What Is a CMU Block? Concrete Masonry Units Explained

That definition covers the block in a garden wall and the block in a four-storey building. What separates them is not the name but the specification: which ASTM standard the unit was made to, what density class it falls in, and what shape it was cast in. This page covers all three. For sizes and dimensions, see the cinder block dimensions reference; to work out how many you need, use the cinder block calculator.

Is a cinder block the same as a concrete block?

In everyday use, yes. In a specification, no.

"Cinder block" is a holdover from an era when coal cinders were a common aggregate. Those units were lighter and weaker than what is produced today. Modern units use sand, gravel, crushed stone, or a lightweight aggregate such as expanded slag, expanded clay or pumice. You will still hear "cinder block," "cement block," "concrete block" and "CMU" used for the same product on the same job site.

The distinction that actually matters is the one printed on the submittal: the ASTM standard and the density class. A unit is not stronger because someone calls it a concrete block rather than a cinder block. It is stronger because it was manufactured to meet a higher minimum.

So when you are buying, ignore the nickname and ask two questions: which ASTM specification, and which density class.

ASTM C90 and ASTM C129: loadbearing versus nonloadbearing

Two specifications cover the great majority of hollow units, and the gap between them is enormous.

ASTM C90, Loadbearing Concrete Masonry Units is the standard you will encounter most often. It requires a minimum net-area compressive strength of 2,000 psi. C90 units can be used in both loadbearing and nonloadbearing applications, and may be manufactured hollow or solid in cross-section.

ASTM C129, Nonloadbearing Concrete Masonry Units covers units intended for nonloadbearing partitions only. The minimum net-area compressive strength is 500 psi — one quarter of the C90 requirement. C129 units must be clearly marked to prevent their use as loadbearing units, and they are not suitable for exterior walls exposed to freezing unless effectively protected from weather.

A note on a figure you will see published elsewhere: 1,900 psi is frequently quoted as the C90 minimum. That value is superseded. The current requirement is 2,000 psi.

Other specifications you may encounter on a submittal:

Units complying with C55, C1634 and C1877 must have a solid cross-section, defined as a net area of at least 75 percent of the gross area in every plane parallel to the bearing surface.

Because the testing procedures differ between the C90/C1877 group and the C55/C1634 group, the compressive strength figures above are not directly comparable across those groups.

Density classes: lightweight, medium weight, normal weight

ASTM standards define three density classes by oven-dry density of the concrete:

Lightweight units are permitted higher absorption because lightweight aggregates are more porous than dense, nonporous aggregates at the same compaction.

Density is not a minor detail. It drives unit weight, thermal performance, sound absorption and fire resistance, and the trade-offs run in opposite directions. Lightweight walls absorb sound better and carry higher R-values. Normal weight walls carry more mass, which matters for sound transmission and for wind resistance. There can be a structural argument for lightweight units in high-seismic regions and normal weight units in areas prone to hurricanes and high winds.

All three classes must meet the same minimum compressive strength. Density and strength are separate properties, and a heavier block is not automatically a stronger one.

If a project needs a particular density class, it has to be named in the project documents. It is not something you can assume from the supplier.

Maximum drying shrinkage for C90 units is 0.065 percent.

Hollow and solid units

The word "solid" in masonry does not mean what it means in plain English.

A unit qualifies as solid when its net cross-sectional area is at least 75 percent of its gross area in every plane parallel to the bearing surface. A unit with small voids can still be classified as solid. A unit below that threshold is hollow.

Most standard 8-inch block is hollow, which is what makes grouting possible. If you are filling cores, see the block fill calculator — the cell volume depends on the unit's width and web configuration.

Common CMU shapes and where each is used

The stretcher unit with two cells and three webs is the one most people picture, but a wall of any complexity needs several shapes.

Stretcher — the standard unit, with flanged ends that receive mortar at the head joint. The default block in a running-bond wall.

Corner or single-bullnose unit — one flat, finished end for exposed corners and the ends of walls.

Half block — half the length of a stretcher, used to maintain running bond at openings and wall ends without cutting.

Open-end or A-shaped unit — one end removed so the unit can be placed around vertical reinforcement already in position, rather than threaded down over it.

H-block or double-open-end unit — both ends open, for the same reason, giving continuous grout across the head joint.

Bond beam unit — a channel-shaped or knockout-web unit that lets horizontal reinforcement and grout run continuously along a course. Bond beams are fully grouted regardless of the wall's vertical grout spacing.

Lintel unit — a U-shaped channel carrying reinforcement over an opening.

Solid top or flat unit — a flat bearing surface for the top course or for supporting a slab.

Pilaster and column units — shapes that build a thickened section within or alongside a wall.

Screen or decorative block — patterned units for non-structural screening.

Slump block — a unit produced with a wetter mix so it bulges as it leaves the mold, giving an irregular, adobe-like face. Used for appearance rather than performance.

Split-face and ground-face units — architectural finishes produced by splitting or grinding the unit face.

Shape availability and exact geometry vary by producer. Specialty units are not standardized across manufacturers the way stretcher dimensions are, so confirm shapes against a local supplier's shape chart before detailing.

The two-cell, three-web unit is no longer the only option

ASTM C90 was revised in 2011 to accommodate unit configurations other than the conventional two-cell, three-web block. Before that revision the standard effectively assumed that geometry.

The change allows producers to optimize units for thermal performance and for material use. Less material per unit means lower resource demand, less manufacturing energy and less fuel to transport units to site.

The practical consequence for estimating: you cannot assume every 8-inch block on the market has the same cell volume. Two units from different producers can both comply with C90 and still hold different amounts of grout. When the quantity matters, check the producer's data.

How to specify a CMU

A complete specification names four things:

1. The ASTM standard — C90 for loadbearing, C129 only where a nonloadbearing partition is genuinely intended
2. Nominal size — width, height and length, with actual dimensions understood to be 3/8 inch less in each direction
3. Density class — lightweight, medium weight or normal weight, stated in the structural documents rather than in architectural sections, to avoid conflicting requirements
4. Shape — stretcher, bond beam, lintel, open-end and so on, referenced to a producer's shape number for anything beyond a standard stretcher

Anything left unstated becomes the supplier's choice, and the supplier will choose what is on the yard.

Sources

• CMHA CMU-TEC-001-23, Concrete Masonry Unit Shapes, Sizes, Properties, and Specifications — ASTM specification comparison, density classes, absorption limits, solid-unit definition, 2011 C90 revision
• CMHA TEK 1-1F, ASTM Specifications for Concrete Masonry Units — ASTM C129 nonloadbearing requirements