100mm vs 140mm vs 215mm Concrete Blocks: UK Size Guide
100mm vs 140mm vs 215mm Concrete Blocks: UK Size Guide
Concrete block size affects the thickness, weight, strength geometry and detailing of a masonry wall. In the current JJ Building Materials range, 100mm and 140mm products are dense 7.3N blocks, while the 215mm product is a hollow 7.3N block. This means a size comparison is also partly a configuration comparison.
This guide explains what each width means, where different sizes can be used, how block width affects wall build-up and why all three can have similar face coverage despite very different thicknesses.
Understanding Concrete Block Dimensions
Many UK concrete blocks have face dimensions around 440mm long by 215mm high. The third dimension is the width through the wall: 100mm, 140mm, 215mm or another specified thickness. Changing width therefore changes wall thickness without necessarily changing the number of blocks visible on the face.
Always check exact product dimensions. Product names often highlight width, but length and height matter for coursing and quantities.
100mm Concrete Blocks
The 100mm BLOC dense concrete block 7.3N is the narrowest current option in the collection. 100mm blocks are widely used in suitable partitions, inner cavity-wall leaves and other general masonry.
At 100mm, the wall has a compact footprint and the individual units are smaller than wider versions. However, structural height, loading, acoustics and fire requirements still need to be checked.
140mm Concrete Blocks
The 140mm BLOC dense concrete block 7.3N provides an extra 40mm of wall thickness. This can improve geometric robustness and mass, but it also occupies more floor area and changes junctions.
140mm blocks may be specified for particular structural walls, higher partitions or other details where 100mm is not appropriate. Use drawings and calculations as the reference.
215mm Concrete Blocks
The 215mm BLOC hollow concrete block 7.3N is a substantially thicker unit. Its hollow configuration reduces material compared with a solid dense block of the same dimensions.
A 215mm wall can form a robust structural masonry element, but foundations, lintels, ties, openings and finishes must all be detailed for that thickness.
100mm vs 140mm Concrete Blocks
Both current products are dense 7.3N units, so the main obvious change is width. A 140mm wall has more cross-sectional area and mass than a 100mm wall, but complete structural capacity depends on height, restraint and load.
The wider block also changes door linings, reveals, floor space and wall junctions. Do not swap sizes late in the build without checking those consequences.
140mm vs 215mm Concrete Blocks
This comparison changes both width and configuration. The 140mm product is dense; the 215mm product is hollow. Although both are 7.3N, their weight, fixing behaviour and structural geometry differ.
Read our dense vs hollow block guide before making a substitution based only on strength grade.
100mm vs 215mm Concrete Blocks
A 215mm wall is more than twice as thick as a 100mm wall, so these products typically serve very different design details. The thicker wall may need broader foundations or supporting elements and reduces available floor area if used internally.
Does a Wider Block Mean Higher Compressive Strength?
No. The current 100mm, 140mm and 215mm BLOC products all carry the same 7.3N designation. Width and material compressive strength are separate properties. Wall capacity can increase with thickness, but it must be calculated as a system.
What Does 7.3N Mean Across Different Sizes?
It indicates the same declared unit-strength level in N/mm², but a 100mm dense block and 215mm hollow block remain physically different products. Our 7.3N strength guide covers this in more detail.
Block Size and Thermal Performance
Increasing concrete thickness adds thermal resistance, but dense concrete remains relatively conductive compared with aircrete and insulation. External wall U-values should be calculated from the complete build-up.
If thermal conductivity is a key driver, compare our aerated concrete blocks as part of the wall-design process.
Block Size and Sound Insulation
Greater mass and thickness can help acoustic performance, but wall finish, cavities and flanking paths matter. A 140mm dense block may be selected over 100mm for specific acoustic or structural reasons, but use tested or calculated assemblies.
Block Size and Fire Resistance
Fire performance depends on block type, thickness, loading and wall finish. Wider masonry can support greater fire resistance in some systems, but do not assign a rating from width alone. Use manufacturer data for the exact construction.
Block Size and Fixings
A 100mm dense block usually offers enough depth for many common anchors, while a 140mm dense block provides greater embedment potential. The 215mm hollow product has deeper overall width but contains voids, so specialist hollow-masonry fixings may be required.
Fixing design should use tested loads, not just available depth.
Block Size and Foundations
The supporting foundation or slab must accommodate the wall thickness and load. A 215mm wall cannot simply sit where a 100mm partition was intended without checking bearing width and structural support.
Block Size Around Doors and Windows
Changing block width affects reveals, lintel bearing details, cavity closers, door linings and finishes. Coordinate the masonry width with the selected windows and doors early, particularly on cavity-wall construction.
How Many Blocks per Square Metre?
If all products use a face around 440 x 215mm, block count per square metre is broadly similar regardless of width. About 10 blocks per m² is a common preliminary estimate with normal mortar joints.
A 20m² wall might therefore need roughly 200 blocks before waste whether the units are 100mm, 140mm or 215mm wide. What changes is pack quantity, delivery weight and wall volume.
Current Pack Sizes
- 100mm BLOC dense 7.3N: pack of 66.
- 140mm BLOC dense 7.3N: pack of 44.
- 215mm BLOC hollow 7.3N: pack of 38.
Always verify live product pages before ordering because pack configurations can change.
Worked Quantity Example
For a net wall face of 35m² using approximately 10 blocks per m²:
35 × 10 = 350 blocks
Add 5% waste:
350 × 1.05 = 367.5, so around 368 blocks before pack rounding. If using 100mm blocks in packs of 66, six full packs would provide 396 units. If using 140mm packs of 44, nine packs would provide 396. The exact order depends on merchant supply and project waste.
Handling Different Block Widths
Wider blocks are heavier and more awkward, even where hollow. Plan pallet placement and lifting so operatives do not carry blocks unnecessarily. Use mechanical handling for large quantities and follow site manual-handling procedures.
Choosing the Right Width
- Start with architectural and structural drawings.
- Confirm dense or hollow block type.
- Check required strength.
- Review wall height and restraint.
- Check thermal, acoustic and fire performance.
- Coordinate foundations, lintels and openings.
- Then calculate quantities and pack requirements.
Concrete Block Size FAQs
What is the standard concrete block size in the UK?
A face around 440 x 215mm is common, with widths such as 100mm, 140mm and 215mm. Other sizes also exist.
Which is more common, 100mm or 140mm?
100mm is very common for general wall leaves and partitions, but 140mm is specified where extra thickness is required.
Is a 215mm block stronger than a 100mm block?
Not necessarily by unit grade. The current products are all designated 7.3N, although wall thickness changes structural geometry.
Do 215mm blocks use fewer blocks per m²?
No if the face dimensions are the same. Width changes wall thickness, not visible face coverage.
Are 215mm blocks solid?
The current JJ Building Materials 215mm BLOC product is hollow.
Can I swap 100mm for 140mm blocks?
Only after checking drawings, wall thickness, support and connected details.
How many 100mm concrete blocks per m²?
Around 10 per m² is a common preliminary estimate for 440 x 215mm face blocks with typical joints.
Where can I buy different block sizes?
Browse JJ Building Materials' concrete blocks for current 100mm, 140mm and 215mm options.
Final Advice
Choose concrete block width from the design rather than from availability alone. 100mm and 140mm dense blocks share the same 7.3N grade in the current range, while the 215mm option is hollow. Width, configuration and strength should all be confirmed before you calculate packs or compare prices.