How do you calculate the number of blocks required?

When undertaking any construction project, it’s important to accurately calculate the number of blocks required. This ensures you purchase the right quantity so there are no shortages or wastage. There are a few key steps involved in working out block requirements correctly:

1. Determine the Dimensions of the Structure

Firstly, you need to confirm the dimensions of the structure you are building. This includes the length, width and height of each wall section. For example, if building a small garden wall that is 2 meters long and 1 meter high, the measurements would be:

  • Length: 2 meters
  • Height: 1 meter

For larger structures like houses, the dimension calculations may be more complex. It’s important to take accurate measurements of each exterior wall section. An architectural plan can provide precise measurements if available.

2. Choose the Block Type and Size

Next, determine what size and type of blocks will be used. Common options include:

  • Concrete blocks – typically 400mm x 200mm x 200mm (length x height x width). Density is normally between 1600 – 2000kg/m3.
  • Brick blocks – often 230mm x 110mm x 76mm. Density is around 1600kg/m3.
  • Lightweight blocks – can be larger like concrete blocks but with lower density, around 500kg/m3.

The block dimensions and density will impact calculations, so be sure to confirm these details.

3. Calculate the Number of Blocks Needed for Each Section

With the measurements and block specifications confirmed, you can now estimate the number of blocks required for each wall section. The calculations will depend on the wall layout, but in general the process is:

  1. Calculate the total length of the wall section in meters – e.g 5m long.
  2. Divide the height by the block height to get the number of courses – e.g 2m high / 0.2m block height = 10 courses.
  3. Multiply the length by the number of courses to determine the total blocks needed – e.g. 5m x 10 courses = 50 blocks.

Repeat these calculations for every wall section in the project. Remember to round up the number of blocks to allow for offsets and wastage.

4. Add up the Total Blocks

Once you’ve calculated the blocks needed for each section, add up these quantities to determine the total number required across the entire project. This gives you the minimum number of blocks to purchase.

As an example, if calculating blocks for a small building with:

  • Front wall – 60 blocks
  • Back wall – 50 blocks
  • Left side – 40 blocks
  • Right side – 45 blocks

The total blocks required would be 60 + 50 + 40 + 45 = 195 blocks.

5. Add a Contingency Margin

It’s smart to add an extra 5-10% contingency margin onto your total block calculation. This caters for any blocks broken during construction or cut downs required on-site. It avoids running short if some wastage occurs.

So if the total was 195 blocks, add 10% contingency which is 20 blocks. Therefore, the final quantity to purchase would be 195 + 20 = 215 blocks.

Key Tips

Follow these tips for an accurate block calculation:

  • Take detailed measurements of all dimensions needed.
  • Confirm the exact block specifications being used.
  • Double check your math calculations.
  • Round up quantities to allow for offsets and cutting.
  • Add a safety margin of surplus blocks.

Block Calculation Example

Let’s look at a detailed example of calculating blocks needed for a small building project.

Project Details

A garden shed will be constructed according to the following specifications:

  • External dimensions: 3m wide x 4m long x 2.2m high
  • Concrete block dimensions: 400mm x 200mm x 200mm
  • One standard door opening: 900mm x 2000mm
  • One window opening: 600mm x 600mm

Wall Sections

There are 4 exterior wall sections that require blocks:

Section Length (m) Height (m)
Front 3 2.2
Back 3 2.2
Left side 4 2.2
Right side 4 2.2

Door Opening Dimensions

The door opening on the front wall measures:

  • Width: 900mm
  • Height: 2000mm

As the standard door height spans 10 courses of blocks, the opening equals approximately 9 blocks (0.9m width x 10 courses).

Window Opening Dimensions

The window opening on the back wall measures:

  • Width: 600mm
  • Height: 600mm

With 3 block courses, the window opening is equivalent to 2 blocks (0.6m x 3 courses).

Block Calculations

Let’s calculate the number of blocks needed for each wall section:

Section Length (m) Courses Blocks Needed
Front 3 2.2 / 0.2 = 11 3 x 11 – 9 = 24
Back 3 2.2 / 0.2 = 11 3 x 11 – 2 = 29
Left side 4 2.2 / 0.2 = 11 4 x 11 = 44
Right side 4 2.2 / 0.2 = 11 4 x 11 = 44

Total blocks required = 24 + 29 + 44 + 44 = 141

Add 10% contingency margin:

0.1 x 141 = 14 extra blocks

Total blocks to purchase = 141 + 14 = 155


Calculating the number of blocks needed for a construction project requires some careful measurements, math calculations and safety margins. By following the step-by-step process, you can accurately estimate the block requirements so you don’t run short or end up with excess wastage on-site. Taking the time to do the block quantity calculations upfront can save time, money and headaches compared to just guessing how many you might need. With the right preparation, your next building project using concrete, brick or other block types can be a great success.

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