Concrete block sizes: 8×8×16 actual size and the full CMU chart
Concrete masonry units — concrete blocks, often called cinder blocks — are named by nominal size, but every one is a little smaller than its name. The difference is deliberate, and once you know it, every block layout and estimate becomes simple arithmetic.
Nominal vs actual size
CMHA (the Concrete Masonry & Hardscapes Association, formerly NCMA) specifies blocks 3/8 in smaller than their nominal size in each direction. That 3/8 in is the standard mortar joint. Lay a 7⅝ in tall block on a ⅜ in bed of mortar and the wall rises exactly 8 in per course; a 15⅝ in block plus a ⅜ in head joint takes exactly 16 in. Walls, door openings and window heights all work out on an 8 in grid.
| Nominal (W × H × L) | Actual (in) | Actual (mm) | Typical use |
|---|---|---|---|
| 4 × 8 × 16 | 3⅝ × 7⅝ × 15⅝ | 92 × 194 × 397 | Partitions, veneer backing |
| 6 × 8 × 16 | 5⅝ × 7⅝ × 15⅝ | 143 × 194 × 397 | Non-load-bearing walls, garden walls |
| 8 × 8 × 16 | 7⅝ × 7⅝ × 15⅝ | 194 × 194 × 397 | The standard block — foundations, walls |
| 10 × 8 × 16 | 9⅝ × 7⅝ × 15⅝ | 244 × 194 × 397 | Taller or heavier walls |
| 12 × 8 × 16 | 11⅝ × 7⅝ × 15⅝ | 295 × 194 × 397 | Basement and retaining walls |
| 8 × 4 × 16 (half-high) | 7⅝ × 3⅝ × 15⅝ | 194 × 92 × 397 | Making up height, caps |
| 8 × 8 × 8 (half block) | 7⅝ × 7⅝ × 7⅝ | 194 × 194 × 194 | Ends of walls, bond |
The answer most people are after is on 8×8×16 block actual size, with an 8 × 10 ft wall worked through.
Blocks per square foot
Because each block plus joints fills an 8 × 16 in rectangle — 8/9 of a square foot — any full-height block lays 1.125 per square foot, whatever its thickness. Half-high blocks lay 2.25. CMHA’s estimating guide (TEK 04-02A) gives 112.5 per 100 sq ft, or 119 with 5% for waste and breakage. Worked through on how many blocks per square foot; for a perimeter wall, measure along the centre line as in how many cinder blocks for a 10×10 shed.
Courses and heights
| Courses of 8 in block | Wall height (with joints) | Dry-stacked height |
|---|---|---|
| 1 | 8 in | 7⅝ in |
| 3 | 24 in (2 ft) | 22⅞ in |
| 6 | 48 in (4 ft) | 45¾ in |
| 9 | 72 in (6 ft) | 68⅝ in |
| 12 | 96 in (8 ft) | 91½ in |
Cores, grout and mortar
Standard hollow blocks have two cores. Filling them solid takes about 36 cu ft of grout per 100 sq ft of 8 in wall (CMHA, 3% waste included) — see how much concrete to fill 100 cinder blocks and how much grout to fill one 8 inch block. For mortar, Quikrete says an 80 lb bag of mortar mix lays up to 13 blocks; see how many bags of mortar per 100 block.
Other shapes you’ll see
- Corner and end blocks have one or both ends finished flat so the end of a wall looks clean.
- Bond beam and lintel blocks have a channel for horizontal rebar and grout, used at the top of walls and over openings.
- Solid and cap blocks finish the top of a wall or carry concentrated loads.
- Split-face and decorative blocks share the same modular sizes but have a textured face.
Availability varies by region, so ask your supplier what shapes they stock before you plan a wall around them. Block weight also varies with the mix and the manufacturer — ask for the figure if you’re planning a delivery or a carry.
Blocks for garden projects
Concrete block is popular for raised beds, benches and fire pits because it needs no cutting if you plan around 8 and 16 in. For a square fire pit laid pinwheel-style, see how many cinder blocks for a fire pit. Remember that standard block isn’t made for direct fire; line it with a steel ring or firebrick.
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Frequently asked questions
Why is an 8×8×16 block not 8 inches?
The nominal size includes a 3/8 in mortar joint. The block itself is 7⅝ × 7⅝ × 15⅝ in, so with joints it fills exactly 8 × 16 in of wall.
How many square feet does one 8×8×16 block cover?
8/9 of a square foot (8 × 16 in including joints), which is why a square foot takes 1.125 blocks.
Sources
- CMHA CMU-TEC-001-23 — Concrete masonry unit shapes, sizes, properties and specifications (opens in a new tab) — Specified dimensions 3/8 in less than nominal; nominal thicknesses 4–16 in; ±1/8 in tolerance.
- CMHA TEK 04-02A — Estimating concrete masonry materials (opens in a new tab) — 8/9 sq ft per unit; 119 per 100 sq ft with 5% waste; grout and mortar tables.
- Quikrete Mortar Mix (No. 1102) data sheet (opens in a new tab) — Up to 13 standard blocks per 80 lb bag.