Concrete Block Sizes and Dimensions

Nominal vs actual size

Concrete masonry units (CMU) are named by their nominal size, which includes one mortar joint. The actual block is 3/8 in smaller in each direction so that, once laid with 3/8 in joints, it fills a neat 8 × 16 in module. The width (the wall thickness) is also 3/8 in under its nominal size.

Whatever the width, the face is the same 8 × 16 in nominal, so the number of blocks per square foot of wall does not change. Only weight, strength and the amount of grout in the cores do.

Metric blocks

In Australia and many metric countries, the common hollow block is 390 mm long and 190 mm high, laid with a 10 mm joint to make a 400 × 200 mm module. Widths are usually 90, 140 and 190 mm (often called 100, 150 and 200 series). Sizes and names vary between manufacturers, so check the product sheet. In Canada, both metric 190 mm blocks and imperial 8 in blocks are sold, so confirm which one your supplier stocks before you plan the wall; the two do not mix in one wall.

Common block shapes

• Stretcher: the standard block with two cores, used along the length of the wall.
• Corner or end block: one flat, finished end for corners and wall ends.
• Half block: 8 in long (nominal), used to finish courses in running bond without cutting.
• Bond beam and lintel blocks: U-shaped or channel blocks that hold horizontal rebar and grout, used at the top of walls and over openings.
• Cap or solid block: solid units for the top of a wall or for bearing points.
• Split-face and decorative block: textured faces for walls that stay exposed.

Plan the wall around the 8 in module

Because every course is 8 in high and every block 16 in long, a wall laid out in multiples of 8 in needs very few cuts. Fewer cuts mean less waste, faster work and stronger walls.

Door and window openings work best when their width is a multiple of 8 in and their height lines up with a course. Many standard metal door frames are made to suit masonry openings for this reason.

Worked example

A wall 24 ft long and 6 ft high: 72 in ÷ 8 = 9 courses, and 288 in ÷ 16 = 18 blocks per course, so 162 blocks. The area method agrees: 24 × 6 = 144 sq ft × 1.125 = 162. Add 5 percent for breakage and you would order 171. Because 24 ft and 6 ft are both multiples of 8 in, the wall needs only half blocks at the ends of alternate courses. You can check any wall in the concrete block calculator.

Choosing a width

Width is a structural decision, not just a quantity one. Taller walls, walls carrying floors or roofs, basement walls and retaining walls need thicker block, reinforcement and grouted cores to resist the loads. Codes specify minimum thickness, reinforcement and footing size for these walls, so check local code, and get an engineer involved for anything that retains soil or carries a structure.

Weight and handling

Block weight depends on the size, the concrete and whether it is normal-weight or lightweight, and an 8 in block is heavy enough that lifting hundreds of them is hard work. Check the supplier's figure, plan your pallet drop near the wall, and use proper lifting technique.

Frequently asked questions