Cinder Block and Concrete Block Dimensions (CMU Sizes)

Cinder block dimensions by width

US blocks are named by nominal size (width × height × length). Every standard unit is 7 5/8 in tall and 15 5/8 in long, and only the width changes. Add a 3/8 in joint and the block occupies exactly 8 × 16 in of wall face.

Weights are calculated from the gross volume, typical percent-solid values for two-core units, and concrete densities from about 100 lb/ft³ (lightweight) to 135 lb/ft³ (normal weight). Actual weights vary by plant; the producer's data sheet gives the exact figure.

8x8x16 CMU weight chart

ASTM C90 sorts loadbearing blocks into three density classes. The density is what makes one 8 × 8 × 16 block noticeably heavier than another of the same size.

Lightweight blocks are easier to lay and insulate slightly better. Normal-weight blocks are denser, block more sound and are standard for foundations. An ungrouted 8 in wall of normal-weight block weighs about 40 to 45 lb per sq ft. One hundred normal-weight blocks weigh 3,400 to 4,000 lb, more than the payload of most half-ton pickups, so check the rating on the door pillar before loading.

Special block shapes and their sizes

A wall is rarely built from stretchers alone. These shapes share the same 8 × 16 in face module so they course with the standard block.

Under ASTM C90, a block counts as "solid" when its net cross-section is at least 75% of the gross area; anything less is hollow. Since the 2014 edition, C90 also requires an average net-area compressive strength of at least 2,000 psi for loadbearing units.

Concrete block sizes in mm in other countries

Most of the world uses metric blocks with a 10 mm joint, and many are not the same size as US blocks. Check before you combine plans or products from different countries.

A metric 390 mm block is 7 mm shorter than a US 15 5/8 in block, so the two are not interchangeable in the same wall. A UK 100 mm dense block weighs roughly 19 to 20 kg, calculated from its 0.0095 m³ volume at 2,000 to 2,100 kg/m³; aerated (aircrete) blocks of the same size weigh about a third as much.

How many cinder blocks you need

Each block covers 8 × 16 in, or 0.889 sq ft, so you need 1.125 blocks per sq ft of wall face.

1. Count courses. Each course is 8 in. A 4 ft wall is 6 courses; an 8 ft basement wall is 12 courses.
2. Count blocks per course. Divide the wall length in inches by 16. A 24 ft wall is 288 ÷ 16 = 18 blocks per course.
3. Multiply. 18 × 6 = 108 blocks for a 24 × 4 ft wall. The area method gives the same: 96 sq ft × 1.125 = 108.
4. Swap in special units. Each wall end needs a corner block on every course and a half block on alternate courses; a bond beam course replaces one row of stretchers.
5. Add 5% for breakage. 108 × 1.05 = 113 blocks.

Plan lengths in multiples of 8 in and heights in multiples of 8 in, and almost nothing needs cutting. Wall heights work out cleanly: 3 courses make 2 ft, 9 courses make 6 ft and 12 courses make 8 ft.

Working concrete block together with brick and lumber

• Brick veneer over block: three courses of modular brick rise 8 in, the same as one block course, so wall ties can be placed in the joints without cutting.
• Wood on top of block: a 2 × 8 sill plate is 7 1/4 in wide, 3/8 in narrower than an 8 in block. The IRC requires preservative-treated or naturally durable wood where a sill sits on masonry. See lumber sizes for actual dimensions.
• Garden projects: an 8 × 8 × 16 block makes a quick raised-bed wall or a square fire pit. Use solid or capped blocks for fire pits and line them with firebrick; ordinary hollow blocks can crack from heat.
• Sheds and decks: solid 4 × 8 × 16 or 8 × 8 × 16 blocks are a common base for small sheds, but deck footings are governed by local code and usually need poured concrete; see deck dimensions.

Blocks are delivered on wood pallets; the count per pallet depends on the plant and the block width, so ask the supplier before you plan a delivery spot. See pallet sizes for deck size and load ratings.

Frequently asked questions