CMU Block Dimensions - Everything You Need to Know

Concrete masonry units, better known as CMU or cinder blocks, are a staple building material used in construction projects of all sizes. But understanding the different dimensions, weights, and sizes of CMU blocks is key to selecting the right units for your specific project goals and requirements.

In this all-inclusive guide, we'll walk through the critical details on CMU block dimensions. You'll learn about the different shapes, sizes, and classes defined in the governing ASTM standard. We'll also cover how factors like wall type, codes, strength needs, and more impact which CMU dimensions are best for your project.

What are CMU Blocks?

CMU blocks are precast concrete masonry units composed of Portland cement, aggregates like sand or gravel, and water. They contain open cells or voids that reduce their weight while still providing structural integrity.

CMUs are incredibly versatile. They're used for structural and non-load bearing walls, foundations, retaining walls, partitions, artistic treatments, and more in all types of buildings. CMUs can be left exposed for their visual appeal or covered with finishes like stucco.

Overview of CMU Block Dimensions

The American Society for Testing and Materials (ASTM) sets the standards for CMU blocks in ASTM C90. This covers the permissible dimensions, densities, finishes, and more.

C90 specifies the nominal dimensions - the named size - as well as actual dimensions . Actual dimensions account for the thickness of mortar joints, typically 1/2 inch total. So an 8 x 8 x 16 inch nominal CMU will be 7 5/8 x 7 5/8 x 15 5/8 inches actual.

CMU Weight Classes

ASTM C90 organizes CMUs into three weight classes based on concrete density:

• Lightweight CMUs - less than 105 lb/ft3
• Medium weight CMUs - 105 to 125 lb/ft3
• Normal weight CMUs - more than 125 lb/ft3

Lower density CMUs are made with lighter aggregates. Though more expensive, lightweight blocks can reduce materials handling and shipping costs. Normal weight CMUs provide maximum compressive strength.

Standard CMU Block Sizes

These are the most common nominal CMU dimensions:

• 8 x 8 x 16 inches - Most popular and versatile size
• 12 x 8 x 16 inches - Used for higher load capacity walls
• 4 x 8 x 16 inches - For non-load bearing partitions

Metric CMU sizes like 200 x 200 x 400 mm are also available. Special sizes like half blocks and bond beam units are made for corners, lintels, and beams.

CMU Block Shapes

CMUs are made in an array of special shapes for construction needs. Common shapes include:

• Stretcher - Standard rectangular block laid lengthwise in walls
• Corner - "L"-shaped corner units
• End - Shorter stretcher for ends of walls
• Bond beam - Has channel for horizontal steel rebar
• Lintel - Extended height for opening spans
• Bullnose - Rounded front edge for decorative reveals
• Column - Cylindrical units for pilasters

The dimensions and form of these shapes optimize them for their specific structural job in walls, columns, and openings.

CMU Block Surface Finishes

Along with shape, CMU blocks also differ in their external finish and texture. Common options include:

• Colored CMUs - Pigments added to concrete mix or mortar for color
• Painted - Factory applied primer/paint for decor and weatherproofing
• Split face - Roughened texture from splitting molds
• Glazed - Smooth, glass-like finish for moisture resistance

Special finishes give architects artistic flexibility with exposed CMU walls. Textured faces like split ribbed blocks also enhance mortar bond.

Hollow vs. Solid CMU Blocks

Another decision is whether to use hollow or solid CMUs. Hollow units with voids are far more common, making up over 95% of CMUs laid:

• Hollow CMUs - Lower weight, better insulator, less expensive
• Solid CMUs - Higher strength, more soundproof, necessary for grouting

Hollow blocks provide adequate strength for many projects while minimizing weight and material use. Solid CMUs are chosen when higher compressive strength or sound isolation are required.

Factors in Selecting CMU Block Size

• Wall type - Larger units for structural walls, smaller for partitions
• Loading - Account for expected live/dead loads
• Seismic - Reinforcing requirements in seismic zones
• Fire rating - Certain ratings dictate block thickness
• Thermal - Insulation needs may modify size
• Acoustic - Interior walls may need soundproofing
• Code - Local building codes set minimums

A structural engineer evaluates these factors to define CMU specifications appropriate for your specific building. Always consult the engineer and local building department.

Tips for Handling and Installing CMU Blocks

Once you've selected your CMU blocks, proper handling and installation are key. Here are best practices to follow:

• Lift blocks with proper equipment - never manually
• Use wheelbarrows, carts, or conveyors to transport units
• Stage blocks near work area for easy access
• Account for mortar joint thickness when laying blocks
• Build corner blocks in alternating directions for strength
• Check level and alignment as you build

Proper CMU installation ensures structurally sound, attractive walls. Planning block staging and movement helps make the process smooth and efficient.

Frequently Asked Questions

What's the difference between nominal and actual CMU size?

Nominal size is the stated dimensions of the CMU. Actual size is around 1/2 inch smaller in each direction to allow for mortar joint thickness when laying blocks.

How much weight can an 8x8x16 CMU block support?

An 8x8x16 normal weight CMU can support loads over 5,000 pounds per linear foot when laid in a wall. Fill cores with grout or concrete for maximum strength.

Can CMU blocks be used for exterior finishing?

Yes, CMUs make durable, energy-efficient exterior walls. Maintain proper installation details and use weather-resistant shapes or finishes.

What's the typical cost per CMU block?

On average, standard 8x8x16 CMU blocks cost $1.20 - $1.60 per block. Prices vary by manufacturer and region.

From sizing and shapes to handling and installation, this guide covers everything you need to know about CMU block dimensions. Understanding ASTM standards and matching CMUs to project needs is key to success.

Armed with this knowledge, you can discuss options with your contractor and start building with CMU blocks. Contact us today if you need an estimate or help with your upcoming masonry project!