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What mix 1:2:4 means, when you see such specification it simply means, ratio of cement to sand and to granite, that is, 1 pan of cement mixed with 2 pan of sand mixed with 4 pan of granite.

Note: 1 bag of cement contains 2 head pan, so if you are to mix 1 bag of cement at once, it will be 1 bag of cement with 4 headpans of sand mix with 8 head pans of granite.

Another example is mix 1:3:6, which means 1 pan of cement mixed with 3 pan of sand mixed with 6 pan of granite.

Note: 1 bag of cement contains 2 head pan, so if you are to mix 1 bag of cement at once, it will be 1 bag of cement with 6 headpans of sand mix with 12 head pans of granite.

*How to Calculate the Total Bags of Cement for a Particular Volume of Concrete simplified version*

As an individual that wants to build a house or any other structural units, you must know the amount of cement require for a particular volume of concrete, knowing this will prevent contractors or bricklayers from extorting you. I will explain to you how to calculate the amount of cement require for any volume of concrete.

Amount of cement = V * k
Where V= volume of concrete
K= Multiplication constant
And the Multiplication factor can be gotten from the picture attached, by checking the mix design that you are using and picking the constant K.

Example. A concrete of 4 meters long, 2 meters tall and 0.225 width of 0.225 meters. Using M15 (1:2:4).

Solution
Amount of cement = V * k
V= length * height *width
= 4 * 2 * 0.225 = 1.8 m3
K= 6.336 ( from the table in the picture)

Bags of Cement require = 1.8 * 6.336 = 11.4048 bags.

How to Calculate the Total Bags of Cement for a Particular Volume of Concrete
As an individual that wants to build a house or any other structural units, you must know the amount of cement require for a particular volume of concrete, knowing this will prevent contractors or bricklayers fro extorting you. I will explain to you how to calculate the amount of cement require for any volume of concrete.
There are different mix design which varies according to the type or purpose of the structure. The calculations below is all about M15 grade ( 1:2:4 mix ratio) of Concrete.

Calculations
TOTAL RATIO = 1+2+4
= 7
Since:
Cement = 1
Sand = 2
Granite = 4

We are dealing with cement, we have 1/7 = 0.143

For 1m³ of concrete = 1m³ * 0.143
= 0.143m³

Addition of waste and shrinkage 54% = (54/100 * 0.143) + 0.143
= 0.0772m³ + 0.143m³
= 0.22m³

Density = Mass/Volume
Constants: Density of cement = 1440 Kg/m³
Mass of 1 bag of cement = 50 kg

1440 = 50/Volume
Volume = 0.03472m³

Calculating for cements required for 1m³ cement Concrete only
1 Bag of Cement = 0.03472m³
Y Bags of Cement = 1m³

Y = 1/0.03472
Y = 28.8 bags of Cement
1m³ of cement concrete only= 28.8 bags

So we can calculate for M15 grade ( Mix design 1:2:4)
0.22m³ of cement = X bags
1m of cement = 28.8 bags

X bags = 28.8 x 0.22
X = 6.336 bags of cement ( should not b up to 6.5 bags of cement)

Example. A fence of 6 meters long, a height of 1.5 meters and a width of 0.225 meters. Calculate the cement required for a grade M15 concrete( 1: 2 :4) .
Volume of the concrete= 6 x 1.5 x 0.225 = 2.025 m³

TOTAL RATIO = 1+2+4= 7
Since:
Cement = 1
Sand = 2
Granite = 4

We are dealing with cement, we have 1/7 = 0.143

For 2.025m³ of concrete = 2.025³ * 0.143
= 0.289575m³

Addition of waste and shrinkage 54% = (54/100 * 0.289575) + 0.289575
= 0.1563705 + 0.289575m³
= 0.4459455m³

0.4459455m³ of cement = X bags
1m of cement = 28.8 bags

X bags = 28.8 x 0.4459455
X = 12.0432 bags of cement