Feature
CMU Sizes Explained — Nominal vs Actual Dimensions, Weights, and Which Block Goes Where
Theo Marchetti · · 2 min
Concrete block sizes are the first trap in masonry: nothing measures what it’s called. An “8-inch block” is 7 5/8 inches wide. The missing 3/8 inch is the mortar joint — the naming assumes the block plus its joint lands on a clean 8-inch module. Once you know that, every size in the yard makes sense.
Nominal vs actual
A block’s nominal size includes one standard 3/8-inch mortar joint; its actual size is what the steel mold produces. The standard full block is nominally 8×8×16 inches — actually 7 5/8 × 7 5/8 × 15 5/8. Lay it with its joint and you get exactly 8 inches of height and 16 of length, which is why masons can count courses in their head.
The standard size chart
| Nominal (W×H×L) | Actual (inches) | Common use |
|---|---|---|
| 4×8×16 | 3 5/8 × 7 5/8 × 15 5/8 | veneer backup, partitions |
| 6×8×16 | 5 5/8 × 7 5/8 × 15 5/8 | interior walls, fences |
| 8×8×16 | 7 5/8 × 7 5/8 × 15 5/8 | the default structural unit |
| 10×8×16 | 9 5/8 × 7 5/8 × 15 5/8 | wider bearing walls |
| 12×8×16 | 11 5/8 × 7 5/8 × 15 5/8 | foundations, tall walls |
Half blocks, bond beams, lintels, sash blocks, and caps all follow the same nominal logic — a “half” is nominally 8 inches long, actually 7 5/8.
Weight, and why it matters
A standard 8-inch stretcher in normal-weight concrete runs about 33-38 pounds; lightweight aggregate versions drop to 24-28. That difference is a production-rate and a back-injury question across a few hundred units, and lightweight block also carries better fire ratings per inch. Structural capacity is the trade — check the spec, not the marketing name.
Picking the unit
Partition with no load: 4- or 6-inch. Typical loadbearing or reinforced wall: 8-inch, grouted at the cores the engineer marks. Foundations and retaining work: 10- or 12-inch, where the extra core space swallows rebar and grout. When a drawing says “8 CMU,” it means the nominal module — order by nominal, measure openings by actual, and let the 3/8-inch joint do the reconciling.
The system is a quiet piece of design: every size in the catalog exists so that walls come out in whole modules with no cutting. The ancient Greeks laying isodomic courses were chasing the same thing — equal units, clean courses, no surprises at the corner.