CMU Block Sizes & Dimensions: Chart, Blocks per Sq Ft, and How to Estimate

That is the size chart most people are looking for. But if you price masonry for a living, the chart is the easy part. The money is lost in what comes after it: counting specialty units as standard block, forgetting bond beams, guessing at grout, and carrying the wrong waste factor. This guide gives you the dimensions first, then walks through how our estimators turn a CMU wall into an order list you can actually bid.

What Is a CMU Block?

A CMU (concrete masonry unit) is a precast block made from portland cement, sand, aggregate and water, molded and cured in a plant. Most CMU are hollow, with two or three vertical cores that cut weight and leave room for rebar and grout. Solid units, split-face and ground-face architectural block, and specialty shapes such as bond beam, lintel and corner units are also made.

What Is a CMU Wall?

A CMU wall is a wall built from concrete blocks laid in mortar, usually in running bond with each course offset by half a block. Structural CMU walls carry vertical rebar in grouted cells and horizontal reinforcement in bond beams or bed joints. They are used for foundations, load-bearing walls, retaining walls, fire walls, and stair and elevator shafts because they are durable, fire resistant and good at blocking sound.

• Unreinforced: non-load-bearing partitions and screen walls.
• Reinforced, partially grouted: most bearing walls, with grout only in cells that hold rebar.
• Fully grouted: retaining walls, shear walls and heavily loaded walls.
• Composite or veneer backup: CMU behind brick or stone veneer.

Nominal vs. Actual CMU Dimensions

Concrete masonry units are sold by nominal size, the size the block occupies in the wall once a standard 3/8″ mortar joint is added. The actual size is 3/8″ smaller in each direction. That is why an “8-inch block” is really 7-5/8″ wide, and why a 16″ block lands exactly on a 16″ module when the joints are included.

This matters for layout and for takeoff. Wall lengths, openings and heights on most US drawings are dimensioned on the 8″ module. If you measure a 40′-0″ wall and divide by 16″, you get a clean 30 blocks per course. If you divide by 15-5/8″, you overcount and your numbers will never tie back to the drawings.

CMU Block Size Chart

Weights vary by manufacturer and aggregate. Lightweight units typically run 20 to 30% lighter. Use the supplier submittal for structural and handling calculations.

The 8″ unit is the workhorse. On most commercial and industrial jobs we take off, it accounts for the large majority of the block count, with 12″ units showing up in retaining walls, tall gym and warehouse walls, and foundation walls carrying soil load.

Common CMU Shapes You Have to Count Separately

A size chart only lists stretcher units. Real walls use several other shapes, and each one is priced and ordered on its own line:

• Corner and end units with one or two flat ends, used at corners, jambs and wall ends.
• Half-length units (8″ long) to finish courses without cutting.
• Bond beam units with a depressed or knock-out web so horizontal rebar and grout can run along a course.
• Lintel units (U-shaped) spanning over doors and windows.
• Pilaster units for columns built into the wall.
• Bullnose units with rounded corners at exposed outside corners.
• Cap units (solid tops) to finish the top of a wall.
• Architectural units: split-face, ground-face and fluted block, which cost far more than gray block and are usually specified only on exposed faces.

If your takeoff lumps all of these into one “8-inch block” count, the quantity may be right and the price will still be wrong. Split-face and ground-face units in particular can cost several times as much as a plain stretcher.

CMU vs. Cinder Block: Is There a Difference?

In everyday use, people say “cinder block” for any gray hollow block. Technically, cinder blocks used coal cinders as aggregate and are rarely made today. Modern CMU is made from portland cement, water and sand, gravel or lightweight aggregate, and is manufactured to ASTM C90 for load-bearing units. The dimensions are the same, so a “standard cinder block size” search and a “CMU size” search lead to the same 8″ x 8″ x 16″ answer.

How Many CMU Blocks per Square Foot?

One standard 8″ x 16″ face covers 128 sq in, which is 0.889 sq ft. Divide 1 by 0.889 and you get 1.125 blocks per square foot of wall. This holds for every standard width because width does not change the face area.

How to Estimate CMU Quantities: Step by Step

This is the same sequence our estimators follow on a masonry takeoff. The math is simple. Discipline in the order of operations is what keeps the numbers right.

