CMU Blocks and Concrete Block Walls in Arizona: Sizes, Weights, Types, and How a Wall Is Actually Built
Almost every wall, fence, planter, BBQ island, and building envelope in the Phoenix valley is built out of the same thing: a concrete masonry unit, or CMU. Most people call it cinder block or concrete block. It looks simple from the sidewalk, but the dimensions, weight, reinforcement, footing, grout, and Arizona conditions all matter.
Almost every wall, fence, planter, BBQ island, and building envelope in the Phoenix valley is built out of the same thing: a concrete masonry unit, or CMU. Most people call it cinder block or concrete block.
This guide covers what a concrete masonry unit actually is, the standard CMU block sizes and weights you will encounter, the specifications that govern them, the unit shapes that do specific jobs, how a concrete masonry wall is assembled from footing to cap, and the Arizona conditions that decide whether it stands straight for decades or cracks in five years.
Key Takeaways
- CMU stands for concrete masonry unit. True cinder block used coal cinders as aggregate and is largely obsolete, though the name stuck.
- Concrete block is sold by nominal dimensions. A nominal 8 by 8 by 16 unit actually measures 7 5/8 inches in each direction, so the standard three eighths inch mortar joint brings it back to the module.
- ASTM C90 is the specification behind load bearing concrete block masonry units, covering face shell thickness, compressive strength, and weight classification.
- Weight varies with aggregate, not just size. A standard 8 inch normal weight unit is heavy enough that unit counts drive labor as much as wall length does.
- The units you can see are the smallest part of the wall. Footing, vertical rebar, grouted cells, and a bond beam are what actually carry load and wind.
- Arizona soil and heat set the rules: caliche in the footing trench, expansive clay under it, and grout that loses water fast on a July afternoon.
- Control joint spacing and a proper cap are the two details most often skipped, and they show up later as cracks and water damage.
What a concrete masonry unit actually is
A concrete masonry unit is a precast block made from portland cement, aggregate, and water, cured under controlled conditions and manufactured to a specification. Load bearing concrete block masonry units are governed by ASTM C90, which sets minimum face shell and web thickness, compressive strength, and moisture content limits. ASTM C90 is the reason a unit from one Arizona yard behaves like a unit from another, and it is the document a plan is referring to when it simply calls for CMU.
The hollow cells are not there to save material. They are the path for vertical steel and grout, which is what turns a stack of individual units into a single reinforced wall. Empty cells and grouted cells look identical once the wall is painted, and they do not perform the same way in a monsoon wind event.
CMU vs. cinder block vs. concrete block: the terminology
The three terms get used interchangeably, and in conversation that is fine. Technically they differ. A true cinder block used coal cinders as aggregate, a byproduct of coal fired power generation. Those units were lighter, more porous, weaker in compression, and no Arizona supplier delivers them today.
What arrives on the pallet is a concrete masonry unit made with portland cement and sand, gravel, or an engineered lightweight aggregate such as expanded shale or pumice. When a client asks us for a cinder block wall, we build a CMU wall. The word is a leftover, not a product choice.
You will also hear the same product called concrete block, CMU concrete block, CMU masonry block, block concrete masonry, and concrete block masonry units depending on whether you are reading a plan set, a supplier price sheet, or a building code section. They all describe the same units. We use concrete masonry unit and CMU block through the rest of this guide.
Nominal vs. actual dimensions: the three eighths inch rule
This is the most common point of confusion, and it matters when you are estimating a wall or laying out an opening. Concrete block is named by its nominal dimensions, which include the mortar joint. The unit itself is manufactured three eighths of an inch smaller in each direction.
So the standard 8 by 8 by 16 unit measures 7 5/8 inches wide, 7 5/8 inches tall, and 15 5/8 inches long. Add a three eighths inch mortar joint on top and at one end and the module comes back to a clean 8 by 16. That is why block dimensions work out to whole numbers on a plan and never on a tape measure.
The practical version: three courses of standard units plus joints equals two feet of height. Three units laid end to end plus joints equals four feet of length. A six foot wall is nine courses. Those two facts let you check a layout, a unit count, or a bid in your head before anyone shows up.
