How to work out how much concrete you will need

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The Concrete Calculator.

 

Ordering how much concrete you need for a Concrete Slab is a very easy task.

I will demonstrate this in both Imperial and Metric in the same size slabs.

Imperial.

Size Of Slab:-

Example One:-  20 feet x 10 feet x 4 inches thick.

Example Two:- 6 metres x 3 metres x 100 mm thick.

The Formula is Length x Width x Depth (Thickness)

Step 1.

20 feet x 10 feet = 200 square feet.

This is your measurement of concrete in square feet, but you still need to find the volume, this is worked out by multiplying the square feet by the thickness.

Step 2.

4 inches (Slab thickness) = 1/3 of 1 foot, which equals .3333 feet.

Step 3.

200 square feet x .3333 feet = 66.66 cubic feet.

Step 4.

As concrete is always ordered by the Square Yard, you need to divide the Cubic Feet by 27, as there is 27 Cubic Feet in a Cubic Yard (3 feet x 3 feet x 3 feet)

66.66 Cubic Feet, divided by 27 = 2.46 Cubic Yards.

Very Important:- Always bear in mind when ordering Concrete, always make allowances up or down for any wastage, variations in depths, pipes penetrating the slab etc.

Metric.

You have a slab 6 metres x 3 metres x 100 mm you want to pour.

The Formula is Length x Width x Depth (Thickness)

Step 1.

6 m x 3 m = 18 square metres.

This is your measurement of concrete in square metres, but you still need to find the volume, this is worked out by multiplying the square metres by the thickness.

Step 2.

100 mm (slab thickness) = 1/10 of 1 metre, which equals .1 metres.

Step 3.

18 square metres x .1 metres = 1.8 cubic metres.

Step 4.

This is not applicable in the Metric system, as every measurement works in units of mm, unlike 12 inches in a foot and 3 feet to a yard.

Very Important:- Always bear in mind when ordering Concrete, always make allowances up or down for any wastage, variations in depths, pipes penetrating the slab etc.

So, as a matter of interest, the two same size slabs of the same thickness in Imperial and Metric are 2.46 Cubic Yards and 1.8 Cubic Metres. 

The Drying / Curing Time.

The final strength of your concrete is what you do during the curing process.  The concrete has to cure as slowly as it can, and nothing you can do can rush the process.

You may have noticed when large area slabs of maybe 6 – 8 inches (150 – 200 mm) have been poured, they are covered with hessian and sprayed with water, this mainly happens in areas of hotter climates.

Slow cured concrete is the best concrete, its better than loosing the strength in your concrete and maybe having the slab crack.

 

If you are reading this in the United States a good reference is also at B4ubuild.

Happy Concreting.