How to Calculate Blockwork: Blocks, Mortar and Accessories

Introduction

Blockwork is the most forgiving material on a building site and the most frequently mis-ordered. Blocks are cheap individually, heavy in bulk, and sold in pallets that are not the same as a square metre of wall. The gap between "the wall is 40 square metres" and "that is 500 blocks, 30 bags of mortar and 4 lintels" is where most small builders lose a day and a delivery charge.

This guide works through the whole calculation for a blockwork wall: the difference between nominal and actual block size, blocks per square metre, the mortar that binds them, and the accessories (wall ties, DPC, lintels, movement joints) that determine whether the wall performs. Worked examples are given in metric and imperial.

Blocks are sold by the unit, walls are measured by the area

The fundamental conversion is blocks per square metre. It depends on the block's face size plus the mortar joint, and on whether the block is solid, hollow or aerated.

Take a common concrete block of 440 × 215 mm with a 10 mm joint. The nominal face is 450 × 225 mm, which is 0.10125 m². One divided by 0.10125 gives 9.88 blocks per square metre, so 10 blocks per m² is the working figure.

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The joint is part of the module. A 10 mm joint consumes roughly 5% of the wall face, which is why ignoring it understates the count by about one block in twenty.

Imperial note. If you are working in feet, convert the module to inches: 450 × 225 mm is 17.7 × 8.86 in, so one block plus joint covers 157 sq in. A square foot is 144 sq in, so there are 0.92 blocks per sq ft, or 9.88 per square metre. The arithmetic is the same; only the units change.

Measure the wall properly

Measure the wall as built: length at the base and at the top, height from the finished floor to the underside of the slab or the wall plate. Walls are rarely plumb and rarely the length shown on the drawing, so take the larger of two readings on each run for material purposes.

Deduct openings over about 1 m², and add back nothing for the reveals: the blocks around an opening are cut and the offcuts are often unusable. For a standard door of 0.9 × 2.1 m, deduct 1.89 m². For a window of 1.2 × 1.2 m, deduct 1.44 m². A small vent or a meter box is not worth deducting, because the cut blocks around it consume more than the hole saves.

Pier and return walls must be measured separately: a 215 mm pier projecting 215 mm from a wall, three metres high, is 0.215 × 3 = 0.65 m² of additional blockwork, plus ties.

Worked example, metric

A single-storey external wall 12.0 m long and 2.7 m high, with two windows of 1.2 × 1.2 m and one door of 0.9 × 2.1 m, built in 440 × 215 mm blocks:

• Gross wall = 12.0 × 2.7 = 32.4 m²
• Deductions = 2 × 1.44 + 1.89 = 4.77 m²
• Net wall = 27.63 m²
• Blocks = 27.63 × 9.9 = 273.5, so 274 blocks
• With 5% breakage and cutting = 274 × 1.05 = 288 blocks, ordered as 300 in pallet quantities

Worked example, imperial

The same wall as 39 ft 4 in × 8 ft 10 in, with the same openings:

• Gross = 39.33 × 8.83 = 347.3 sq ft
• Deductions = 2 × 15.5 + 20.4 = 51.4 sq ft
• Net = 295.9 sq ft
• Blocks at 0.92 per sq ft = 272.2, so 273 blocks, or 287 with 5% waste

The two methods agree within a block, which is the check worth doing whenever you switch units.

Mortar: the quantity that decides the pallet count

Mortar for blockwork is usually measured as volume per square metre of wall, then converted to bags of cement and sand using the mix.

For 100 mm blocks with 10 mm joints, mortar volume is roughly 0.02-0.025 m³ per square metre of wall. For 200 mm blocks, allow 0.04-0.05 m³ per square metre, because the bed area is wider even though the face area is similar.

Worked example. The 27.63 m² of 215 mm wall above:

• Mortar volume = 27.63 × 0.035 (interpolating for a 215 mm block) = 0.97 m³
• A 1 : 5 cement : sand mix by volume, with a 1.33 bulking allowance for the sand, needs about 0.97 × 1.33 ÷ 6 = 0.215 m³ of cement
• At 1,440 kg/m³ that is 310 kg, or 6.2 bags of 50 kg, and about 1.3 m³ of sand

For a typical house with 150 m² of 215 mm external wall and 120 m² of 100 mm internal partitions, mortar comes to roughly 6-7 m³, which is 40-45 bags of cement and 9 m³ of sand. Ordering that alongside the blocks is how you avoid three separate delivery charges.

Accessories that are not optional

A block wall without these is not a wall:

• Wall ties, one per 2.5 blocks per square metre of cavity area, or roughly 2.5-4 ties per m² depending on the tie type and spacing rules in your region.
• Damp-proof course, in the length of the wall at the base, plus a DPC at any change of material.
• Lintels over every opening: measured in linear metres and often sold by the length, with 150 mm minimum bearing each end.
• Movement joints every 6-9 m on long walls, and at changes of height or thickness.
• Reinforcement, in the form of bed-joint reinforcement at openings and at the base of tall panels.
• Fire stops and cavity barriers, where the wall meets a floor or a compartment line.

