Concrete Blocks Buying Guide: Dense, Hollow, Sizes & Strength

Concrete Blocks Buying Guide: Dense, Hollow, Sizes and Strength

Concrete blocks are used across UK housing, extensions, commercial construction, retaining structures and internal masonry. The phrase covers several different products, so choosing correctly means looking at density, configuration, thickness, compressive strength, wall type and the rest of the construction rather than simply ordering a generic 'block'.

JJ Building Materials' live range includes 100mm BLOC dense concrete blocks 7.3N, 140mm BLOC dense concrete blocks 7.3N and 215mm BLOC hollow concrete blocks 7.3N. This guide explains what those descriptions mean and how to choose the right block for your project.

What Are Concrete Blocks?

Concrete masonry blocks are manufactured units made from cementitious binders and aggregates. Their density, geometry and strength vary by product. Dense blocks use a relatively high-mass concrete, while hollow blocks contain substantial formed voids. Lightweight aggregate and aerated concrete are separate categories with different characteristics.

Blocks are used because their larger face size allows walls to be built quickly compared with individual bricks. They can form load-bearing walls, partitions, inner leaves, foundations and other construction where the exact product and design are appropriate.

Dense Concrete Blocks

Dense concrete blocks are solid or substantially solid high-mass masonry units. Their mass can be useful where structural strength, robustness and sound insulation are priorities. They are also familiar fixing substrates when the correct masonry anchors are used.

JJ Building Materials currently carries dense BLOC products in 100mm and 140mm widths, both with a 7.3N compressive-strength designation. Width changes wall geometry and unit weight, so choose according to the drawing rather than automatically selecting the thicker block.

Hollow Concrete Blocks

Hollow concrete blocks contain large internal voids. These reduce unit weight and can create opportunities for reinforcement or grouting in systems specifically designed that way. The current JJ Building Materials live range includes a 215mm BLOC hollow concrete block with a 7.3N designation.

Hollow blocks require different fixing considerations from solid dense blocks because an anchor may encounter the shell or a void. They should also not be confused with lightweight aircrete; a hollow dense-concrete unit can still be a substantial masonry product.

Dense vs Hollow Concrete Blocks

The best choice depends on the wall. Dense blocks offer continuous solid material through most of the unit and high mass. Hollow blocks reduce weight and can suit thicker structural wall systems. Neither is automatically stronger, lighter in every size or better for every application.

Read our dense vs hollow concrete blocks guide for a detailed side-by-side comparison.

100mm Concrete Blocks

100mm concrete blocks are one of the most common widths for general masonry and can be used in suitable internal walls and cavity-wall leaves. The current JJ Building Materials 100mm BLOC dense block carries a 7.3N designation and is supplied in packs of 66.

A 100mm width keeps the wall footprint relatively compact but must still satisfy structural, acoustic, fire and thermal requirements. For partitions, consider what will be fixed to the wall as well as its height and support.

140mm Concrete Blocks

140mm concrete blocks add 40mm of thickness compared with a 100mm unit. The wider leaf changes wall slenderness, mass, bearing area and junction details. JJ Building Materials stocks a 140mm BLOC dense 7.3N block in packs of 44.

Where a drawing calls for 140mm, do not reduce to 100mm simply to save space. Conversely, moving to 140mm can interfere with designed cavities, reveals and internal dimensions if it was not planned.

215mm Hollow Concrete Blocks

The current 215mm hollow concrete block is a large-format unit intended for wall constructions that need that thickness and configuration. It is supplied in packs of 38 and carries a 7.3N designation.

At 215mm wide, unit handling, foundation width, wall ties, lintels and finishes all need to coordinate with the block. Read our concrete block size guide before changing widths.

What Does 7.3N Mean?

A 7.3N concrete block is commonly shorthand for a declared compressive strength of 7.3 N/mm². This is a unit material property measured under standard test conditions. It does not mean every 7.3N block can be used interchangeably because density, configuration and dimensions may differ.

Wall capacity depends on mortar, width, height, restraint, loading and workmanship as well as block strength. Our 7.3N block strength guide explains the distinction.

Concrete Blocks vs Aerated Blocks

Dense or hollow concrete blocks should be compared separately from aerated blocks such as Thermalite. Aircrete is much lower density and generally offers better material thermal conductivity and easier cutting. Dense concrete provides more mass and can offer different structural and acoustic characteristics.

The correct product is the one designed into the wall. A lightweight block should not replace a dense 7.3N block without checking strength and system performance, and a dense block should not replace aircrete without checking thermal and weight consequences.

