (26.03 x 8.89 x 17.78cm)
- 11″ x 4″ x 6″ (27.94 x 10.16 x 15.24cm)
Here are several common firepit diameters you may consider for your new fire pit, with the number of retaining wall blocks needed for one full circular layer. You will likely want to build up several layers, anywhere from 2up to 6. So go ahead and multiply the numbers below by the number of rows you will have in total.
Block Size: 8″x 3″x 4″ (20.32 x 7.62 x 10.16cm)
27″ Diameter Pit: 15 blocks per row
30″ Diameter Pit: 16 blocks per row
32″ Diameter Pit: 17 blocks per row
36″ Diameter Pit: 19 blocks per row
39″ Diameter Pit: 21 blocks per row
42″ Diameter Pit: 22 blocks per row
48″ Diameter Pit: 25 blocks per row
54″ Diameter Pit: 29 blocks per row
57″ Diameter Pit: 30 blocks per row
60″ Diameter Pit: 32 blocks per row
Block Size: 10.25″ x 3.5″ x 7″ (26.03 x 8.89 x 17.78cm)
27″ Diameter Pit: 12 blocks per row
30″ Diameter Pit: 13 blocks per row
32″ Diameter Pit: 14 blocks per row
36″ Diameter Pit: 16 blocks per row
39″ Diameter Pit: 17 blocks per row
42″ Diameter Pit: 18 blocks per row
48″ Diameter Pit: 21 blocks per row
54″ Diameter Pit: 23 blocks per row
57″ Diameter Pit: 24 blocks per row
60″ Diameter Pit: 25 blocks per row
Block Size: 11″ x 4″ x 6″ (27.94 x 10.16 x 15.24cm)
27″ Diameter Pit: 11 blocks per row
30″ Diameter Pit: 12 blocks per row
32″ Diameter Pit: 13 blocks per row
36″ Diameter Pit: 14 blocks per row
39″ Diameter Pit: 16 blocks per row
42″ Diameter Pit: 17 blocks per row
48″ Diameter Pit: 19 blocks per row
54″ Diameter Pit: 22 blocks per row
57″ Diameter Pit: 23 blocks per row
60″ Diameter Pit: 24 blocks per row
Once you have determined the number of trapezoidal blocks needed for your fire pit, the following process is pretty straightforward. These blocks are designed to fit snuggly in place and don’t need mortar or cement to keep them in place. All you need is a rubber mallet and the blocks will lock together with ease, creating your new fire pit wall without any problems.
How Can I Extend the Life of my Retaining Wall Fire Pit?
Although these blocks are designed to withstand the elements, it will be in your best interest to protect them from exposure. A simple way of doing this is by adding a fire pit to the centre of your retaining wall.
This way, the blocks will always be protected from the intensity of the fire heat. Fire pit inserts are available in many predetermined sizes, and many of the above-mentioned diameter sizes are compatible with pit sizes. For a higher cost, you can find and purchase a pit liner at a custom size. You’ll also want to make sure that the height of your pit aligns with the height of your retaining wall bricks.
To keep your fire pit burning brightly for many years to come, you may want to invest in a cover or lid. This is helpful for both safety and convenience. A mesh cover will keep sparks from flying out of the pit. It also provides a safer environment for you and your guests to enjoy an evening around the fire.
A lid is great because it keeps critters like grasshoppers, spiders, wasps & bees from crawling inside, as well as larger debris such as leaves or twigs. This prevents your fire from going out and wasting all of that work you’ve done to create a beautiful new outdoor space.
Can You Build a Fire Pit on Grass?
Since fire pits are designed to contain fires and prevent their spread, you can build a fire pit on top of grass. However, it is important to consider the safety implications and how this will affect your lawn in the long term. There are alternatives to placing your pit directly on the grass that can help to preserve the health and aesthetic of your lawn.
An inexpensive option to consider is adding sand to the base of your retainer wall pit. Sand helps to hold the heat and make sure that it distributes evenly throughout the base of the fire. It also helps by absorbing some of the heat that would otherwise go into the metal pit liner or retaining bricks.