1. Measure each wall by type. Separate walls by block width, block type (gray, split-face, ground-face) and grout requirement. A single elevation often has two or three wall types.
2. Calculate gross area. Length x height for each wall type, using nominal dimensions from the drawings.
3. Deduct openings. Subtract doors, windows, louvers and other openings. Many estimators skip deducting openings under about 2 sq ft because the cutting waste offsets the saved block.
4. Convert area to blocks. Net area x 1.125 for standard units.
5. Count specialty units separately. Bond beam courses, lintels over each opening, corners, jambs and caps come out of the stretcher count and go onto their own lines.
6. Add waste. 5% on long plain walls with an experienced crew, 7% as a general default, and 10% on walls with many openings, corners or architectural units.
7. Take off mortar, grout and reinforcement from the final block count (see the table below).
8. Check against coursing. Wall length divided by 16″ times the number of 8″ courses should land close to your area-based count. If it does not, something was missed.

If you are measuring from PDFs or paper sets, accurate wall lengths are the foundation of everything above. Our guide on how to measure linear feet from blueprints covers scale checks and the mistakes that throw off long runs of wall.

Mortar and Grout Quantities for CMU

Mortar and grout are where CMU estimates drift the most, because they depend on how the wall is built, not just how big it is. These rules of thumb are a good starting point for budgeting and checking a takeoff:

Core fill volumes from the Titan America Block Estimating Guide. Per-sq-ft figures = cu ft per block x 1.125.

• Mortar: about 3 bags of masonry cement per 100 standard blocks, with roughly 1 cu yd of sand for every 7 bags (Titan America rule of thumb). Face-shell bedding uses about 8.5 cu ft of mortar per 100 blocks; full bedding runs closer to 11 cu ft.
• Grout: fully grouted walls use the full core volume above. Partially grouted walls only fill the reinforced cells. With vertical bars at 32″ on center, that is one core in four, or about 25% of the fully grouted volume, plus bond beams, jambs and any cells the structural drawings call out.
• Grout waste: carry 10% or more. Grout runs into webs, joints and voids that never show up in a core-volume calculation.
• Joint reinforcement: ladder or truss wire is typically placed every other course (16″ on center). Count runs per wall and add about 10% for laps.

Always read the structural notes before using any of these numbers. Grout spacing, bond beam locations and whether a wall is solid-grouted are design decisions, and they change from one wall type to the next on the same job.

Worked Example: Takeoff for a 120 ft CMU Wall

Here is a simple wall worked all the way through, so you can see how the steps connect. The wall is 120 ft long and 10 ft high, built with 8″ gray CMU, with two 3′ x 7′ doors and one 4′ x 4′ window. Vertical bars are at 32″ on center with a grouted bond beam at the top course.

Two checks are worth doing on every wall like this. First, the coursing check: 120 ft is 90 blocks per course, and 10 ft is 15 courses, so the gross count is 1,350 blocks, which matches 1,200 sq ft x 1.125. Second, the specialty check: the top course of 90 units is bond beam block, not stretchers, and each opening needs lintel units and jamb units. Those come off the stretcher count and go onto their own lines before you price the wall.

CMU Takeoff Mistakes That Cost Contractors Money

• Mixing nominal and actual dimensions. Measuring with 15-5/8″ instead of the 16″ module inflates the count on every wall.
• Counting every unit as a stretcher. Bond beams, lintels, corners and caps are different products at different prices.
• Pricing architectural block as gray block. Split-face and ground-face units are often specified only on exterior faces, so one wall may need two different units.
• Guessing grout. Partially versus fully grouted walls can change grout quantities by a factor of three or four. Pull the grout schedule from the structural sheets.
• Ignoring wall height breaks. Walls that change height, step with grade or end at sloped roofs need course-by-course counts, not a single area.
• Using one waste factor for the whole job. A long plain warehouse wall and a stair core full of openings should not carry the same waste.

CMU Wall Thickness and Fire Rating

The fire resistance of a CMU wall is not determined solely by its nominal width. It depends on the unit’s equivalent thickness—the amount of solid material within the block, which increases when the cores are grouted—as well as the type of aggregate used. The IBC and TMS 216 specify the required equivalent thickness for each fire rating, and manufacturers provide the equivalent thickness values for their specific units. As a general guideline, hollow 8-inch units typically achieve ratings in the 1- to 2-hour range; this can be increased through grouting or by using different types of aggregate. Always verify the fire rating for the specific masonry unit and ensure compliance with the building codes applicable to the project.

Need a CMU Takeoff You Can Bid With?

Design Estimation prepares CMU and masonry takeoffs for general contractors and masonry subcontractors across the US, with block counts split by size and unit type, specialty units, mortar, grout and reinforcement. Turnaround is typically 24 to 48 hours. See our concrete and masonry takeoff services, or send your drawings for a quote.