Building materials · Reference
A nominal 8×8×16 in concrete block (CMU) actually measures 7 5/8 × 7 5/8 × 15 5/8 in (194 × 194 × 397 mm) — 3/8 in (10 mm) smaller each way so a standard mortar joint fills the 8×16 in module.
CMU is specified by nominal width × height × length; the actual unit is 3/8 in (10 mm) smaller in each direction to allow for the mortar joint.
| Nominal size (W × H × L) | Actual / specified size | Metric equivalent |
|---|---|---|
| 4 × 8 × 16 in | 3 5/8 × 7 5/8 × 15 5/8 in | 92 × 194 × 397 mm |
| 6 × 8 × 16 in | 5 5/8 × 7 5/8 × 15 5/8 in | 143 × 194 × 397 mm |
| 8 × 8 × 16 inMost common CMU | 7 5/8 × 7 5/8 × 15 5/8 in | 194 × 194 × 397 mm |
| 10 × 8 × 16 in | 9 5/8 × 7 5/8 × 15 5/8 in | 244 × 194 × 397 mm |
| 12 × 8 × 16 in | 11 5/8 × 7 5/8 × 15 5/8 in | 295 × 194 × 397 mm |
| Half block (8 × 8 × 8 in) | 7 5/8 × 7 5/8 × 7 5/8 in | 194 × 194 × 194 mm |
Source: NCMA TEK 02-01B — Typical Sizes and Shapes of Concrete Masonry Units (2020)
| Nominal width | Weight each | Typical use |
|---|---|---|
| 4 in | 24 lb (11 kg) | Partitions, veneer |
| 6 in | 25 lb (11 kg) | Partitions |
| 8 in | 38 lb (17 kg) | Most common structural unit |
| 10 in | 43 lb (20 kg) | Tall / retaining walls |
| 12 in | 52 lb (24 kg) | Foundations, tall walls |
Source: Civil Sir — How much do cinder blocks weigh (4, 6, 8, 10 & 12 in)
| Density class | Oven-dry density of concrete |
|---|---|
| Lightweight | < 105 lb/ft³ (< 1,680 kg/m³) |
| Medium weight | 105–125 lb/ft³ (1,680–2,000 kg/m³) |
| Normal weight | ≥ 125 lb/ft³ (≥ 2,000 kg/m³) |
Source: CMHA (NCMA) CMU-TEC-001 — CMU Shapes, Sizes, Properties & Specifications (2023)
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A nominal 8 × 8 × 16 in CMU is actually 7 5/8 × 7 5/8 × 15 5/8 in (194 × 194 × 397 mm). The missing 3/8 in (10 mm) on each dimension is taken up by the mortar joint, so laid units return to the 8 × 16 in module.
Blocks are cast 3/8 in (10 mm) undersized so that the unit plus a standard 3/8 in mortar joint equals the nominal modular dimension. ASTM C90 also allows a ±1/8 in manufacturing tolerance.
Standard nominal widths are 4, 6, 8, 10 and 12 in, all 8 in high and 16 in long. Half-length (8 in) and half-height (4 in) units and a metric 190 × 190 × 390 mm block are also common.
A standard 8 × 8 × 16 in hollow block weighs about 38 lb (17 kg). A 4 in block is ~24 lb and a 12 in block ~52 lb; lightweight-aggregate units weigh less.
Nominal is the planning module (e.g. 8 in); actual/specified is what you measure (7 5/8 in). Actual = nominal − 3/8 in mortar joint.
Every figure on this page is traced to a named source, linked under each table. Standard sizes come from published standards and established industry references; where a size is a typical range or varies by manufacturer, we say so. Sizes vary by region, model and revision — treat these as the standard reference, and confirm the exact spec of the item you are selling.
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