Engineers, site managers, and quantity surveyors are always faced with the challenge of specifying as accurately as possible, the quantity of materials needed to execute a specific item of work. In this article, we are going to explain how you can estimate the quantity of mortar (cement and sand) needed to lay blocks per square metre of wall.

It is well known that we need about 10 blocks to build one square metre of wall.

Simple Proof;

Area of wall = 1 m^{2}

Planar area of one standard block (Nigeria) = 450mm x 225mm = 0.101 m^{2}

Therefore, the number of blocks required (disregarding mortar) = 1/0.101 = 9.99 (say 10 blocks)

Now, how do we estimate the quantity of mortar needed to lay the blocks? First of all, let us look at the dimensions of a typical 9 inches block with holes (the most popular block for building in Nigeria).

From the image above, we can say that the cross-sectional area of the block that receive horizontal mortar is;

A_{b} = (0.45 x 0.225) – 2(0.15 x 0.125) = 0.0637 m^{2}

Now, let us assume that the mortar will be 25 mm (1 inch) thick

The typical arrangement of blocks in a one square metre wall is shown below;

Therefore, we can estimate the volume of mortar required to build one square metre (1 m^{2}) of wall as follows;

Vertical mortar = 8 x (0.025 x 0.225 x 0.275) = 0.0123 m^{3}

Horizontal mortar = 10 x (0.0637 x 0.025) = 0.0159 m^{3}

Total = 0.0282 m^{3}

Therefore the volume of mortar required to lay one square metre (1 m^{2}) of 9 inches block (with hole) can be taken as 0.03 m^{3} for all practical purposes.

To go further, let us assume that we have 150 m^{2} of wall, and we want to estimate the quantity of cement and sand needed to lay the blocks.

The volume of mortar required = 0.03 x 150 = 4.5 m^{3}

Typical mix ratio for mortar (laying of blocks) is 1:6

**Cement requirement**

Quantity of cement required = 1/7 x 1440 kg/m^{3} = 205.71 kg

Making allowance for shrinkage between fresh and wet concrete = 1.54 x 205.71 = 316.79 kg

Quantity of cement required in bags = 316.79/50 = 6.33 bags

For 4.5m^{3} of mortar;

Provide = 4.5 x 6.33 = 29 bags of cement + allowance for waste as appropriate.

*Sand*

Quantity of sand required = 6/7 x 1600 kg/m^{3} = 1371.428 kg

Making allowance for shrinkage between fresh and wet concrete = 1.54 x 1371.428 = 2112 kg

For 4.5m^{3} of mortar;

Provide = 4.5 x 2112 = 9504 kg of sharp sand + allowance for waste as appropriate.

Therefore you need about 10 tonnes of sharp sand and 29 bags of cement for laying 150m^{2} of 9 inches block wall.

Thank you for visiting Structville today, and God bless.

Nice post.

Dry volume = 1.54 x Wet volume for concrete

Dry volume = 1.35 x Wet volume for mortar please confirm a lot of arguments on this

This is an awesome one bro.

It makes sense but truth still remains that no one will accept that on site.

The site standard we know is that 1 bag of cement should lay about 50 blocks. This calculation basically confirms that.

No client will accept this, this is one of the problems of Engineering in Nigeria. Your estimation is according to standard, but that's only for Company Jobs where a workman is entitled to just 70 blocks per day.

No common man will accept this, before you know it, one person that doesn't even know standard will come and quote nonsense.

Kudos bro

Good job Brother. I truly love your analysis. It’s practically correct and applicable in sites in Nigeria.

Let’s check to see if this is possible.

By your calculation, 29 bags of cement is required to build 150m² wall. It’s true because in 150m² we have 1500 blocks, since the area of one block is 0.10m².

Now dividing 1500 by 29 the answer is 51 block. It means 1 bag of cement can build 51 block which to Me its ok!

But my question is, for 1m² you gave 8 vertical portions of mortal and 10 for horizontal. For horizontal, I have no problem since there 10 blocks per square metre. But for vertical is there a formula that relate?

God bless you greatly Sir.

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I think this calculation does not take into account the practical application.

If 1 bag of cement is to be mixed with 12 head pans of sand.

32 bags of cement will be mixed with 384 head pans of sand. Using the 1: 6.

Now you said for 32 bags, 10 tonnes of sand should be ordered, 10 tonnes of sand is 40 wheel barrow, i.e 160 head pans of sand.

That’s my view, I want you to shed more light on this.

Thanks.

Hi Raheem,

Look at it this way. Our basic calculation was 9504 kg of sand and 29 bags of cement (that is 1450 kg of cement).

Density = Mass/Volume

For sand; 1600 = 9504/Volume (Hence volume of sand = 5.94 cumecs)

For cement; 1440 = 1450/Volume (Hence volume of cement = 0.99 cumecs)

Therefore, you can see that the ratio of 1:6 is in order. Kindly confirm that 10 tonnes of sand is equal to 40 wheelbarrows.

This is because the volume of 10 tonnes of sand is 6.25 cumecs

The volume of a standard builder’s wheel barrow = 0.065 cumecs

Therefore, there are about 96 unheaped wheelbarrows in 10 tonnes of sand, which is 384 headpans of sand.

So I think we are very much in order.

Kindly look at the calculation again. I think there is a problem. If 4.5m3 is required to lay 150m2 of 225mm blocks, I think to calculate the cement and sand contents should be

Cement 1/7×4.5=0.643m3 x 1440=925.92kg/50kg= 18.5bags

Sand 6/7×4.5=3.857m3 x 1450=5592.65kg

Approximately one bag of cement lays 80 blocks.

Thanks

I tend to agree with your calculation

Hi Adewunmi,

Remember that I applied a factor of 1.54 to account for shrinkage between fresh and dry and mortar, and you didn’t do that. That is just the difference in our result.

Thank you for this suitable article about how to calculate the quantity of mortar for laying blocks, it will help me and people like me looking for the same. I appreciate your effort for taking time to do your research and present these details before us. Really nice way to present this content, very appreciative!!

Well presented analysis. Very helpful.