Concrete Block Sizes and Dimensions

Standard CMU block sizes and actual dimensions

Concrete masonry units, CMU, are described by nominal size, written width x height x length. The industry body's specification guide, CMHA CMU-TEC-001-23, states that "Typically, concrete masonry units have nominal face dimensions of 8 in. (203 mm) high by 16 in. (406 mm) long," that "Nominal thicknesses include 4, 6, 8, 10, 12, 14, and 16 in.," and that "Specified dimensions of concrete masonry units are typically 3/8 in. (9.5 mm) less than nominal dimensions."

Actual sizes are the nominal less 3/8 inch, and producers list the same figures: Basalite's cut sheet gives the 8 x 8 x 16 as 7-5/8 x 15-5/8 x 7-5/8 inches, 194 x 397 x 194 mm. CMHA's guide to specialty units notes that "Specialty concrete masonry unit lengths include 18-in. and 24-in." and that "specialty widths may include 14 and 16 in." Half-high and 12 inch high units are specialties too. Ask your yard what it actually stocks; most stock the 4 to 12 inch widths and order the rest.

One more number worth knowing: ASTM C90 allows a block to vary from its specified dimensions by up to 1/8 inch, with exceptions for split and slumped faces, according to a block producer's summary of the standard. That tolerance is why a course of blocks never lines up perfectly and why the mortar joint is adjustable.

Why a block is 7-5/8 inches and not 8

Because the joint is part of the module. CMU-TEC-001 says the 3/8 inch reduction is there "so that a 4 or 8 in. module is maintained with 3/8 in. mortar joints." CMHA's glossary defines the nominal dimension the same way: "The specified dimension plus an allowance for mortar joints, typically ⅜ in." One block plus one joint is 8 inches high and 16 inches long, so walls, openings and corners all fall on an 8 inch grid.

CMHA's guide to modular layout puts it as "Standard concrete masonry modules are typically 8 in. (203 mm) vertically and horizontally," and adds that "Modular window heights are any multiple of 8 in." Plan wall lengths, heights and openings in multiples of 8 inches and you never cut a block except for something you meant to.

Course heights and blocks per square foot

The module makes the layout arithmetic simple. This is our arithmetic from the 8 inch module, not a published table:

• Blocks per square foot: one 8 x 16 face covers 8 x 16 = 128 square inches including its joints, so it takes 1.125 blocks per square foot of wall, or 112.5 per 100 square feet.
• Blocks per course: wall length in inches divided by 16.
• Half-high units fill a 4 inch difference when a wall height is not a multiple of 8.

The concrete block calculator does this for a whole wall, with openings, waste, mortar and grout.

Concrete block shapes and what each is for

Every width comes in a family of shapes. The definitions below are from CMHA's masonry glossary and CMU-TEC-001, with the stretcher and corner names from the Canadian industry's metric technical manual.

"Solid" and "hollow" also have precise meanings. CMHA defines a solid unit as one "whose net cross-sectional area in every plane parallel to the bearing surface is 75 percent or more" of the gross area; anything less is hollow. So a "solid" block can still have small cores, and its weight is set by how much concrete is in it, which the concrete block weight page works through size by size.

Metric block sizes: Canada, the UK and Europe

Canada builds to a metric module. The Canadian Concrete Masonry Producers Association's technical manual gives the standard 200 mm unit as 190 x 190 x 390 mm, and says "The actual dimensions of width, height and length are 10 mm less than the nominal dimensions." A 1993 federal metrication newsletter, hosted by the US Department of Veterans Affairs, made the comparison: "Metric modular block is 190 by 190 by 390 mm. This equates to 7-1/2 by 7-1/2 by 15-3/8 inches," while the US inch block is 194 by 194 by 397 mm, "quite similar to metric block." Close, but not interchangeable in one wall. A metric unit covers 0.08 square meters with its joint, so 12.5 blocks per square meter.

The UK uses a 440 x 215 mm face, which with a 10 mm joint makes a 450 x 225 mm module: two standard bricks long and three brick courses high. The width is the variable: 100 mm is the common size, with 140, 190 and 215 mm for heavier walls. UK block is sold by type rather than ASTM density class: dense, medium dense and aircrete (autoclaved aerated concrete). A 100 mm dense block weighs about 19 kg and an aircrete one 6 to 8 kg, on Tarmac's and Forterra's published figures.

AAC block in the US is its own size system. The International Masonry Institute's brief on AAC units gives nominal faces of 8 inches high by 24 long, "with nominal thickness of 2”, to 16”," actually 7-7/8 x 23-7/8 inches to allow for a thin-bed joint of 1/16 to 1/8 inch rather than 3/8.

Which block size to use

The width of block in a structural wall is set by the design, and for a house the building code and your engineer decide it. The code floor is IRC R606.4.1: masonry bearing walls more than one story high must be at least 8 inches thick, and 6 inches is allowed only for a one-story dwelling or garage no more than 9 feet tall. For the jobs most people are pricing:

• Garden and planter walls a few courses high: 6 or 8 inch block, capped. Use 8 inch if the wall retains any soil.
• Foundation and basement walls: 8 inch at minimum, 10 or 12 where the wall is tall or holds back more soil. Size these to the code tables or an engineer.
• Veneer and non-loadbearing partitions: 4 inch.
• Openings: plan them on the 8 inch grid, and buy lintel or bond beam units for the course above, plus sash blocks if the frame needs a slot.

Once you know the size, the block calculator counts the units, mortar and grout, and the weight guide tells you what a pallet of them weighs before it arrives.