Standard CMU block sizes and weights
Width is the dimension people quote, and it refers to the thickness of the wall. Six, eight, and twelve inch are the common widths in residential and light commercial work here, with four inch used for veneer backing and low planters. The standard CMU block sizes below are the units we handle most often. Weights are approximate and vary by manufacturer and aggregate.
| Nominal Dimensions | Actual Dimensions | Approximate Weight | Typical Use |
|---|---|---|---|
| 4 x 8 x 16 | 3 5/8 x 7 5/8 x 15 5/8 | Roughly 22 to 28 pounds | Planters, low garden walls, backing for veneer |
| 6 x 8 x 16 | 5 5/8 x 7 5/8 x 15 5/8 | Roughly 28 to 35 pounds | Short perimeter walls, screen walls, equipment enclosures |
| 8 x 8 x 16 | 7 5/8 x 7 5/8 x 15 5/8 | Roughly 33 to 43 pounds | The workhorse. Most perimeter and property line walls |
| 12 x 8 x 16 | 11 5/8 x 7 5/8 x 15 5/8 | Roughly 50 to 60 pounds | Tall walls, retaining walls, load bearing commercial |
| 8 x 4 x 16 (half height) | 7 5/8 x 3 5/8 x 15 5/8 | Roughly 18 to 22 pounds | Course adjustment, seat walls, detail work |
| Cap unit, 8 inch | Varies by profile | Roughly 20 to 30 pounds | Finishing the top course and shedding water |
What makes a size standard
A size is standard because the units are manufactured to the modular dimensions the specification and the trade are built around, in the weight classifications ASTM C90 defines. Ask a yard for standard CMU block sizes and you will be shown this list. Ask for something outside it and you are into special order, longer lead time, and a higher unit price. On a typical Arizona perimeter wall there is rarely a reason to leave the standard sizes.
Why the weights matter
Two units of the same dimensions can differ by ten pounds or more depending on aggregate, which changes how many units a mason lays in a day. Lightweight units are easier to handle and cut. Normal weight units carry more compressive strength, which is why they are typical for tall or load bearing concrete masonry construction. ASTM C90 sorts units into lightweight, medium weight, and normal weight by oven dry density, and that classification is what a specification is pointing at when it calls out a weight.
The specifications behind concrete masonry
Concrete masonry construction in Arizona is not improvised. Three documents sit behind almost everything a mason does, and any contractor building masonry walls should be able to name them.
ASTM C90: the units
ASTM C90 is the standard specification for load bearing concrete masonry units. It sets minimum face shell and web thickness by unit width, minimum compressive strength, maximum moisture content, and the lightweight, medium weight, and normal weight classifications. When a plan calls for CMU with no further description, ASTM C90 is the assumed baseline.
ASTM C270: the mortar
ASTM C270 specifies mortar for unit masonry, including the mortar types and their proportions of portland cement, lime, and sand. Mortar type is a specification decision, not a preference, because the wrong type in an exterior structural wall is a defect.
The Masonry Society: the assembly
The Masonry Society publishes TMS 402 and TMS 602, the building code requirements and specification for masonry structures that the model building codes adopt by reference. Reinforcement spacing, grouting requirements, lap lengths, and inspection all trace back to these documents. The National Concrete Masonry Association publishes the technical notes most masons work from day to day.
Block types and shapes
| Unit Type | What It Does | Where It Goes |
|---|---|---|
| Stretcher | Standard unit with two notched ends | The body of a running bond wall |
| Corner or single end | One finished flat end | Corners and wall terminations |
| Half block | Full height at half length | Keeping the bond pattern correct at corners and openings |
| Bond beam | Low or knocked down web | Continuous horizontal steel in a grouted course |
| Lintel or U block | Channel shaped | Carrying steel and grout across a gate or window opening |
| Open end or A block | Open on one end | Placed around standing vertical rebar |
| Cap unit | Solid finishing course | Top of the wall, shedding water out of the cells |
Face finishes on the units themselves
Precision or standard face: the smooth gray face most perimeter masonry walls start as, ready for paint or stucco.
Split face: the face is mechanically split after curing for a rough stone texture, often left exposed.
Slump block: a softer, adobe like unit with irregular bulging faces, a classic Arizona look on older custom homes.
Ground or burnished face: the face is ground to expose the aggregate for a polished, terrazzo like surface.
Scored and ribbed: units cast with vertical grooves or ribs that read as a smaller module or a linear texture.
Screen or decorative: patterned units used for privacy panels and courtyard walls that still pass light and air.
Architectural CMU block
Architectural CMU block is the category name for units selected for how they look, not only for what they carry. Split face, ground face, scored, ribbed, and integrally colored units all fall under it. Structurally they are still concrete block masonry units meeting the same specification. The difference is the face, the color consistency, and the price.
