Concrete Hollow Blocks: US Sizes and Specifications

Standard Concrete Hollow Block Sizes in the US

Concrete masonry units (CMU) are ordered by nominal size. For conventional modular units, specified dimensions are typically 3/8 inch smaller to allow for a mortar joint. The table shows width × height × length; actual supplied dimensions should be checked against the manufacturer’s product sheet.

These dimensions follow the modular convention described by the Concrete Masonry & Hardscapes Association. Special shapes and local product ranges can differ.

Hollow Blocks vs. Solid Blocks

Both hollow and solid units can fall under ASTM C90. Choose the unit required by the project specification; do not infer wall strength, insulation or fire performance from the hollow/solid label alone. CMHA unit guidance.

For a broader comparison, see hollow blocks vs. solid blocks.

How to Check Block Weight Before Ordering

Ask the supplier to identify the exact block model and its published unit weight. A nominal size alone is not a sufficient shipping-weight specification.

Use this purchasing checklist:

• Exact product name, nominal size and specified dimensions.
• Published weight per unit and the condition to which that weight applies.
• Number of units on the quoted pallet.
• Total loaded pallet weight, including packaging.
• Delivery and unloading requirements for your order.

For a preliminary goods-weight calculation, multiply the supplier’s stated unit weight by the confirmed number of units. Use the supplier’s loaded weight for delivery planning.

Specifications to Check for Your Project

For US loadbearing CMU, ask which edition of ASTM C90 the product is documented to meet. Product compliance and the design of the finished masonry wall are separate questions. A block’s dimensions or individual strength figure is not a wall load rating. CMHA specifications reference.

Before requesting a quotation, state the required block size, quantity, project specification and delivery location. For budget context, see our concrete block cost guide; confirm actual prices with your supplier.

How Are Concrete Hollow Blocks Made?

Concrete hollow blocks are primarily produced using a concrete brick making machine. First, the raw materials for the hollow blocks are mixed in specific proportions, then transported to the machine’s mold. The material is vibrated on the platform, and the upper mold presses down to shape the block.

Hollow blocks made by a concrete brick machine offer high output and efficiency while saving on labor and raw materials. Additionally, the blocks produced have high strength and precise dimensions.

Depending on production requirements, different types of brick-making machines can be used to produce concrete hollow blocks.

Features to Compare When Selecting Hollow Blocks

Compare the documented properties of the actual unit and proposed wall assembly. Ask for relevant product and assembly data for strength, thermal performance, sound performance and fire resistance. The hollow-block label alone does not guarantee these outcomes.

GB Standards and MU Grades: Regional Manufacturing References

The following manufacturing references use GB standards, MU grades and metric mold dimensions. These are separate from the US nominal sizes shown above. A grade label or mold size alone does not establish ASTM compliance or the load capacity of a finished wall. For a US project, check the specified standard and the manufacturer’s product test documentation.

According to its size deviation, the appearance quality is divided into: excellent products (A), first-class products (B), qualified products (C).

According to its intensity level: MU3.5, MU5.0, MU7.5, MU10.0, MU15.0, MU20.0

Mark in the order of product name (codename NHB), strength level, appearance quality level and standard number.

Examples of a grade tag:

The block with strength grade MU7.5 and appearance quality of excellent product (A) is marked as: NHB MU7.5A GB8239

Technical requirements of concrete hollow brick

This regional manufacturing example uses a 390 × 90 × 190 mm block. It is separate from the US nominal-size convention above; confirm the available mold dimensions for your production requirements.

The minimum outer wall thickness should not be less than 30mm, and the interval between holes should not be less than 25mm. The hollowness should not be less than 25%.

Size tolerance（mm）

Strength Class ( MPa)

Relative water content (%)

Raw Material Standards for Concrete Hollow Blocks

The following are the standards to ensure the quality and durability of the raw materials used in producing concrete hollow blocks.

1. Cement:
   • Must comply with the standards GB/T 175, GB/T 1344, and GB 12958.
2. Fine Aggregate:
   • Must meet the requirements of GB/T 14684.
3. Coarse Aggregates:
   • Crushed stones, pebbles, and heavy slag are acceptable.
   • Crushed stones and pebbles must comply with GB/T 14685 standards.
   • Heavy slag must comply with YBJ 20584, with a maximum particle size of 10mm.
   • If using broken stones like stone chips, the fine stone powder content (particles smaller than 0.15mm) should not exceed 20%.
4. Admixtures:
   • Must meet the standards set by GB 8076.

Production Requirements of Concrete Hollow Brick

The following are the materials needed hollow bricks that meet the requirements,

• Required raw materials for mortar: cement, sand, water.
• Admixture: Lime, the purpose of adding lime is to increase the adhesion between the brick and the setting material.

What is the ratio of mortar for concrete hollow block?

When you use mortar strength M7.5, cement strength 32.5, the ratio of mortar and cement is:

For Mortar Strength M7.5 (Cement Strength 32.5):

• Weight ratio: Cement : Water : Sand = 1 : 0.9 : 6.72
• Amount per cubic meter:
  • Cement: 250kg
  • Water: 220kg
  • Sand: 1680kg

For Mortar Strength M10 (Cement Strength 32.5):

• Weight ratio: Cement : Water : Sand = 1 : 1.07 : 6.35
• Amount per cubic meter:
  • Cement: 260kg
  • Water: 280kg
  • Sand: 1650kg

Additional Masonry Requirements for Concrete Hollow Blocks

1. Masonry Below Indoor Floor:
   • Use small blocks of ordinary concrete and cement mortar with a strength grade of at least M5.
2. Bottom Walls of Civil Buildings (Five Floors or More):
   • Small blocks with a strength grade of at least MU5 and M5 masonry mortar are required.
3. Painting and Waterproofing:
   • Concrete hollow block walls should be double-painted.
   • Outdoor footings should be coated with cement mortar for protection.
4. Moisture-Proofing for Humid Environments:
   • In humid conditions, walls made from lightweight aggregate concrete small blocks should be plastered with cement mortar for effective moisture-proofing.
5. Thermal Insulation for Cold Regions:
   • In northern cold regions, when using light aggregate concrete small blocks for exterior walls, additional thermal insulation measures should be applied where the insulation performance is weakened, such as around ring beams, lintels, and core columns.

Project requirements must follow the applicable specifications and approved design details.

Producing Hollow Blocks with Lontto Equipment

All our concrete block machines can produce hollow bricks in various sizes. The most common sizes include 8-inch, 6-inch, 5-inch, and 4-inch hollow blocks. A single brick machine can be fitted with multiple molds, allowing it to produce concrete bricks of different sizes.

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