How Many Concrete Bags needed For Your Slab Size ? | Calculations Explained

Determining the number of concrete bags for a slab is somewhat challenging but is quite possible when you follow steps correctly. The number of concrete bags depends on the size of slab (length and width), thickness of slab, and size of the substantial bag you choose.

The process to estimate the concrete bags’ exact count is pretty cumbersome. The number of bags varies with the size of the slab as well as the concrete bag size. Let us go through the steps you need to follow for determining the bag count for any given slab of any height, width, or length.

What Is The General Evaluation Process To Determine Number Of Concrete Bags For A Specific Slab Size?

Following are the steps that will give you an accurate count of concrete bags needed for your slab –

Step 1: Find Volume Of Slab

The foremost step is to find out the slab’s volume in cubic feet. Suppose M x N is the size of the slab having D as the thickness or depth of the slab. The standard thickness is 4 inches, but some slabs have a depth of 5 inches, 6 inches, or 8 inches. However, you need to convert inches of depth to cubic feet before calculating the volume.

The formula to calculate the volume of the slab is –
Volume Of Slab = (Size Of Slab in Length And Width) x Depth/Thickness of slab

Thus,
Volume of slab = M x N x D
With this, you will get the volume result in cubic feet. In the United States, one cubic yard is the unit measurement of the volume represented by 3 feet long by 3 feet depth by 3 feet wide based on imperial and US customary size.

So, volume = 3 * 3 * 3 = 27 cubic feet. One cubic yard is equal to 27 cubic feet.

Step 2: Convert Cubic Feet Volume Result Into Cubic Yard

To convert cubic feet into cubic yards, all you need to do is divide the volume in cubic feet by 27 (the volume of the cubic yard in cubic feet). Mathematically,
The volume of M x N slab in the cubic yard at D thickness = Volume Found in the above step divided by 27. I.e. V / 27

Step 3: Calculate Cubic Yards Of All Sizes Of Concrete Bags

As we all know, concrete bags are available in various sizes, such as 40 lbs, 50 lbs, 60 lbs, 80 lbs, and 90 lbs; one bag comprises different cubic feet. Thus to find out the cubic feet in concrete bags of different sizes is quite easy with the calculation below –

Cubic Feet in S size of concrete bag = S (in lbs)/133 lbs.
Here, 133 lbs. is the standard weight of concrete per cubic foot.
So, you have got the cubic feet of S size concrete bags that you need to convert into cubic yards. For this, all you need to do is divide it by 27 cubic feet shown below,

Cubic yard Of S size concrete bag = cubic feet in S size concrete bag divided by 27 or V / 27

Step 4: Divide The Volume Of Slab In Cubic Yard By Cubic Yard Of Concrete Bag’s Size

It is the last step to get the number of concrete bags you need for your slab size. To calculate the number of concrete bags, divide the Volume of the slab in the cubic yard that we got in step 2 by the cubic yard of the S size concrete bag obtained from step 3.
So,
Number of concrete bags = Volume of the slab in cubic yard/Cubic yard of S size concrete bag

Thus, this is the four-step process you must follow to evaluate the number of S-size concrete bags for your M x N size slab having D thickness.

Let us get a more precise idea by taking different slab sizes examples. Various slabs are constructed, such as 2×2, 4×4, 8×8, 4×8, 10×8, 10×10, etc.

How Many Concrete Bags Do You Need For 4 x 4 Slab?

To find out the number of concrete bags for 4 x 4 slab, let us follow the steps one by one and get the accurate result –

As per step 1 –

4 x 4 slabs may have 4 inches, 5 inches or 6 inches. So by converting these thickness values in feet, we will get –
4 inches = 0.33 feet
5 inches = 0.416 feet
6 inches = 0.5 feet

Evaluating the volume of the slab in cubic feet having different depth –
Volume of 4 x 4 slab having 4 inches thickness will be 4 * 4 * 0.33 = 5.28 cubic feet; 5 inches thickness will be 4 x 4 x 0.416 = 6.656 cubic feet; and 6 inches thickness slab volume will be 4 x 4 x 0.5 = 8 cubic feet.

Converting cubic feet Value in cubic yard according to step 2,

Volume of a 4 x 4 slab in a cubic yard having four inches depth will be 5.28/27 = 0.195 cubic yard; five inches depth will be 6.656/27 = 0.246 cubic yard; and six inches depth will be 8/27 = 0.296 cubic yard.

Calculating cubic yards of every size of concrete bag available with the formula mentioned in step 3,

First of all, you have to find cubic feet in different sizes of concrete bags such as –
Cubic feet in S size concrete bags will be as follows,
40 lbs. bag = 40/133 = 0.3007 cubic feet
50 lbs. bag = 50/133 = 0.375 cubic feet
60 lbs. bag = 60/133 = 0.451 cubic feet
80 lbs. bag = 80/133 = 0.601 cubic feet
90 lbs. bag = 90/133 = 0.676 cubic feet

After evaluating cubic feet in S size bags, convert those into cubic yards by dividing it by 27.
40 lbs bag = 0.3007/27 = 0.011 cubic yard
50 lbs bag = 0.375/27 = 0.0138 cubic yard
60 lbs bag = 0.451/27 = 0.0167 cubic yard
80 lbs bag = 0.601/27 = 0.022 cubic yard
90 lbs bag = 0.676/27 = 0.025 cubic yard

These values are the same for any size slab every time you calculate. So you can directly refer to the above values.

