How Many Bags Of Concrete in A Square Foot ? | Every Size

Evaluating the number of concrete bags for one square foot or the total square feet available varies with the height or depth decided for slabs, footings, etc.

For instance, you will require 160 bags of concrete for 144 square feet slab (12 by 12 ) having 4 inches depth. Height/ depth varies from 4 inches to 12 inches.

We all know that square footage is calculated by multiplying length and width or dividing the volume in cubic feet by the height. Thus, there are two methods to calculate the bags of concrete in a square foot discussed further.


But before that let us have a look at the table below listing the number of concrete bags in given square feet having various heights.

SizeHeight/Depth40 lbs50 lbs60 lbs80 lbs90 lbs
1 sq.ft.4”1.104 1.132 0.72 0.552 0.49
1 sq.ft.6”22211
1 sq.ft.8”22211
1 sq.ft.10”33222
1 sq.ft.12”43322
4 * 46”2722181412
8 * 84”7257483632
10 * 108”22317614911299
12 * 124”1611281078171
10 * 2012”667531445334295
800 sq.ft.8”177814141185889786
1000 sq. ft6”166713251112834736
600 sq.ft12”2000159213341000883
20*304”667530445334295

How To Calculate The Count Of Concrete Bags In A Square Feet?

There are two methods to evaluate the number of bags. Hence you decide which method to choose that you think is easier.

The only difference in both methods is the area found in square feet and cubic feet. Follow the detailed procedure step by step carefully to get an accurate answer.


Method 1:

Go for this method to get the square feet in every size of concrete bags with the exact number of bags for required square feet. Again, here you have two options for this method; let us see –

Option 1:

Step 1: Find Out The Area By Its Length And Width

You already have the length and width of the slab or footing; all you need to do is find out the area. You will get the result in square feet. Skip this step to find the number of bags required in one square foot.

Area = Length (ft) * Width (ft) = A (result in square feet)

Step 2: Find Out The Height/ Depth And Convert It To Feet

Determine the height or depth you are planning for your construction. It varies from 4 inches to 6 inches for the standard slab, 8 inches to 12 inches for the standard footing, and 4 inches to more than 8 inches for driveways, depending on the choice and plan.

After finding the depth in inches, convert it into feet. As 1 inch = 0.083 feet; you need to multiply the given inches to 0.083 –

Depth in feet = depth in inches * 0.083

For instance,
4 inches height will be,
Height in feet = 4 * 0.083 = 0.332 feet

Step 3: Evaluate Cubic Feet Coverage In Each Size Of Concrete Bag Available

You cannot get the square feet coverage directly; hence, you will first determine the cubic feet in S size concrete bags. We know that one concrete bag contains 133 lbs weightage, so simply divide the size of the concrete bag by 133.

Cubic feet in S size concrete bag = S / 133

For example,
Cubic feet in 40 lbs concrete bag = 40 / 133 = 0.3008 cubic feet

*Note* – You can skip this step once calculated for all sizes of the concrete bag as this will remain the same.

Step 4: Calculate Area

When you have length and width in feet with you, determine the area by the basic formula. It will give you an answer in square feet for which you want to find out the count of concrete bags.

Area = length * width

Step 5: Determine Square Feet In Each Bag

It is time to determine the square feet coverage in every S-size concrete bag. Dividing the cubic feet result obtained in the above step by height in feet will give you the square footage of S size bag.

Square feet in S size concrete bag = cubic feet in S size concrete bag / depth in feet.

For instance,
Finding square feet in 40 lbs bag having 4 inches height,
Square feet in 40 lbs = 0.3008 cubic feet / 0.332 feet height = 0.906 square feet

Step 6: Calculating Number Of Concrete Bags In Square Feet

Now all you need is to divide the total square feet obtained in step 4 by square feet in S size concrete bags obtained in above step four to get the exact count of bags you need.