There are other materials that will work well in your pit, including lava rock, dirt, gravel, and even a thin layer of cement. Just make sure to double-check that the material doesn’t contain air pockets, which could cause it to explode and project hot material into your space.
How Deep Should You Dig for an In-Ground Fire Pit?
If you would like an inground fire pit, you will want to remove anywhere from 6″ to 12″ (15.24cm x 30.48cm) of earth from the ground underneath where you will be placing your pit. While you can technically use a soil hole for fire, many people fit this space with a pit or liner so that the shape retains its size and symmetry.
You can then use retaining wall blocks to build the fire pit wall, above ground. Since the pit will begin in the ground, you may choose to use fewer rows of bricks so that you can easily see the fire and it looks like it’s emerging from the ground.
Since an in-ground pit is lower, you’ll want to be careful and make sure that animals and small children cannot harm themselves around the fire.
Do You Need Fire Brick for Fire Pit?
Fire brick is made to withstand high temperatures and is unlikely to crack, explode, or degrade with regular use. This is a great choice if you don’t plan on using a fire pit insert, since they do the same job.
Fire bricks are not necessary for a safe fire pit, and most regular retaining wall bricks will be able to withstand the heat and flames of a fire. The advantage of fire brick is its longevity. If you invest in this material, you can rest assured that your pit will last several years longer with a reasonable amount of maintenance.
Can You Use Mortar for a Fire Pit?
Absolutely! Mortar is not necessary when using retaining wall blocks. However, it can help to prevent excessive heat from escaping as well as help to hold the bricks together. This is a great option for a more permanent fire pit that will be used frequently and outdoors year-round, as it can stand up well against large amounts of heat and last much longer than non-mortared brick walls.
If you purchase a back of 60 pounds of mortar, this should be enough material for one fire pit. You can mix your material in a wheelbarrow, an industrial bag, or a large bucket. Purchase a masonry trowel and proceed to apply the matter between each block. Use a brush or towel to wipe off the remaining mortar that doesn’t smoothly adhere.
If you’re using mortar, your bricks will be permanently placed and you can’t adjust their shape later on. While this may be a benefit for some, it can be a drawback if your fire pit is built on land that regularly shifts due to environmental conditions.
You’ll want to make sure to prep your ground beforehand by removing any sticks, rocks, or other debris that may make a trip hazard. Dig out the shape of your pit and fill it with sand before you begin building to ensure an even foundation for this project.
Where Is the Best Place To Put a Fire Pit in the Backyard?
If you’re placing your fire pit in the backyard, there are a few factors to consider. You may want it near your patio or outdoor entertainment areas so that guests can gather around and enjoy its warmth on chilly evenings.
Another option is to place it away from these spaces but within a clear view of them if possible. This gives you options for fire pit seating, safety, and warmth. You can also put it away from your home under a canopy of trees to make the atmosphere more private while still enjoying its use. Make sure that your trees are at least 20 feet (approx. six meters) away from the pit itself so that they will not be in danger of catching fire or being burned by flying
When you’re deciding on where to place an in-ground fire pit, consider whether any structures or landscaping would be affected by heat damage when they are exposed during your fire. If you have a brick patio, for example, it may be at risk of cracking due to the heat. You’ll want to place your fire pit away from any sensitive areas so that they are not affected by the heat.
A rule of thumb is to keep your fire pit at least 10 feet (3.05 meters) away from any structures in your home or trees. This gives you enough room to enjoy the fire and keeps it from accidentally catching anything on fire as well.
Final Thoughts
A huge benefit of using retaining wall blocks to build your fire pit is that they are very easy to use and you can have a completed project in just a few hours. Retaining wall blocks are the perfect solution for novice DIYers who want to build their own fire pit without any experience since they won’t need any additional materials such as a liner, ground cover, or mortar.
However, if you want to continue to improve your fire pit’s structure and longevity, you can add these elements over time. Make sure to discuss any details with the manufacturer or company that you will be purchasing your retaining wall blocks from to ensure that your fire pit is the best it can be.
Check out our other handy guides for all things outdoor here