What is the standard size of a concrete block?

Nominal size

8 × 8 × 16 inwidth × height × length

Actual size

7-5/8 × 7-5/8 × 15-5/8 instandard stretcher unit

Difference

3/8 inthe mortar joint

Product standard

ASTM C90loadbearing concrete masonry units

Short version

• Nominal 8 × 8 × 16 inches is what the block is called; 7-5/8 × 7-5/8 × 15-5/8 is what it measures.
• The 3/8-inch gap between the two numbers is the mortar joint thickness.
• Dimensions are quoted as width × height × length, and width is the wall thickness.
• Nominal widths of 4, 6, 8, 10, and 12 inches are all standard, and all share the 8 × 16 face.
• Sizing is a manufacturing convention, not a code requirement.

What is the difference between nominal and actual block size?

Nominal size includes the mortar joint; actual size is the block by itself. A unit called 8 × 8 × 16 actually measures 7-5/8 × 7-5/8 × 15-5/8 inches, because 3/8 inch has been subtracted from each dimension to leave room for mortar.

The reason is layout arithmetic. Add the 3/8-inch joint back and every block occupies exactly 8 inches of wall height and 16 inches of wall length. A wall three courses high is exactly 24 inches, and ten blocks long is exactly 160 inches, with no accumulating fractions to track.

That is why block, brick, and masonry generally are described as modular: the trade works in whole nominal units and the joints absorb the difference. A mason quoting a wall as “12 courses” means 96 inches of height without doing any arithmetic.

Order matters in how the dimensions are written. The convention is width × height × length, so the first number is the thickness of the finished wall. An 8-inch block builds an 8-inch wall.

What are the common concrete block sizes?

Nominal widths of 4, 6, 8, 10, and 12 inches are all standard, and every one of them keeps the same 8-inch nominal height and 16-inch nominal length. Only the thickness changes, which is what keeps the modular layout consistent across wall types.

Beyond the plain stretcher unit, manufacturers produce corner blocks with one finished end, bond beam units with a channel for horizontal reinforcement, and sash, lintel, and column shapes. Availability differs by region and by plant, so confirm what your local supplier stocks before detailing a wall around a specialty unit.

ASTM C90 is the product specification for loadbearing concrete masonry units. It covers dimensional tolerances, minimum face shell and web thicknesses, and density classifications that separate lightweight, medium weight, and normal weight units.

How do block dimensions let you lay out a wall without cutting?

Design wall lengths in multiples of 8 inches and most cutting disappears. Each block plus its head joint occupies 16 nominal inches, and a half block covers 8, so any length divisible by 8 can be built from full and half units.

Heights work the same way in 8-inch increments. Eight courses is 64 inches, twelve courses is 96 inches, and a wall specified at those numbers needs no split courses.

Openings follow the same logic. Positioning door and window openings on the 8-inch grid means jambs land on unit ends rather than in the middle of a block, which saves cutting and produces a cleaner bond pattern.

Where a dimension cannot land on the grid, plan the odd unit deliberately rather than discovering it at the last course. Masons generally place the cut block away from corners and openings where it is least visible and least structurally awkward.

Estimating follows directly from the module: one block covers 8/9 of a square foot of wall face, so wall area in square feet multiplied by 1.125 gives the block count before waste.

Is the 8x8x16 dimension a code requirement?

Block dimensions are an industry manufacturing convention, not something a building code prescribes. Codes do not say a block must be 8 × 8 × 16; they address how masonry assemblies must perform, and the units themselves are governed by product standards such as ASTM C90.

Structural requirements for masonry come through the building code adopted in your jurisdiction, which references masonry design and construction standards. Wall thickness, reinforcement, grouting, and footing requirements for a specific wall depend on the loads, the soil, the seismic and wind conditions, and the code edition your area has adopted.

Because adoption varies by state and by municipality, treat any specific requirement you read online as a starting point and confirm it with your local building department. That is also true of frost depth, which governs footings under a block wall.

General information, not engineering advice. Structural work should be designed by a licensed engineer.

What is the difference between a concrete block and a cinder block?

Both terms describe the same category of product, and the correct current name is concrete masonry unit, or CMU. “Cinder block” is a historical term from the era when units were made using cinders, a byproduct of burning coal, as the aggregate.

Modern units are made with concrete aggregates and are classified by density as lightweight, medium weight, or normal weight, depending on the aggregate used. The old cinder-aggregate units are not what a supplier sells today, though the name survives in everyday speech across much of the country.

The practical consequence is that “cinder block” and “concrete block” at a building supply yard mean the same thing, and asking for either gets you a CMU. What actually distinguishes one unit from another is the density classification, the nominal size, and whether it is loadbearing under ASTM C90 or a non-loadbearing unit under a different specification.

For fastening into block walls, see how to hang pictures on concrete walls, since hollow units need different anchors than solid concrete.

Common mistakes

• Ordering by nominal size and building to actual — the 3/8-inch difference per unit compounds fast and throws a long wall out of layout.
• Designing wall lengths off the 8-inch module — every dimension that misses the grid turns into cut block and extra labor.
• Assuming a block wall is solid — most units are hollow, which changes anchoring, insulation, and grouting entirely.
• Treating dimensions as a code requirement — sizing is a manufacturing convention; structural requirements come from the adopted code and an engineer.
• Specifying a specialty unit without checking local stock — bond beam, sash, and column shapes vary by plant and region.