Cinder blocks share standard dimensions with other concrete units. Some units have the same shapes, but are made from other materials for projects with different density requirements.
This guide reviews and compares the standard dimensions, shapes and uses for cinder blocks and other concrete masonry units.
Standard Dimensions for Concrete Masonry Units
Cinder blocks are a type of concrete masonry unit (CMU). Standard dimensions are set by the American Society for Testing and Materials (ASTM C90). The standard cinder block dimensions are 8 in. x 8 in. x 16 in. However, this dimension includes the length of the average mortar joint, which is 3/8 in. Standard cinder block dimensions that include mortar joint length are sometimes known as nominal dimensions. The true dimensions of the standard cinder block are 7 5/8 in. x 7 5/8 in. x 15 5/8 in.
Cinder blocks typically have two or three large hollow cells with a 1-inch divider between them. The cells are tapered on one side to aid in removal from the mold during manufacturing. The dimensions of indented ends of a cinder block are typically 1 1/4 in. and can vary in width from 1/2 in. to multiple inches.
All CMUs have the same standard dimensions, but their concrete mix may vary to suit different projects such as cinder for a flower bed or concrete for an outdoor wall. The standard types of CMU concrete mixes are as follows:
- Lightweight concrete mix weighs less than 105 lb./cu. ft. using materials such as cinder, shale or slag. Lightweight CMUs are hollow and more porous, but provide better protection against fire, rapid temperature change and sound.
- Medium weight concrete mix weighs between 105-125 lb./cu. ft. using heavier materials such as air-cooled blast furnace slag. These CMUs offer more sound resistance and are the most common weight for structural projects.
- Normal weight concrete mix contains sand and gravel, or crushed limestone that weighs more than 125 lb./cu. ft. Type of aggregate used is typically based on local availability.
Cinder blocks are considered lightweight CMUs and may be more expensive in high volumes. However, the lighter material typically results in lower freight and labor costs for a project, which can compensate for the higher material cost.
Cinder Block and CMU Size Chart
Standard CMUs can also vary in width. In addition to the most common 8 in. width, cinder blocks can also have 4 in., 6 in., 10 in. and 12 in. widths. For larger projects such as foundations and load-bearing walls, a larger width CMU more easily fills with concrete once the structure is built.
The cinder block and CMU size chart above shows the nominal and specified dimensions of each size of CMU. Half-block sizes are also included, and these units resemble a cube with a single hollow cell similar to full blocks. The CMU size is in the left column, and each number refers to the block width. Half blocks are half the length of a standard CMU, making them 8 in. long.
Height is the most consistent with 8 in. across all CMUs. However, half height blocks are available for exterior load bearing purposes. Half height blocks are often used as an opportunity to make construction more efficient because they can create an exterior masonry finish with a single wytheload-bearing design. A single wythe design uses only one layer of blocks to create load-bearing walls and structures. Load-bearing designs for standard CMUs typically use a triple wythe design, which requires more materials and labor.
Cinder Block Shape Chart
There are many types of cinder block shapes, and each serves a different purpose in the construction of masonry walls and features. The most common type of cinder block is called a stretcher unit and is recognizable by the connecting pieces on both sides. While stretcher units often make up the majority of the wall, they are best suited for starting connections with other units that serve other functions.
The most common types of cinder and concrete blocks include:
- Stretcher unit: A standard CMU laid with its length parallel to the face of the wall. It is used to build the framework of the wall while other pieces form more complex connections.
- Single corner unit: A CMU with only one pair of connectors. The other side is even for placement on the corner of a masonry wall without protrusion.
- Double corner unit: A CMU with no connectors and even corners on each side for creating piers and pillars.
- Sash unit: A CMU with an end slot to create openings that receive metal window frames and pre-molded expansion joints, which prevent separation by bracing the units prior to adding mortar joints and concrete fill.
- Lintel unit: A hollow CMU with openings that allow construction workers to build a continuous channel for reinforcing steel and grout. This is vital for distributing the load on concrete doorways and windowsills.
- Kerf unit: Also known as an all-purpose unit, this CMU has no connectors on the corners and a slit cut through the middle. These can be split in half on the job site to fill openings or corners with half blocks.
- Open-ended unit: A CMU with one or both ends open for placement around vertical reinforcement bars during construction. Reinforcements are placed before the masonry and are surrounded by concrete and mortar when complete.
- Bond beam units: A CMU made to accommodate horizontal bond beams in a masonry structure. Some are open ended, and some have “knockout” ends that can be easily broken away on the job site to fit a bond beam.
Pro Tip: An extra double corner can be used in place of a missing kerf unit, provided you will not need to split it in half.
Specialized Cinder Block Shapes
Some cinder and concrete blocks are made for specialized building purposes and can connect construction elements such as reinforcements, control joints and rounded corners to the structure. Each specialized CMU is available in standard sizes and materials and is designed for a specific wall application.
- Control Joint unit: A CMU with a protrusion that fits into the stretch or single corner unit connectors to fasten control joints. Control joints are added to prevent cracking in areas where it is likely to occur by inserting a separator.
- Bevelled unit: These CMUs for a 45-degree angle with the face of the adjacent unit and are used to form walls that intersect at a 135-degree angle. Adjacent rows overlap to form a running bond pattern along the corner.
- Bull-nosed unit: A CMU with one or two rounded edges on one side to create a softer corner in the structure.
- Corner Return unit: A CMU used to complete a corner from both sides. One side is smaller than the other to connect to the next wall block on the other side of the corner. Corner return blocks are useful for anticipated corner work, as opposed to the incidental kerf block.
- Pilaster units: A CMU made to create a space for a wall pilaster interface, which is an open section an array of larger vertical reinforcements like rebar.
- Screen units: A CMU made for energy efficiency and air circulation. Common uses include fences, partitions and openings in interior concrete masonry walls.
Pro Tip: If you are planning for significant vertical reinforcement, consider the quantity of open-ended and pilaster units necessary. Larger structures like apartment buildings may require both.
More Products. More Tools. More Perks.
Be more competitive and boost your bottom line with Pro Xtra, The Home Depot’s free loyalty program built for Pros. Sign up today to access the enhanced Pro Online Experience, built with the online business tools and time-saving features Pros need.
Save money on large orders with the Volume Pricing Program and your free Pro Xtra account. Just assemble your project list, bring it to the Pro Desk at The Home Depot and let our Pro associates get you the lowest price possible.