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Project guide

How many concrete blocks do I need?

Concrete blocks are quicker to lay than bricks and are used for garden walls that will be rendered, garage and outbuilding walls, and the inner leaf of cavity walls. Measure the face of the wall and multiply by 10 blocks per square metre.

Updated Based on manufacturer guidance, sources below

The short answer

Standard UK concrete blocks (440 x 215mm face, 100mm thick) lay at 10 blocks per m² with 10mm joints. Add 5% for cuts and breakages. A wall 5m long and 2.4m high is 12 m², so it needs about 126 blocks, 3 bags of cement and 1 bulk bag of building sand at a 1:4 mix.

The formula

  • Wall area (m²) = length x height, minus doors and windows
  • Blocks = area x 10, plus 5% (440 x 215mm face with 10mm joints)
  • Mortar (m³) = area x 0.012 for 100mm blocks, plus 10%
  • Cement (25kg bags) = mortar m³ x 14 at 1:4; building sand (tonnes) = mortar m³ x 1.5

Worked example

A 100mm blockwork wall 5m long and 2.4m high, 1:4 mortar:

  • Area: 5 x 2.4 = 12 m²
  • Blocks: 12 x 10 = 120, plus 5% = 126 blocks
  • Mortar: 12 x 0.012 x 1.1 = 0.16 m³
  • Cement: 0.16 x 14 = 2.2, so 3 bags
  • Building sand: 0.16 x 1.5 = 0.24 tonnes, so 1 bulk bag

Work it out for your job

Change the measurements and the quantities update straight away.

Length x height, minus doors and windows.
Blocks
126 (10 per m² + 5%)
Mortar
0.16 m³ (including 10%)
Cement
3 x 25kg bags (1:4)
Building sand
0.24 tonnes, about 1 bulk bag

Your list

  • 3Cement 25kg
  • 1Building sand, bulk bags
  • 126Concrete blocks 7N 100mm
  • 1Mortar plasticiser 1L
Price the blocks, cement and sand

Opens SmartBuild with this list added, priced from supplier product feeds.

Materials checklist

  • Concrete blocksDense 7N blocks, 440 x 215 x 100mm, suit most garden, garage and below-ground walls.
  • Cement25kg bags of general-purpose cement.
  • Building (soft) sandFor the mortar. Sold in bulk bags of about 800kg or in 25kg bags.
  • Mortar plasticiserMakes the mortar easier to work. Follow the dose on the bottle.
  • Wall ties or starter kitsWhere the new wall joins an existing one.
  • Concrete for the footingA strip foundation under the wall, sized for the load and the ground.

You'll also need

  • Brick trowel, pointing trowel and jointer
  • Spirit level, string line and line pins
  • Bolster chisel and club hammer, or a masonry saw, for cutting blocks
  • Mixer or mixing tub, shovel and buckets
  • Gloves, goggles and safety boots: dense blocks weigh about 20kg each

Step by step

  1. Measure the wallMultiply the length by the height in metres and take off any doors and windows.
  2. Count the blocksMultiply the area by 10 and add 5% for cuts and breakages.
  3. Work out the mortarAllow 0.012 m³ per m² for 100mm blocks plus 10%, then split it into cement and sand by the mix.
  4. Plan the coursesEach course of blocks with a 10mm joint is 225mm high. Set the height so you avoid a thin cut course at the top.
  5. Order and stackHave the blocks delivered close to the work and stacked on a level base, off the soil.

Tips from the manufacturers

  • Ten blocks cover the same face as about 60 bricks, so blockwork goes up much faster.
  • Dense blocks are heavy. Lightweight aerated blocks are easier to handle, but check they suit the job, especially below ground.
  • Bond each course by half a block over the one below, and tie into existing walls with wall starter kits.
  • Blocks that will show in a garden are usually rendered or clad. Allow for render and a coping on top.

Mistakes to avoid

  • Forgetting the courses below ground between the footing and ground level.
  • Laying blocks that are wet through, which makes the mortar slump and the wall move out of line.
  • Ordering the exact count with nothing for cuts at corners and openings.
  • Building load-bearing or extension walls without checking Building Regulations and the structural design.

Frequently asked questions

How many blocks are in a square metre?

About 10 standard 440 x 215mm concrete blocks per m² with 10mm joints.

How many blocks do I need for a garage wall?

Multiply the wall length by the height and take off the door and window openings, then multiply by 10 and add 5%. A 5 x 2.4m wall with no openings needs about 126 blocks.

How much mortar do I need for 100 blocks?

100 blocks is about 10 m² of wall. At 0.012 m³ per m² plus 10%, that is about 0.13 m³: 2 bags of cement and about 0.2 tonnes of building sand at a 1:4 mix.

What mortar mix should I use for blocks?

A 1:4 cement to building sand mix suits most general blockwork. Use 1:3 below the damp-proof course or in very exposed places.

Sources

Quantities are estimates for typical jobs, using the coverage figures above. Products vary, so check the data sheet for the product you buy, and talk to a professional for structural, electrical or gas work. SmartBuild is independent and not affiliated with any of the manufacturers listed.

How many bricks do I need for a wall?To work out how many bricks you need, measure the face of the wall, multiply by the number of bricks per square metre for the wall thickness and add a little for cuts and breakages. Then work out the mortar to lay them. How much sand and cement do I need for bricks, blocks or slabs?Work out the volume of mortar first, then split it into cement and sand by the mix. Here are the standard UK rates for brickwork, blockwork and laying paving slabs. How much concrete for a shed base?Make the base a little bigger than the shed, work out the volume, and convert it to bags of cement and ballast for a 1:5 mix.

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