Concrete Masonry Units (CMU)
Masonry & Structural · 8 specs
Direct Answer
Concrete Masonry Units (CMU), commonly called cinder blocks or concrete blocks, are standardized hollow load-bearing structural blocks governed by ASTM C90. A standard nominal 8×8×16-inch block is manufactured to actual dimensions of 7-5/8 × 7-5/8 × 15-5/8 inches (194 × 194 × 397 mm), designed to incorporate standard 3/8-inch (9.5 mm) mortar joints into a precise 8-inch modular grid.
Source: ASTM C90-22 (Standard Specification for Loadbearing Concrete Masonry Units) · Last reviewed Aug 28, 2026
Specs
| Governing Standard | ASTM C90 / NCMA TEK / CSA A165.1 |
| Standard 8" Block (Nominal) | 8 × 8 × 16 inches (203 × 203 × 406 mm) |
| Standard 8" Block (Actual) | 7-5/8 × 7-5/8 × 15-5/8 inches (194 × 194 × 397 mm) |
| Standard Mortar Joint Thickness | 3/8 inch (9.5 mm) bed and head joints |
| 4" Stretcher Block (Actual) | 3-5/8 × 7-5/8 × 15-5/8 inches (92 × 194 × 397 mm) |
| 6" Stretcher Block (Actual) | 5-5/8 × 7-5/8 × 15-5/8 inches (143 × 194 × 397 mm) |
| 12" Stretcher Block (Actual) | 11-5/8 × 7-5/8 × 15-5/8 inches (295 × 194 × 397 mm) |
| Compressive Strength Minimum | ASTM C90 requires a minimum average net-area compressive strength around 1,900 PSI (13.1 MPa); most manufactured units test well above this, commonly in the 2,000–2,800 PSI range |
The 3/8-Inch Mortar Joint and 8-Inch Modular Grid
In architectural drafting and masonry construction, all dimensions are planned on an 8-inch (203 mm) modular grid. Because every block is manufactured exactly 3/8 inch (9.5 mm) smaller than its nominal name in all three axes (7-5/8" actual), adding the standard 3/8-inch mortar joint results in a finished block-plus-joint dimension of exactly 8.0 inches high and 16.0 inches long (7-5/8 + 3/8 = 8"). This eliminates block cutting when framing window and door rough openings.
Hollow Core Geometry, Rebar, and Grouting
Standard two-core CMU blocks feature tapered interior cell walls with thicker face shells at the top to provide a larger mortar bed. Vertical steel reinforcing bars (rebar, typically #4 or #5) are placed inside aligned cores and filled with fluid pea-gravel concrete (grout, ≥ 2,000 PSI) to create composite reinforced masonry walls capable of resisting extreme hurricane wind and seismic shear forces.