How Many Concrete Blocks Do I Need? UK Quantity & Pack Guide
How Many Concrete Blocks Do I Need?
Calculating how many concrete blocks you need starts with the net wall area and the face size of the chosen block. Many UK concrete blocks use a face around 440 x 215mm. With typical mortar joints, about 10 blocks per square metre is a widely used preliminary estimating figure. That makes early budgeting simple, but a final order should use exact product dimensions, openings, wall details and waste.
This guide works through the calculation for the current JJ Building Materials concrete blocks, including 100mm and 140mm dense BLOC blocks and 215mm hollow BLOC blocks.
Step 1: Confirm the Block Face Size
Do not start with wall thickness. Block count per square metre is governed mainly by the visible face dimensions: length and height. If a 100mm, 140mm and 215mm block all have the same 440 x 215mm face, they cover broadly the same wall area per unit despite containing very different amounts of concrete.
Check the product page before using the 10-per-m² rule because some block formats differ.
Step 2: Measure Wall Length
Measure each wall section in metres. On a rectangular room or structure, record each elevation separately if heights or openings differ. Accurate dimensions are more useful than one rounded perimeter number.
Step 3: Measure Wall Height
Measure from the supporting level to the intended top of blockwork. If the wall steps, has gables or changes height, split it into simple rectangles or triangles and calculate each area separately.
Step 4: Calculate Gross Wall Area
Multiply length by height. For example, a wall 10m long and 2.4m high has a gross area of:
10 × 2.4 = 24m²
If there are several walls, add their gross areas.
Step 5: Subtract Openings
Calculate the area of large doors and windows and subtract it from the gross wall. A 1.2m x 1.2m window is 1.44m². Two identical windows remove 2.88m² from the gross blockwork area.
Do not over-deduct small service penetrations because waste and cutting around them can use as much material as a full block.
Step 6: Multiply by Blocks per m²
Using 10 blocks per m² as an early figure, a 21.12m² net wall would need around:
21.12 × 10 = 211.2 blocks
Round the base quantity up before adding waste.
Step 7: Add a Waste Allowance
Waste covers cuts, breakages and damaged units. Around 5% is a common starting point for straightforward blockwork, while walls with many openings, returns and complex cuts may need more.
Using 212 base blocks and 5% waste:
212 × 1.05 = 222.6
Round to 223 blocks, then convert to pack quantities.
Step 8: Round to Whole Packs
Current JJ Building Materials pack sizes differ:
- 100mm dense 7.3N: pack of 66.
- 140mm dense 7.3N: pack of 44.
- 215mm hollow 7.3N: pack of 38.
For 223 blocks, four packs of 66 provide 264 100mm units; six packs of 44 provide 264 140mm units; six packs of 38 provide 228 215mm units. The most efficient pack rounding therefore varies with product.
Worked Example: 100mm Dense Block Wall
Suppose a partition has a net face area of 30m².
- 30m² × 10 blocks/m² = 300 blocks.
- Add 5% waste = 315 blocks.
- 100mm product pack size = 66.
- 315 ÷ 66 = 4.77 packs.
- Round to 5 packs = 330 blocks.
This leaves 15 blocks beyond the calculated requirement, providing a small site contingency.
Worked Example: 140mm Dense Block Wall
For the same 30m² face area, the base and waste quantity is still around 315 because the face dimensions are similar. With packs of 44:
315 ÷ 44 = 7.16 packs, so eight full packs provide 352 blocks.
Wall thickness changes delivery weight and block pack count, but not the initial face-area calculation.
Worked Example: 215mm Hollow Block Wall
Again using 30m² and 315 blocks after waste, with packs of 38:
315 ÷ 38 = 8.29 packs, so nine packs provide 342 blocks.
Always verify whether the merchant can supply partial packs before assuming whole-pack rounding is compulsory.
Do 100mm Blocks Need Fewer Blocks Than 140mm?
No if length and height are identical. 100mm and 140mm refer to wall thickness, not face coverage. Both may use about 10 blocks per m².
Do 215mm Hollow Blocks Need Fewer per m²?
Again, not if their face is 440 x 215mm. Their width and voids affect wall thickness and weight, not the number of units seen on the wall face.
Read our concrete block size guide for a detailed explanation.
How to Calculate Blocks for a One-Leaf Wall
Use net wall face area directly. If the wall is 12m long and 2.5m high with no openings, 30m² × approximately 10 blocks/m² gives about 300 blocks before waste.
How to Calculate Blocks for Two Leaves
If both leaves of a cavity wall are blockwork, calculate each leaf separately because they may use different block types or dimensions. Do not simply double one figure unless the two leaves genuinely use identical products and geometry.
Calculating Blocks Around Openings
Subtract large openings but remember that jambs, reveals, returns and piers can add blockwork back into the design. A simple net-area deduction can underestimate complex masonry around wide openings.
Calculating Blocks for Piers
Piers add wall volume beyond a flat elevation. Calculate their dimensions separately or count units from the bond detail. This is especially important on retaining, boundary and structural walls with regular thickened piers.
Cut Blocks and Half Blocks
Most wall layouts require cuts at openings and returns. Offcuts can sometimes be reused, but do not assume every half piece is usable. Keeping a modest waste allowance is safer than designing the quantity around perfect reuse.
Block Strength and Quantity
A 7.3N designation does not change the number per square metre. Strength and coverage are separate. See our 7.3N concrete block guide for strength information.
Dense vs Hollow and Quantity
Dense and hollow blocks with the same face dimensions cover the same wall area, though pack sizes and site handling differ. Our dense vs hollow guide compares them.
Delivery and Storage Planning
Concrete blocks are heavy bulk materials. The quantity calculation should be followed by a delivery plan: check vehicle access, unloading space, stable storage ground and how packs will be moved close to the work area.
Avoid stacking loose blocks unsafely or storing heavy packs on surfaces not designed for the load.
Should You Order Extra Blocks?
Yes, a reasonable waste allowance is sensible. Spare matching blocks can also be useful for late service changes or future repairs. Do not over-order excessively, because returning heavy building materials can be costly or impractical.
Concrete Block Quantity Checklist
- Confirm exact product face dimensions.
- Measure wall lengths and heights.
- Subtract substantial openings.
- Add piers and returns.
- Multiply by an appropriate blocks-per-m² figure.
- Add realistic waste.
- Check current pack quantity.
- Round up to practical supply quantities.
- Plan delivery and storage.
Concrete Block Quantity FAQs
How many concrete blocks are in 1m²?
About 10 blocks per m² is a common preliminary estimate for 440 x 215mm face blocks with typical mortar joints.
How many blocks do I need for 20m²?
Approximately 200 before waste using the 10-per-m² rule. Add a waste allowance and round to the chosen pack size.
Does block thickness affect how many I need?
Not if length and height stay the same. Thickness affects wall width and weight, not face coverage.
How much waste should I allow?
Around 5% can be a reasonable starting allowance for simple walls, with more for complex cutting and details.
Should I subtract door openings?
Yes, substantial openings should normally be deducted, while returns and piers are added separately.
How many 100mm blocks are in a pack?
The current BLOC 100mm dense 7.3N product at JJ Building Materials is sold in packs of 66.
How many 140mm blocks are in a pack?
The current 140mm dense BLOC product is sold in packs of 44.
How many 215mm hollow blocks are in a pack?
The current 215mm hollow BLOC product is sold in packs of 38.
Final Advice
Use wall area for the first calculation, exact face dimensions for accuracy, and pack size for the final order. Around 10 blocks per square metre is a useful shortcut for common 440 x 215mm units, but openings, piers, waste and product-specific packs determine what you actually need delivered.