How to Calculate Quantities of Cement, Sand, Granite for Concrete Foundation from Building Plan

To begin this calculation you should know the concrete strength class you want to achieve and in turn the concrete mix you are using. The right concrete mix depends on the cement grade that you are using. The minimum concrete strength class recommended for the construction of the load-bearing building structural members is concrete strength class 20/25. In the Nigerian open market, the two cement grades/strength classes that are available and which are used for building construction are cement grade 32.5 and cement grade 42.5.

To produce concrete with strength class 20/25 which is the minimum concrete strength class recommended for the construction of the load-bearing building structural members, you should use one of the following concrete mix ratios/cement grade pairs:

1. A mix-ratio of 1:1.5:3 using Portland-limestone cement grade 32.5 to produce concrete strength Class 20/25
2. A mix-ratio of 1:2:4 using Portland-limestone cement grade 42.5 to produce concrete strength Class 20/25

What is Concrete Mix Ratio?

Concrete mix ratio simply means the amount by volume of cement, fine aggregate (which can be sharp sand) and coarse aggregate (which can be granite, gravel, etc). For example, a concrete mix ratio of 1:2:4 simple means 1 head-pan of cement you need to mix 2 head-pans of sand and 4 head-pans of granite. And a concrete mix ratio of 1:1.5:3 simple means for 1 head-pan of cement you need to mix 1.5 head-pans of sand and 3 head-pans of granite.

Estimating the Quantities of Cement, Sand, and Gravel from Building Plan

Generally, cement grade 42.5 represents a higher strength cement than cement grade 32.5. Hence, we will use 42.5 Portland-limestone cement grade and a concrete mix ratio of 1:2:4. Follow as we proceed to estimate the quantities of cement, sand, and granite that we need.

Calculate the Volume of Concrete

To know the quantity of cement, sand, and granite that you need for a foundation, you need to know the volume of concrete.
Foundation thickness = 150 mm = 0.15 m (which is the recommended minimum).
Foundation width = 450 mm= 0.45 m (which is the minimum recommended for a firm soil)
Foundation length from the plan above = 39.45 m (see how to calculate the total length of the foundation from building plan).
Volume of Concrete = length x thickness x width = 39.45 m x 0.15 m x 0.45 m = 2.6629 m³

Calculate the Volume of Cement in the Concrete

To get the volume of cement in the concrete using mix ratio 1:2:4,

• Add the numbers in the mix-ratio together

Total parts in the mix-ratio = 1 + 2 + 4 = 7.

• Divide the volume of the concrete by the total parts in the mix-ratio

Volume of cement = 2.6629 m³ ÷ 7 = 0.3804 m³

Calculate the Number of Bags of Cement you Need

Density of 1 cement bag (50 kg) = 1440 kg/m³
Density = mass/volume (ρ = m ÷ V)
V = m ÷ ρ
V= 50 kg ÷ 1440 kg/m³
Volume of one bag of Cement=0.0347 m³ = 0.035 m³
To get the numbers of bags of cement that you need simply divide the volume of cement in concrete by the volume of one bag which is 0.035 m³.
0.3804 m³÷0.035 m³ = 10.8685 = 11 bags of cement
Adding 1 bag of cement which is 10% of 11 bags of cement as wastage, we get the total number of bags of cement needed for the sample plan as:

You need 11 bags + 1 bags = 12 bags of cement.

Calculate the Quantity of Sand you Need

Recall that from our mix ratio for 1 part cement, you need 2 parts of sand. That is, for 0.3804 m³ of cement, you need:

0.3804 m³ x 2 = 0.7608 m³ of sand.
The dry loose bulk density (DLBD) of sand = 1762 kg/m³.

The quantity of sand that you need = 0.7608 m³ x 1762 kg/m³ = 1217.28 kg of sand (about 1.5 tonnes).

Calculate the Quantity of Aggregate (Gravel or Granite) you Need

Remember that from our mix ratio for 1 part cement, you need 4 parts of granite. That is, for 0.3804 m³ of cement, you need:

0.3804 m³ x 4 = 1.5216 m³ of gravel.
Bulk Density of Gravel= 1762 Kg/m³

The quantity of Gravel you need = 1762 Kg/m³ x 1.5216 m³ = 2681.06 kg of gravel (about 3 tonnes).

Quantity of Water that is Required

Using a water/cement (W/C) ratio of 0.50,
50 kg of Cement will require = 50 kg * 0.5

Quantity of Water required = 25 kg = 25 litres.