Concrete Block Strength and Load-Bearing Walls

Concrete blocks are commonly used in load-bearing masonry, but the phrase 'load-bearing' does not define one universal required block strength. A single-storey wall with modest loading differs from a multi-storey wall, pier or heavily loaded bearing detail.

Use structural drawings and calculations where provided. Pay particular attention beneath beams, lintels and concentrated loads because local bearing requirements can differ from the general wall.

Thermal Performance

Dense concrete has higher thermal conductivity than aircrete, so cavity insulation or other insulation layers often do more of the work in thermally efficient external walls. Do not estimate a whole-wall U-value from block type alone. Cavity width, insulation product, mortar, finishes and thermal bridges all matter.

Sound Insulation and Mass

Higher-mass masonry can be useful in acoustic construction, but sound performance is a system property. Wall thickness, plaster, cavities, junctions and flanking routes can be just as important. Use tested or calculated wall build-ups for separating walls and acoustically sensitive spaces.

Fire Performance

Concrete masonry is non-combustible, but required fire resistance depends on block thickness, load condition, finish and complete wall construction. Refer to manufacturer and design data rather than assuming all concrete blocks have the same fire rating.

Cutting Concrete Blocks

Dense concrete blocks are harder to cut than aerated blocks. Use appropriate masonry saws or disc cutters, with dust control, guards and PPE. Plan cuts to minimise waste and avoid leaving small fragile pieces at openings and corners.

Hollow blocks require careful cutting because removing shell material can alter the intended unit geometry. Follow project guidance around reinforcement and openings.

Fixing into Dense Concrete Blocks

Dense blocks can accept a wide range of masonry anchors, but fixing selection still depends on load, block strength and edge distance. Drill the specified diameter and depth and use anchor data relevant to concrete masonry rather than choosing by screw size alone.

Fixing into Hollow Blocks

Hollow blocks need anchors designed to work with shells and voids. Options can include specialist hollow-masonry fixings, sleeves or resin systems with mesh components, depending on load. The fixing manufacturer should provide substrate-specific data.

How Many Concrete Blocks per Square Metre?

Many UK concrete blocks have face dimensions around 440 x 215mm. With typical mortar joints, around 10 blocks per m² is a widely used preliminary estimate. For final ordering, calculate from the exact product dimensions and wall area.

Subtract large openings, add returns and waste, then round to whole packs. Read our concrete block quantity guide for worked examples.

Mortar for Concrete Blocks

Use the mortar specified for the wall and block. Stronger is not automatically better because masonry needs a compatible balance of strength and movement. Maintain consistent joints and full bedding as required by the block configuration.

Storage and Site Handling

Store packs on stable ground and protect them from contamination and severe weather. Dense blocks are heavy, especially at larger widths, so plan pallet positions close to the workface and use mechanical handling where appropriate.

Concrete Block Buying Checklist

• Confirm dense or hollow block type.
• Check specified width: 100mm, 140mm, 215mm or other.
• Verify minimum compressive strength.
• Check wall loading and bearing details.
• Review thermal, acoustic and fire requirements.
• Choose fixings for the exact substrate.
• Calculate wall area and block quantity.
• Add waste and round to pack sizes.
• Plan delivery access and handling.

Concrete Blocks FAQs

What are dense concrete blocks used for?

They are used in many structural and non-structural walls where their strength, mass and dimensions meet the design.

Are hollow blocks weaker than dense blocks?

Not automatically. Strength is declared for each product. Configuration, width and intended wall system also differ.

What does 7.3N mean?

It refers to a declared compressive-strength level of 7.3 N/mm².

How many concrete blocks are needed per m²?

Around 10 blocks per m² is a common early estimate for blocks with a 440 x 215mm face and typical joints.

Can I use dense blocks instead of Thermalite?

Only if the wall design is checked because thermal performance, weight, strength and detailing change.

Can I fix heavy items to concrete blocks?

Yes with appropriately rated masonry anchors. Hollow blocks require fixings designed for their configuration.

Which size concrete block should I use?

Use the width stated on the wall drawing or specification. Thickness affects much more than strength.

Where can I buy concrete blocks online?

Browse the JJ Building Materials concrete blocks collection for current dense and hollow products.

Final Advice

Start with the wall specification, then choose configuration, width and strength. Dense 100mm and 140mm 7.3N blocks and 215mm hollow 7.3N blocks serve different construction needs. Once the technical requirements are satisfied, compare pack sizes, handling and cost to plan the order efficiently.