Architects specify architectural CMU block on commercial frontages, screen walls, and custom residential work because it delivers finished masonry in a single operation, with no stucco or veneer layer to fail later. In Arizona the practical considerations are integral color holding up better than surface paint under constant UV, a rougher face collecting more dust and requiring occasional washing, and sealing being more important on exposed units because there is no coating protecting the face.
architectural units show variation between production runs more than precision block does. On any exposed wall we pull from multiple pallets as we lay, which blends the color across the face instead of banding it. It costs nothing at the time and is impossible to fix afterward.
How a concrete masonry wall is built, step by step
The short version of a properly built residential block wall. Every step in it is something a bid can quietly leave out.
The line is set, the trench is dug to the depth and width the wall and the soil require, and the bottom is cleaned to undisturbed bearing.
Reinforced concrete is poured in the trench with vertical dowels set at the spacing the wall calls for, so the steel in the wall is tied to the footing from the start.
Units are laid in a full mortar bed on the cured footing and checked for level and line. Everything above inherits whatever this course does.
Successive courses are laid with joints offset by half a unit, which distributes load across the wall instead of creating a continuous vertical seam.
Rebar rises from the footing dowels through the cells at the required spacing and is lapped where lengths join.
Grout is placed in the reinforced cells in controlled lifts and consolidated so no voids are left around the steel.
A horizontal grouted course with continuous steel, typically at the top and over openings, ties the wall together along its length.
Vertical joints are placed at planned intervals so the wall cracks where it was designed to move rather than randomly.
The top course is capped, joints are tooled, and the face is cleaned before mortar sets on it.
the first course takes the longest and it should. A quarter inch out of level at the footing becomes a visible wave nine courses up, and no amount of care in the middle of the wall corrects it. When a crew has the first course laid before mid morning on a long wall, they did not check it enough.
Grout, rebar, and what a cheap wall leaves out
Two walls can be quoted for the same height and length and be built very differently. The difference is inside the cells. A fully grouted wall has every cell filled. A partially grouted wall has grout only in the cells that contain steel. Both are legitimate approaches when they are engineered that way. An ungrouted wall with no vertical steel is a stack of units held together by mortar, and it is the thing you get when the bid comes in unusually low.
Mortar and grout are also different materials. Mortar is the bedding between units, a mix of portland cement, lime, and sand specified by type under ASTM C270. Grout is a fluid concrete mix placed into cells to bond with the rebar. Using one in place of the other is a defect. Reinforcement requirements come from the TMS documents the building code adopts.
when we are called to look at a failing wall, the first thing we do is tap the face and listen. A grouted cell answers dull and solid. An empty cell rings hollow. On more than a few leaning walls in this valley, the tap test finds hollow cells all the way along a wall that was sold as reinforced.
Arizona conditions that change the build
A block wall in the Phoenix valley is not a national wall. The soil, the heat, and the monsoon each add a requirement.
This cemented layer can stop a footing trench cold and often needs specialized equipment. A crew that hits caliche and simply shallows the trench has moved the problem into the future.
Soils that swell and shrink with moisture lift and drop the footing seasonally, which is a common source of stair step cracking through mortar joints.
On a summer afternoon, units and grout lose water to evaporation quickly. Units may need to be handled differently, grout lifts timed, and the wall protected so the mix cures rather than dries.
A tall, unreinforced wall is a sail. Wind loading is the reason height, steel spacing, and pilaster placement are engineered rather than guessed.
Drip lines and sprinkler overspray at the base of a wall keep the footing wet and the bottom course saturated, which is where efflorescence and spalling begin.
Wall height triggers permitting and property line rules that vary by jurisdiction. Check with the City of Phoenix or Maricopa County Planning and Development before building.
Finishes on block
Most walls get a finish. Paint needs a masonry rated product and a clean, cured surface. Stucco gives the smooth look common on Phoenix perimeter walls and hides coursing entirely. Architectural units are usually left exposed because the texture is the point. Stone or brick veneer can go over concrete block when the wall is designed to carry it, which is a different assembly with its own drainage details.
On repairs and additions, matching an existing finish is its own skill. Sun fading, original mix, and joint tooling all have to be read before new work goes in, which is why masonry color matching is a service line rather than an afterthought.
Where each unit type belongs
What a real quote should include
A written proposal should tell you what goes into the ground and into the cells, not just height and length.
- Wall height, length, and unit width, stated in nominal dimensions.
- Footing dimensions and depth, plus how caliche or unsuitable soil will be handled.
- Vertical rebar size and spacing, horizontal steel, and bond beam locations.
- Whether the wall is fully grouted or partially grouted.
- Unit type and face finish, and the planned surface finish.
