Concrete Masonry Units (CMU) are sometimes used in the garden for raised garden beds, retaining walls, along with a few other uses.
There are a lot of styles and sizes for concrete blocks that you can find at you home center. The most basic type is the precision block and it is probably the least expensive. These are also known as “common block” or “standard cored concrete block”.
The standard size for this CMU block is nominally 8″ deep x 8″ high x 16″ long. This size is not their actual size, they call this their common size! Their actual dimensions are 7-5/8″ deep x 7-5/8″ high x 15-5/8″ long. This allows for 3/8″ mortar joints between the blocks. This article is going to be on using this size of block for a garden planter.
Concrete block garden planters
Lets say you are wanting to build a raised planter made with CMU blocks and need to figure out the dimensions and how many blocks you need, without cutting them.
Typically, CMU is laid out in 8″ modules, also known as “block module”. This is so that the blocks do not have to be cut, creating waste and costing more time to assemble.
8″ modules includes the 3/8″ mortar joint. This is all fine and dandy if you are building a retaining wall, screen wall, or anything else that requires these joints for mortar.
But if you are building a small raised planter, you might not be building it with these joints, whether it is because it is too much work to do, you just don’t know how to do it, or just don’t want to do it because it is much easier and quicker to lay them out to their actual sizes. That is fine, low planters typically do not need to have these joints.
CMU planter or wall with mortar joints
Lets say that you are building the planter or wall with these mortar joints. For 3/8″ mortar joints, you would follow these dimensions, the first column being the number of 8″ modules:
| 1 | = | 8″ | = | 0′-8″ |
| 2 | = | 16″ | = | 1′-4″ |
| 3 | = | 24″ | = | 2′-0″ |
| 4 | = | 32″ | = | 2′-8″ |
| 5 | = | 40″ | = | 3′-4″ |
| 6 | = | 48″ | = | 4′-0″ |
| 7 | = | 56″ | = | 4′-8″ |
| 8 | = | 64″ | = | 5′-4″ |
| 9 | = | 72″ | = | 6′-0″ |
| 10 | = | 80″ | = | 6′-8″ |
| 11 | = | 88″ | = | 7′-4″ |
| 12 | = | 96″ | = | 8′-0″ |
| 13 | = | 104″ | = | 8′-8″ |
| 14 | = | 112″ | = | 9′-4″ |
| 15 | = | 120″ | = | 10′-0″ |
If you notice the overall dimensions, all the odd feet end in 4″ (1′-4″, 5′-4″, 9′-4″, etc.) and all the even feet end in either 0″ or 8″ (2′-0″, 2′-8″, 8′-0″, 8′-8″, etc.). Recognizing this, you can instantly know if the planter or wall dimensions require cutting block or not.
CMU Planter or wall without mortar joints
Figuring out the overall dimensions using no joints gets much trickier. I have not seen or heard of an easy way to figure out the dimensions other than using basic math to figure it out.
However, in most cases, this small difference in size will not matter much because it is so minimal and there is a possibility that the blocks will not be in direct contact with each other anyways because it takes just a little dirt/soil to get between the blocks and throw off the dimension .
If you really want to get these blocks really snug together, below is a chart you can go off of for what the dimensions would be if everything went together perfectly. I would add a little to the overall dimension the larger the planter gets to allow for the imperfections.
Here are the dimensions, the first column being the number of 7-5/8″ modules:
| 1 | = | 7.625″ | = | 0′-7-5/8″ |
| 2 | = | 15.25″ | = | 1′-3-1/4″ |
| 3 | = | 22.875″ | = | 1′-10-7/8″ |
| 4 | = | 30.5″ | = | 2′-6-1/2″ |
| 5 | = | 38.125″ | = | 3′-2-1/8″ |
| 6 | = | 45.75″ | = | 3′-9-3/4″ |
| 7 | = | 53.375″ | = | 4′-5-3/8″ |
| 8 | = | 61″ | = | 5′-1″ |
| 9 | = | 68.625″ | = | 5′-8-5/8″ |
| 10 | = | 76.25″ | = | 6′-4-1/4″ |
| 11 | = | 83.875″ | = | 6′-11-7/8″ |
| 12 | = | 91.5″ | = | 7′-7-1/2″ |
| 13 | = | 99.125″ | = | 8′-3-1/8″ |
| 14 | = | 106.75″ | = | 8′-10-3/4″ |
| 15 | = | 114.375″ | = | 9′-6-3/8″ |
As you can see, these dimensions are not as nicely defined as using the 8″ module.
Calculating out the number of blocks you need
If you have a construction calculator, you can easily enter the fractions and multiply by the number of blocks.
If you have a standard decimal calculator, you can convert the 7-5/8″ to 7.625″ and 15-5/8″ to 15.625″ and multiply the number of blocks. It is not too hard if you could remember the fraction conversion of 5/8 is 0.625.
Any other concrete block that is a different size will need to be calculated out with its own dimensions.
If you have neither type of calculator on hand, then I hope you can remember how to do the math manually because I am not going to show you how to do it here. Besides, there is a very good chance that if you are reading this article on your computer, phone, or tablet, then you already have access to a calculator.
If you don’t want to figure it out on your calculator and don’t want to do it manually, I have created a calculator just for this to make it super simple for 8″ modules. You can access it here.
If you have neither type of calculator on hand and can’t figure it out manually, here is a handy dandy chart you can use to easily determine the number of blocks when using the 8″ module system:
If you have more than one row of concrete block, multiply the number of rows by the number of blocks in the chart above to get the number you need.