What is the actual size of an 8×8×16 block?
An 8×8×16 concrete block actually measures 7-5/8 × 7-5/8 × 15-5/8 in (about 194 × 194 × 397 mm). The name is the size with a ⅜ inch mortar joint added, so each block fills 8 × 16 inches of wall: 1.125 per sq ft. An 8 ft by 10 ft wall takes 95 blocks with 5% extra.
How to work it out
- 12 courses of 8 in (block + ⅜ in joint) · 10 ft ÷ 16 in = 7.5 blocks per course
- 80 sq ft ÷ 0.889 sq ft per block face + 5% = 95 blocks
- Mortar: 90 blocks ÷ 13 per 80 lb bag + 15% (BIA suggests 15–25%) = 8 bags
Like a 2×4, a concrete block is named for a size it never quite is. The difference is deliberate: it leaves room for a ⅜ inch mortar joint so walls lay out on a neat 8 inch grid.
Adjust for your project
More options: Openings · Extra
- Courses (rows)
- 12 courses
- Blocks per course (to the next half)
- 7.5
- Blocks per square foot
- 1.125 per sq ft
- Wall area (net of openings)
- 80 sq ft
- Mortar mix, 80 lb bags (incl. 15% extra)
- 8 bags
- Actual block size
- 7-5/8 × 7-5/8 × 15-5/8 in
- Count corner, end and bond-beam units separately if your wall needs them; ask the supplier which shapes they stock.
Blocks for an 8 ft high wall by length (5% extra)
| Wall length | Blocks to buy | Blocks per course (to the next half) | Mortar mix, 80 lb bags (incl. 15% extra) |
|---|---|---|---|
| 4 ft | 38 blocks | 3 | 4 bags |
| 8 ft | 76 blocks | 6 | 7 bags |
| 10 ft | 95 blocks | 7.5 | 8 bags |
| 16 ft | 152 blocks | 12 | 13 bags |
| 20 ft | 189 blocks | 15 | 16 bags |
Why the 3/8 inch matters
CMHA (the industry group formerly called NCMA) specifies concrete block 3/8 in smaller than nominal in every direction, with a tolerance of ±1/8 in. Lay the block with a 3/8 in joint and the wall grows in exact 8 in steps up and 16 in steps along. That makes layout simple: a wall 10 ft long is 7½ blocks per course, and 12 courses build 8 ft.
It also means dry-stacked blocks (no mortar) don’t follow the 8-inch grid: without joints, 12 courses stand 91½ in tall, not 96, and a course of 7½ blocks spans 117 in instead of 120. Garden walls and fire pits built dry end up smaller than the nominal math suggests.
Other dimensions to know
The thickness is the first number: an 8×8×16 is 7⅝ in thick, a 6×8×16 is 5⅝ in and a 12×8×16 is 11⅝ in, but all have the same 8 × 16 face, so the count per square foot doesn’t change with thickness. A standard 8 in block has two cores. For the count over a whole wall, see how many blocks per square foot; for filling those cores, how much concrete to fill 100 cinder blocks.
Different size? Use the full Concrete Block Calculator.
What you’ll need
- Mason’s line and line blocksBudget price
Bright braided line stretched between line blocks keeps each course of block or brick straight and level, and marks out slab edges.
See options on Amazon (opens in a new tab) - 4 ft levelMid-range price
Checks forms for level and slope, and plumbs block, brick and posts. A 4 ft level bridges small bumps that fool a short one.
See options on Amazon (opens in a new tab) - Brick trowelBudget price
Spreads and furrows the mortar bed and butters the ends of each block or brick. A 10–11 in trowel suits most hands.
See options on Amazon (opens in a new tab)
As an Amazon Associate we earn from qualifying purchases. Links to products are affiliate links — they cost you nothing extra and help keep these calculators free.
Frequently asked questions
What is the actual size of a 4×8×16 block?
3-5/8 × 7-5/8 × 15-5/8 in — the same face as an 8 inch block, just 3⅝ inches thick.
What is the actual size of a 12×8×16 block?
11-5/8 × 7-5/8 × 15-5/8 in, about 295 × 194 × 397 mm.
What is the actual size of an 8×4×16 half-high block?
7-5/8 × 3-5/8 × 15-5/8 in: 2.25 per sq ft, twice as many as a full-height block.
Other common sizes
- How many blocks per square foot — 1.125 blocks per sq ft (119 per 100 sq ft with 5% extra)
- How many cinder blocks for a 10×10 shed — 89 blocks for a 3-course wall (28 per course)
Sources
- CMHA (formerly NCMA) CMU-TEC-001-23 — Concrete masonry unit shapes, sizes, properties and specifications (opens in a new tab) — Specified dimensions are 3/8 in less than nominal, so an 8×8×16 block is 7-5/8 × 7-5/8 × 15-5/8 in; nominal thicknesses 4–16 in; ±1/8 in tolerance.
- CMHA (formerly NCMA) TEK 04-02A — Estimating concrete masonry materials (opens in a new tab) — Each 8×16 unit covers 8/9 sq ft, so 112.5 units per 100 sq ft (119 with 5% waste); grout per 100 sq ft by unit width and core spacing (Table 3); cores filled per 80 lb grout bag (Table 4); one 80 lb bag of preblended mortar lays about 16 CMU with face-shell bedding (Table 2).
- Quikrete Mortar Mix (No. 1102) data sheet (opens in a new tab) — Each 80 lb bag lays up to 37 standard bricks or 13 standard 8×8×16 blocks.