Number of S size bags for X square feet area = X / square feet in S size bag

Option 2:

Follow every step discussed above for method one except step five. Instead of step 5 discussed above, you can go for the below two steps –

Step 6: Calculating Number Of Concrete Bags For One Square Feet

Here, you will have to find out the count of S size concrete bags for one square foot having d depth –

Count of S size concrete bag = 1 / square feet in S size bag (obtained in step 4 above)

For instance,
Finding out the number of 40 lbs in a square foot having 4 inches depth,
Number of 40 lbs bags = 1 / 0.906 (square feet in 40 lbs bags) = 1.104 bags

Step 7: Multiply Concrete Bags Count With The Given Square Feet Coverage

You simply need to multiply the found number of bags in one square foot to the total area in square feet. Suppose you have 100 square feet, you will need –

Number of 40 lbs concrete bags for 100 square feet = 1.104 (obtained in above step) * 100 (given total square feet) = 110.4 bags.

Method 2:

You can also go with this method, where you need to multiply the area in square feet with the planned height to get the total volume in cubic feet. But the first four steps are the same as method one. Let us discuss after those three steps –

Step 5: Determining The Volume

You will be required to evaluate the volume of the entire slab, footings, or driveway by multiplying the area obtained in step four to the h height in feet.

Volume = Area * h (height)

Step 6: Finding The Exact Count Of Bags

This is the final step where you need the values or result found in step 3 and step 5 of this method.

Number of S size concrete bags = Volume / cubic feet in S size bag

How Many Bags Of Concrete Needed In 12 * 12 Slab?

We will try with both methods individually step by step to find the number of different sizes concrete bags needed in 12 * 12 slab having 6 inches height –

Method 1:

Let’s start with determining length, width and height already given –
Length = 12 feet
Width = 12 feet
Height = 6 inches

Converting height in inches to feet,
Height = 6 * 0.083 = 0.5 feet

Finding cubic feet in S sizes concrete bags available in the market,
Cubic feet in 40 lbs bag = 40 / 133 = 0.3008
Cubic feet in 50 lbs bag = 50 / 133 = 0.376
Cubic feet in 60 lbs bag = 60 / 133 = 0.451
Cubic feet in 80 lbs bag = 80 / 133 = 0.602
Cubic feet in 90 lbs bag = 90 / 133 = 0.677

Checking the area of slab,
Area = length * width = 12 * 12 = 144 square feet

Evaluating square feet in S size concrete bags,
Square feet in 40 lbs bag = 0.3008 / 0.5 = 0.602
Square feet in 50 lbs bag = 0.376 / 0.5 = 0.752
Square feet in 60 lbs bag = 0.451 / 0.5 = 0.902
Square feet in 80 lbs bag = 0.602 / 0.5 = 1.204
Square feet in 90 lbs bag = 0.677 / 0.5 = 1.354

Now there are two options to go, but here we will be going with option one. You can go with option two if you want the bags count needed in one square foot. Calculating the number of S size bags needed for 12 * 12 slab size having 6 inches depth –

Number of 40 lbs bags = 144 / 0.602 = 239.2 = 240 bags
Number of 50 lbs bags = 144 / 0.752 = 191.5 = 192 bags
Number of 60 lbs bags = 144 / 0.902 = 159.6 = 160 bags
Number of 80 lbs bags = 144 / 1.204 = 119.6 = 120 bags
Number of 90 lbs bags = 144 / 1.354 = 106.35 = 107 bags

Let’s go with the second method –
Here, we have the results of the first four steps. So now we only have to start the calculation from step five –

Finding out the volume of 12 * 12 slab having 6 inches height,
Volume = Area * height = 144 * 0.5 = 72 cubic feet

Determining the number of S size concrete bags,
Number of 40 lbs bags = 72 / 0.3008 = 239.3 = 240 bags
Number of 50 lbs bags = 72 / 0.376 = 191.48 = 192 bags
Number of 60 lbs bags = 72 / 0.451 = 159.6 = 160 bags
Number of 80 lbs bags = 72 / 0.602 = 119.6 = 120 bags
Number of 90 lbs bags = 72 / 0.677 = 106.3 = 107 bags