Bags of concrete are the lifeblood of any home improvement project. Each square foot of ground is held in place by the sheer weight and denseness of these hardworking bags, which have been carefully calculated and poured by professional contractors.
So, just how many bags of concrete are in a single square foot?
There are several factors affecting the amount of concrete needed to fill one square foot, including the type of concrete used and the depth required.
Generally speaking, a single bag of pre-mixed concrete is approximately 1/4 cubic foot in volume. Therefore, four bags would equal one cubic foot and 16 bags would equal one square foot.
However, these numbers can change depending on the type of concrete mixture and the desired finish.
For instance, if you’re using a ready-mix bagged concrete with higher water content, then it will require more bags to fill one square foot than a mix with less water. Similarly, you may need fewer bags if you are pouring an extra thick layer of concrete.
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.
| Size | Concrete Height/Depth | 40 lbs | 50 lbs | 60 lbs | 80 lbs | 90 lbs |
|---|---|---|---|---|---|---|
| 1 sq.ft. | 4” | 1.104 | 1.132 | 0.72 | 0.552 | 0.49 |
| 1 sq.ft. | 6” | 2 | 2 | 2 | 1 | 1 |
| 1 sq.ft. | 8” | 2 | 2 | 2 | 1 | 1 |
| 1 sq.ft. | 10” | 3 | 3 | 2 | 2 | 2 |
| 1 sq.ft. | 12” | 4 | 3 | 3 | 2 | 2 |
| 4 * 4 | 6” | 27 | 22 | 18 | 14 | 12 |
| 8 * 8 | 4” | 72 | 57 | 48 | 36 | 32 |
| 10 * 10 | 8” | 223 | 176 | 149 | 112 | 99 |
| 12 * 12 | 4” | 161 | 128 | 107 | 81 | 71 |
| 10 * 20 | 12” | 667 | 531 | 445 | 334 | 295 |
| 800 sq.ft. | 8” | 1778 | 1414 | 1185 | 889 | 786 |
| 1000 sq. ft | 6” | 1667 | 1325 | 1112 | 834 | 736 |
| 600 sq.ft | 12” | 2000 | 1592 | 1334 | 1000 | 883 |
| 20*30 | 4” | 667 | 530 | 445 | 334 | 295 |
How To Calculate The Count Of Concrete Bags In A Square Feet?
There are two methods to evaluate the number of bags. Although both methods are correct, they have various methods and it’s up to you to decide which one is easier for you.
Method 1:
Go for this method to get the square feet in every size of the concrete bag with the exact number of bags for the required square feet. Again, here you have two options for this method:
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 by 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 the 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 lb, 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 the 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 the 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 4 inches in height,
Square feet in 40 lbs = 0.3008 cubic feet / 0.332 feet height = 0.906 square feet
Step 6: Calculating the 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 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 the 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 the Concrete Bags Count With The Given Square Feet Coverage
You simply need to multiply the found number of bags in one square foot by 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 the 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 by the planned height to get the total volume in cubic feet. But the first four steps are the same as method one.
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 by 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 results found in steps 3 and 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 both methods individually step by step to find the number of different sizes of concrete bags needed in a 12 * 12 slab 6 inches in height:
Method 1:
Let’s start with determining the 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 the 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 to 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