The estimating lesson is that blocks are about 60% of the material cost of a blockwork wall. The other 40% is mortar, ties, lintels, DPC and reinforcement, and it is the part that gets forgotten.

Block types change the weight, the labour and the fixings

Dense concrete blocks are strong and cheap but heavy: 18-20 kg for a 440 × 215 × 100 mm unit, which is 180-200 kg per square metre. That weight sets the pallet load, the handling method and sometimes the foundation design.

Lightweight aggregate blocks are 30-40% lighter for similar strength, which speeds up laying and reduces structural loads. They cost more per block and often need specific fixings.

Aerated (AAC) blocks are lighter again and give better thermal performance, but they are friable: they need a thin-bed adhesive rather than mortar, specific fixings, and careful handling to avoid chipped arrises.

For an estimate, the block type changes the labour rate and the accessory list as much as the unit price. Two quotes for the same wall in different block types are not comparable.

Waste, breakage and pallet quantities

Allow 5% on a straightforward rectangular wall, 7-10% where there are many openings, piers or a curved wall. Blocks break in handling and cutting, and a broken block is not reusable.

Blocks are sold in pallets of 60-80 units depending on size. Round up to full pallets for the main run, and keep a few loose blocks for the cuts. A wall that ends needing four more blocks means a trip and a minimum order charge.

Labour and time

A two-person gang lays 100-150 blocks per day on a straightforward wall with a good supply, which is 10-15 m². Productivity falls sharply with:

• high lifts, which need scaffolding
• many openings and returns
• cutting at every course
• a block change mid-wall
• hot or windy weather, which dries the mortar

Time is not a material line but it drives the cost, so a takeoff that assumes 150 blocks a day on a wall with twelve openings will understate labour by a third.

Quality checks that protect the quantity you bought

1. Check the block strength matches the specification, especially for load-bearing walls.
2. Check the mortar class: a 1 : 5 mix is normal for general blockwork; stronger is not better and can crack the blocks.
3. Keep the courses level: a wall that climbs out of level consumes extra mortar and extra time.
4. Secure ties as you go; retro-fitting ties into a finished cavity is expensive.
5. Cure the mortar: in hot climates, work that dries too fast loses strength.

Cavity walls, solid walls and what changes in the takeoff

A solid 215 mm wall is one skin, one set of blocks and one face to finish. A cavity wall is two skins with a gap between them, tied together, and that changes every line in the estimate.

Take a wall of the same face area in cavity construction: 100 mm inner leaf, 100 mm cavity, 100 mm outer leaf. The block count roughly doubles because there are two leaves. The tie count appears as a new line. The DPC appears twice, once per leaf, and cavity barriers are added at floor zones.

The thermal performance improves and the weather resistance improves, which is why cavity construction dominates in wet, temperate climates, while solid blockwork with a rendered face is common in warmer regions where rain is intense but brief.

For estimating, the practical rule is that a cavity wall consumes about 1.9 times the blocks of a solid wall of the same thickness, because the two leaves are thinner but there are two of them. Get this wrong and the order is short by half.

Comparing blockwork quotes line by line

Blockwork quotes are frequently presented as a single rate per square metre, which hides the differences that matter. Normalise every quote into the same six lines: blocks, mortar materials, accessories, plant and delivery, labour, and wastage.

Two quotes that differ by 20% on the headline rate are usually identical once these lines are visible, with one including lintels and the other excluding them. The most common omissions are lintels, ties and scaffolding, in that order.

A useful check is the block count itself. Ask each contractor how many blocks they allow for the wall. A quote built on 9 blocks per square metre rather than 9.9 will be short by a pallet on a house, and that shortfall appears later as a variation.

Frequently asked questions

How many blocks are in a square metre? About 10 for a 440 × 215 mm block with a 10 mm joint, 12.5 for a 390 × 190 mm block, and 7.8 for a 600 × 200 mm aerated block.

Do I subtract windows and doors? Deduct openings larger than about one square metre. Small openings cost more in cutting than they save.

How much mortar per square metre? Roughly 0.02-0.025 m³ for a 100 mm wall and 0.04-0.05 m³ for a 200 mm wall.

How many blocks does a 50 kg bag of cement lay? With a 1 : 5 mix, one bag of cement makes enough mortar for about 30-40 blocks.

Can I use a 100 mm block for an external wall? Only with a structural design that allows it. External walls are usually 215 mm or a cavity construction for weather resistance.

How long does a block wall take? Allow 10-15 m² per day for a two-person gang on straightforward walls, less where there are many openings.

Estimate it with FRELUX

The FRELUX blockwork calculator converts wall dimensions and block size into blocks, mortar, cement and sand, with breakage allowance and a cost in your market's currency. Enter the opening sizes and it deducts them for you.