Will A 4 x 4 Fit In A Cinder Block?

If you're building a fence using 4 x 4 posts and cinder blocks for your garden, you'll want to make sure the pieces fit together correctly. You may wonder if a 4 x 4 will fit in a cinder block. We have done the research and will answer your question in this post!

The size of the holes in a cinder block hole can vary. To fit a 4 x 4 in a cinder block, you will need a block that is at least 8 by 16 inches. This type of block typically has holes that are about 5 1/2 inches wide. A 4 x 4 should fit perfectly in this cinder block, as the additional space will allow it to pass through.

Since there are different cinder block sizes, you may be wondering if there are other options you can use for fitting a 4 x 4 or if it is possible to widen the holes of a cinder block. We will discuss these questions and more in the next sections, so keep on reading!

How Big Are The Holes In A Cinder Block?

cinder blocks of gray concrete are neatly stacked in a pile, slender rows of bricks

Cinder blocks are also referred to as masonry units. They are similar to concrete blocks but are a little more lightweight and porous. Masonry units are produced in standard sizes to keep them uniform. The average (or nominal) masonry unit size is 8 x 8 x 16 inches.

Depending on the manufacturer, there is a tendency for masonry units to run small. Depending on how you use the 4 x 4, you may need to factor this into your project—for example, when you seal the 4 x 4 in the cinder block.

As long as the hole is around 5 inches, a 4 x 4 should fit just fine. Wooden 4 x 4 posts and cinder blocks together can create a lot of friction, so you want the cinder block hole to be a little larger than 4 x 4 inches.

Ideally, you want cinder blocks that have square holes. Some cinder blocks are at near average measurements but have narrow, rectangular holes smaller than 5 inches.

If you have to choose another option, you should size up. It is also useful to note that the divider between cinder block holes is one inch thick.

Aside from cinder block dimensions, you should consider the shape of the cinder block. Concrete masonry units can come in the following shapes:

  • solid corners (all-purpose)
  • with an indent on either side (stretcher unit)
  • with an indent on only one side (single-comer unit)
  • with a wide indent on one side and a narrow indent on the other (sash unit)
  • with open ends (either one end, both ends, over the top)

You may be able to find other special unit shapes in addition to these. However, these are uncommon and cannot accommodate a 4 x 4.

How Do You Attach A 4 x 4 To A Cinder Block?

Side view of caucasian man in his yard guiding a 4x4 wooden post through the blade of a table saw that is cutting wood for a DIY proje

No matter how you attach a 4 x 4 post to a cinder block, the method is the same. You can attach the 4 x 4 through the hole or against the side. Here are the steps for doing this:

  1. Prepare your tools and materials
  2. Position the 4 x 4 on or against the cinder block
  3. Nail the post into the cinder block
  4. Secure the post

Prepare Your Tools And Materials

A concrete post holding a fence panel, Will A 4 x 4 Fit In A Cinder Block?

Aside from the 4 x 4 and the cinder block, you will need a nail gun, nails, a mallet or hammer, measuring tools, and a workbench. Be sure to wear safety equipment such as goggles and gloves. 

You should use nails that will penetrate the post 1 1/2 inches into the cinder block. Use nails made specifically for masonry that are at least 5 1/2 inches long.

Position The 4 x 4 Against Cinder Block

Working on a steady surface, pass the 4 x 4 through or onto the cinder block. You can get an assistant to help keep things in place. Measure and level everything out as needed. It also helps to mark things for more precise work.

Nail The Post Into The Cinder Block

Once you have finalized the position of your 4 x 4, carefully nail it in place. Make sure that the post is attached securely. You can use the mallet to hammer the nails in place.

Secure The Post

When you are satisfied with your work, you can secure the post and cinder block in place as part of a fence or wall.

For more details on the process, check out this video.

Can You Set A 4 x 4 Post Directly In Concrete?

Man people person working digging up dirt for vegetable winter garden post raised bed cold frame in Ukraine dacha and closeup of shove

You can set a post in concrete. This will help prop up the post, and it can also keep it from moving out of place if you opt not to nail it down. 

Here are the basic steps for setting a 4 x 4 post into concrete:

  1. Check the area to make sure you will not be digging into any utility lines.
  2. Prepare the materials (post, concrete, gravel, brackets, screws, tools).
  3. Dig a 12-inch-wide hole with a depth that is 1/3 of the post's height + 6 inches
  4. Fill the base with gravel (around 6 inches).
  5. Set the post in the hole. It's a good idea to use a level for this task. You may need to support the post with pieces of wood.
  6. Pour mixed concrete into the hole. (Make sure to use fast-setting concrete.)
  7. Double-check to make sure the post is plumb and remove the supports.
  8. Leave it to dry.

How To Make Cinder Block Holes Bigger

You can make cinder block holes bigger in case the hole is too small for the 4 x 4. Tools such as a core drill or hole saw are helpful for this but are not always necessary. A chisel or file will do. With these tools, you can manually take down bits of cinder block until the 4 x 4 fits.

To make a smaller hole, you do not necessarily need heavy-duty tools. Unless you are trying to make a significantly larger hole, you can use a drill with a small bit if you find that a tool is necessary. 

Is A Cinder Block And 4 x 4 Fence Expensive?

The cost of a cinder block starts at around $2. However, you will usually have to buy these blocks in bulk. The price of a 4 x 4 post will vary depending on the length. A 4 x 4 that's 8 feet long can cost around $9. You should check with your local dealer to find out the cost of the amount you need. 

A fence made of cinder blocks and 4 x 4 posts can be as affordable or as expensive as your budget allows, depending on the materials and labor. Just make sure to get sturdy masonry and treated posts to avoid any issues in the future.

Cinder Block Fence Ideas

Horizontal shot of stacks of cinder blocks and other building materials at a construction site

You can make several things with cinder blocks and 4 x 4 posts. Cinder block walls are common, but reinforcing them with 4 x 4 posts can make for interesting designs. Additionally, you can make a fence of 4 x 4s that uses cinderblocks in between.

Both materials are also useful in square-foot gardening. Wooden posts are commonly used in square-foot gardening, but cinder blocks are an option in building raised garden beds as well! You can even plant directly into the cinder blocks.

Here are some cinder block and 4 x 4 ideas you might want to try.

This fence uses only cinder blocks and 4 x 4 posts. The posts can be moved, as you do not need to nail them, use cement, or secure them through the cinder block. The blocks are nominal size with 5-inch holes. Notice how the blocks have a wooden platform to keep them steady on the ground.

Since the wood can be moved, this design makes for a versatile fence that you can easily customize and change up.

Using the same concept as the fence, you can come up with benches like this one! They are easy to put together and perfect for outdoor seating. You have the option of painting the cinder blocks or staining the 4 x 4 to make the most of the design.

Click here to see this masonry paint on Amazon.

Take the bench concept up a notch by adding some cushions and throw pillows! The wood can be a little uncomfortable, so adding soft materials boosts the usability.

For the final idea, here is a square-foot garden using cinder blocks. There is no construction required aside from laying down the cinder blocks. With enough blocks, you can form any number of shapes and paths perfect for your gardening needs.

Wrapping Things Up

A concrete post holding a fence panel

A 4 x 4 will fit easily into a standard 8 x 16-inch cinder block. Make sure the holes are at least 5 inches wide for the best fit. You can construct a variety of fences, walls, and benches for your yard and garden using cinder blocks and 4 x 4 posts.

For added support, you can nail the posts into the cinder blocks or set them in concrete. However, this is not always necessary. If you plan to try any of the ideas mentioned above, you can make do with just the two materials.

Did you find this post helpful? If you did, check out our other articles before you go!

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