As per last step 4,

All you need to do is divide the cubic yard we obtained from step 2 by cubic yard in all S size concrete bags,
Several concrete bags for 4 x 4 slab having D inches thickness for S size slab is Volume of 4 x 4 slab having D inches thickness in the cubic yard/cubic yard of S lbs bag.

For 4 inches, concrete bags count for S size will be –
40 lbs bag = 0.195/0.011 = 17.72 bags
50 lbs bag = 0.195/0.0138 = 14.13 bags
60 lbs bag = 0.195/0.0167 = 11.67 bags
80 lbs bag= 0.195/0.022 = 8.86 bags
90 lbs bag = 0.195/0.025 = 7.80 bags

Similarly for 5 inches, count of S size concrete bags will be –
40 lbs. bag = 0.246/0.011 = 22.36 bags
50 lbs. bag = 0.246/0.0138 = 17.80 bags
60 lbs. bag = 0.246/0.0167 = 14.70 bags
80 lbs. bag = 0.246/0.022 = 11.18 bags
90 lbs. bag = 0.246/0.025 = 9.84 bags

For 6 inches thickness of 4 x 4 slab, number of concrete bags of every size will be –
40 lbs = 0.296/0.011 = 26.90 bags
50 lbs = 0.296/0.0138 =21.44 bags
60 lbs = 0.296/0.0167 = 17.72 bags
80 lbs = 0.296/0.022 = 13.45 bags
90 lbs = 0.296/0.025 = 11.84 bags

This way, you can find out the number of concrete bags where there is no need to evaluate step 3 again and again when done. So, above was a brief explanation of steps for 4 x 4 slab size. We will check out the concrete bags count for more slab sizes following steps directly with the standard 4 inches thickness.

How Many Concrete Bags Do You Need For 8 x 8 slab?

To determine the number of concrete bags for a 8 x 8 slab, first, you will have to evaluate the Volume of the slab in cubic feet and convert it into a cubic yard. You can skip step 3 and jump to step 4 to determine the concrete bags count.

The Volume of an 8 x 8 slab having 4 inches depth will be 21.12 cubic feet; having 5 inches depth will be 26.62 cubic feet, and having 6 inches depth will be 32.00 cubic feet. Converting cubic feet into the cubic yard will give 0.78 cubic yards for 4 inches thickness.

By skipping step 3, the number of 40 lbs concrete bags having 4 inches thickness will need 70.9 sacks; 56.50 bags of 50 lbs size; 46.70 bags of 60 lbs size; 35.45 bags of 80 lbs size; and 31.20 bags of 90 lbs size.

How Many Concrete Bags Do You Need For 10 * 10 Slab?

The Volume of 10 x 10 slabs having 4 inches will be 33.00 cubic feet, and after converting it into the cubic yard, it will be 1.22 cubic yards.

The number of 40 lbs concrete bags needed for 10 x 10 slabs will be 110.90 bags; 88.40 bags of 50 lbs size; 73.05 bags of 60 lbs size; 55.45 bags of 80 lbs size, 48.80 bags of 90 lbs.

How Many Concrete Bags Are Needed For 4 by 8 Slab?

The Volume of 4 x 8 slab having 4 inches (0.33 cubic feet) will be 10.56 cubic feet, and converting it into a cubic yard will give 0.39 cubic yards.

The number of 40 lbs concrete bags needed for 4 x 8 slabs will be 35.45 bags; 28.26 bags of 50 lbs size; 23.35 bags of 60 lbs size; 17.72 bags of 80 and 15.60 bags of 90 lbs size.

How Many Concrete Bags Do You Need For 12 x 12 Slab?

The Volume of a 12 x 12 slab having 4 inches thickness will be 47.52 cubic feet or 1.76 cubic yards. Thus, for 12 x 12 slabs, you will need 160 bags of 40 lbs size; 127.53 bags of 50 lbs; 105.38 bags of 60 lbs; 80 bags of 80 lbs size; and 70.40 bags of 90 lbs size.

How Many Concrete Bags Do You Require For 10 x 12 Slab?

The Volume of a 10 x 12 slab having 4 inches thickness will be around 39.60 cubic feet or 1.466 cubic yards. Hence, you will need 133.27 bags of 40 lbs size; 106.23 bags of 50 lbs size; 87.78 bags of 60 lbs; 66.63 bags of 80 lbs; and 58.64 bags of 90 lbs size.

How Many Concrete Bags Do You Need For 10 x 8 Slab?

The Volume of 10 x 8 slabs having 4 inches thickness will be around 26.4 cubic feet or 0.97 cubic yards. So, you will need 88.18 bags of 40 lbs size; 70.28 bags of 50 lbs; 58.08 bags of 60 lbs; 44.09 bags of 80 lbs; and 38.80 bags of 90 lbs size.

How Many Concrete Bags Do You Need For 10 x 20 Slab?

The Volume of a 10 x 20 slab having 4 inches thickness will be about 66 cubic feet or 2.44 cubic yards. Therefore, you will need 221.81 bags of 40 lbs size; 176.81 bags of 50 lbs size; 146.10 bags of 60 lbs size; 110.90 bags of 80 lbs size; and 97.60 bags of 90 lbs size.
More slab sizes differ according to the requirement. If you follow all these steps one by one carefully, you will get the perfect count of concrete bags required for your slab.