Concrete Masonry Block Dimensions: Read the Size Correctly

The standard CMU people call an “8-inch block” is nominally 8 × 8 × 16 inches but is commonly manufactured to specified dimensions of 7 5/8 × 7 5/8 × 15 5/8 inches. The missing 3/8 inch allows one unit plus a typical 3/8-inch mortar joint to occupy an 8-by-16-inch module. The Concrete Masonry & Hardscapes Association identifies 8-by-16 inches as the typical nominal face size and lists nominal widths of 4, 6, 8, 10, 12, 14 and 16 inches (CMHA).

Common CMU dimensions

CMU dimensions are stated width × height × length. Width is the wall thickness—not the face height. A unit described as 8 × 4 × 16 is therefore 8 inches wide, 4 inches high and 16 inches long; it is not a 4-inch-thick block (Angelus Block).

For standard precision units with an 8-by-16-inch nominal face, the common dimensions are:

Common name	Nominal size, W × H × L	Specified size, W × H × L
4-inch CMU	4 × 8 × 16 in.	3 5/8 × 7 5/8 × 15 5/8 in.
6-inch CMU	6 × 8 × 16 in.	5 5/8 × 7 5/8 × 15 5/8 in.
8-inch CMU	8 × 8 × 16 in.	7 5/8 × 7 5/8 × 15 5/8 in.
10-inch CMU	10 × 8 × 16 in.	9 5/8 × 7 5/8 × 15 5/8 in.
12-inch CMU	12 × 8 × 16 in.	11 5/8 × 7 5/8 × 15 5/8 in.
16-inch CMU	16 × 8 × 16 in.	15 5/8 × 7 5/8 × 15 5/8 in.

These specified sizes come from a manufacturer’s precision-unit schedule (Angelus Block). A local producer may not stock every width, and 14-inch units are part of the recognized nominal range even though they do not appear in that particular schedule. Confirm the product catalog before completing takeoffs or opening layouts.

Nominal, specified and actual are different

• Nominal dimension: the modular size used for naming, coordination and layout. It includes the intended mortar-joint allowance.
• Specified dimension: the target manufacturing dimension written into the unit specification. For conventional modular CMU, it is typically 3/8 inch less than nominal.
• Actual dimension: the measurement of a particular delivered unit.

Do not automatically call 7 5/8 inches the “exact actual” dimension. ASTM C90 units may vary by up to ±1/8 inch from the specified width, height and length; textured products have stated exceptions (CMHA). ASTM C90 covers hollow and solid concrete masonry units suitable for loadbearing and nonloadbearing work, but project requirements can also call for particular finishes or performance properties (ASTM International).

That distinction matters when checking a wall against a slab edge, steel opening, veneer line or equipment clearance. Use nominal dimensions for the masonry module, specified dimensions for drawings and calculations where required, and approved product data or field measurements where physical clearance is tight.

Fast layout and quantity calculations

A nominal 8-inch course gives 1.5 courses per vertical foot. A nominal 16-inch unit gives 0.75 full units per horizontal foot. For a simple wall using full 8-by-16 modules:

Gross full-size-unit equivalents = wall area in square feet × 1.125

Example: a wall 20 feet long and 8 feet high has 160 square feet of gross area:

160 × 1.125 = 180 full-size-unit equivalents

Subtract openings by their modular area, then break the result into the actual schedule of stretchers, half blocks, corners, sash units, bond beams and lintel units. Do not treat 180 as a purchase order by itself. Bond pattern, cuts, corners, reinforcement, lintels and breakage change the mix.

Half-length and half-height units preserve the module but are separate products. One manufacturer, for example, lists an 8-inch half block at a nominal 8 × 8 × 8 inches and an 8-inch half-height unit at 8 × 4 × 16 inches, both with corresponding 7 5/8- or 3 5/8-inch specified dimensions (Jandris Block).

What to verify before ordering

1. Wall width: Use the drawings and masonry schedule; “standard block” does not establish 4-, 6-, 8-, 10- or 12-inch width.
2. Face module: Confirm whether the work uses the common 8-by-16 module or another height or length.
3. Unit configuration: Separate regular units from corners, halves, bond beams, lintels, sash units and open-end units.
4. Finish and tolerance: Precision, split-face and slump units do not share identical dimensional behavior.
5. Local availability: CMHA notes that sizes and shapes vary by region and recommends consulting local manufacturers about available or custom units (CMHA).
6. Handling plan: Order and stage by unit type, not just total count, so reinforcement and opening work do not stop while crews search mixed cubes.

Dimensions establish the module, but they are only one part of the assembly. For reinforcement, grout, bond beams, lintels, ties and moisture details, see the companion guide to a concrete blockwork wall.