- Control joint spacing and cap detail.
- Permit responsibility and any engineering required for the height.
- Demolition and haul off of an existing wall or fence, if applicable.
- License number, insurance, and warranty terms in writing.
no mention of footing depth, no rebar spacing, a refusal to state whether cells are grouted, a promise to build tall without engineering, and a start date that assumes the footing can be poured and laid on the same day.
Frequently Asked Questions
What is a CMU wall?
A CMU wall is a wall built from concrete masonry units laid in mortar, typically reinforced with vertical steel and grout in the cells and tied to a concrete footing. It is the standard wall system for perimeter walls, fences, and many commercial buildings in Arizona.
How much does a concrete block weigh?
It depends on dimensions and aggregate. A standard 8 by 8 by 16 unit generally falls in the range of roughly 33 to 43 pounds, with lightweight aggregate units at the low end and normal weight units at the high end. A 12 inch unit can approach 60 pounds.
What are the standard CMU block sizes?
Nominal 4, 6, 8, and 12 inch widths, nearly always 8 inches high and 16 inches long, with half height and half length units available. Actual dimensions are three eighths of an inch less in each direction. Those standard CMU block sizes cover the overwhelming majority of concrete masonry construction in the valley.
How thick are CMU walls?
Nominal thickness is usually 4, 6, 8, or 12 inches, with the actual unit measuring three eighths of an inch less. Most residential perimeter walls here are 6 or 8 inch. Taller and load bearing walls move to 12 inch.
Are CMU walls filled with concrete?
Some are and some are not. Reinforced cells are filled with grout, which is a fluid concrete mix. A wall can be fully grouted or grouted only at the reinforced cells, depending on what the design requires. Empty cells with no steel are the sign of a wall built without reinforcement.
What is architectural CMU block?
It is concrete block selected for appearance as well as structure: split face, ground face, scored, ribbed, and integrally colored units. It meets the same load bearing specification as precision block and is normally left exposed rather than stuccoed or painted.
How do you insulate CMU walls?
On conditioned buildings, the common approaches are rigid insulation on the interior or exterior face, insulated inserts placed in the cells before grouting, or loose fill in ungrouted cells. Grouted cells cannot be insulated, so insulation strategy has to be decided before the wall goes up, not after.
How many blocks are in a square foot of wall?
Using the nominal 8 by 16 face, one unit covers about 0.89 square feet, which works out to roughly 1.125 units per square foot. Multiply wall height by length in feet, then by 1.125, for a quick estimate before waste and specialty units.
How do you build a CMU wall?
In order: lay out the line, pour a reinforced footing with dowels, lay a level first course in a full mortar bed, build up in running bond, run vertical rebar through the cells, grout in lifts, add a bond beam, place control joints, and cap the top. Anything that skips the footing or the steel is not a wall you want on a property line.
How long does a block wall last in Arizona?
A properly footed, reinforced, and capped wall is a multi decade structure. The walls that fail early almost always fail for the same reasons: an inadequate footing, missing reinforcement, no control joints, or constant irrigation water at the base.
Building or repairing concrete masonry in the Phoenix valley
New Era Masonry builds and repairs concrete masonry throughout Phoenix, Gilbert, Mesa, Scottsdale, Paradise Valley, and the surrounding valley, residential and commercial. That includes new perimeter and property line walls, retaining walls, planters and outdoor living structures, commercial concrete masonry construction, repairs, tuck pointing, color matching, and replacement when a wall is past saving. Our in house masonry designer works through layout, unit type, and finish before anything is ordered.
Building or Repairing Concrete Masonry in the Phoenix Valley
New Era Masonry builds and repairs concrete masonry throughout Phoenix, Gilbert, Mesa, Scottsdale, Paradise Valley, and the surrounding valley, residential and commercial. That includes new perimeter and property line walls, retaining walls, planters and outdoor living structures, commercial concrete masonry construction, repairs, tuck pointing, color matching, and replacement when a wall is past saving. Our in house masonry designer works through layout, unit type, and finish before anything is ordered.
If you are planning a wall or looking at cracks in one you already have, reach out for a written proposal that spells out footing, steel, grout, and finish. Licensed, bonded and insured, ROC #339091.
Planning a CMU Wall or Repairing an Existing One?
New Era Masonry can evaluate the wall, footing, reinforcement, grout, drainage, finish, and repair requirements before the scope is finalized.
Request a Written ProposalView Masonry RepairsWritten by the New Era Masonry team, an Arizona masonry contractor with more than 40 years of block, brick, stone, and concrete work across three generations of masons.