How Many Bags of Concrete Do I Need: Explained

It is often easier and more economic in small applications to use bags of concrete instead of ready mix concrete. These applications include post holes, overlays, repairs, and small steps, patios, walkways, ramps, and others.

But how can i calculate how many bags of do i need for my project?

The easy formula for calculating how many concrete bags you need for any application

There are several ways to estimate how many pre-blended sacks you need for any application. One of the simplest methods is summarized in the following two steps:

• Step 1: Calculate the required volume of concrete.
• Step 2: Divide the required volume by the yield of the concrete bag.

With this easy method, the yield is often assumed, unless tested or mentioned in the product’s specifications.

Besides, it is important to note that our engineering tools and calculators, specifically the concrete volume and bag calculators, are integrated with more advanced methods for high accuracy and precision.

Step 1: Calculating the volume of concrete

The first step is to determine the volume of concrete. In fact, its formula largely depends on the shape of the element. For example, the formula of determining the volume of a rectangular prism is simply length multiplied by width multiplied by thickness. The following table shows the volume formulas for different shapes.

It is important to note that the units should be consistent. For example, if you are calculating concrete yardage, you should convert all units from feet and inches to yards. Each 3 feet convert to one yard, and each 36 inches convert to one yard. Or, if you are calculating cubic meters of concrete, you should convert all the units from centimeters and millimeters to meters.

In addition to the aforementioned regular shapes, irregular shapes also often exist in many concrete applications. In brief, you should divide these shapes into two or more regular shapes to simplify your volume calculations.

To know more about how to calculate the volume of concrete for various applications and shapes, you can read the following article, “Calculating Yards of Concrete: Explained“.

Step 2: Dividing the required calculated volume in step 1 by the yield

The yield of concrete bags is estimated as shown in the following table.

By using this table, you can easily calculate the number of premixed concrete sacks you need for a specified volume. Simply, divide the volume by the yield.
Note: It is important to use constant units in your calculations.

You can also calculate both the volume and the number of concrete bags for various applications with the following engineering calculator:
The Ultimate Concrete Calculator (Cubic Meters, Cubic Yards, & Bags).

Imperial Questions

How many bags of concrete do I need for a 10×10 slab 2 inches thick?

A 10×10 slab that is 2 inches thick requires 57 40-lb bags, 38 60-lb bags, or 29 80-lb bags.
In fact, the volume of this slab = 10/3 x 10/3 x 2/36 = 0.617 cubic yards.
And, the estimated yield is 0.01099 cubic yards for 40-lb bags, 0.01648 cubic yards for 60-lb bags, and 0.02197 cubic yards for 80-lb bags.
Hence, the number of 40-lb bags = 0.617 / 0.1099 = 56.14, rounding up to 57 bags, and so on.

How many 80lb bags of concrete do I need for a 10×10 slab 4 inches thick?

A 10×10 slab that is 4 inches thick needs 57 80-lb bags. Basically, this slab has a volume of 1.2345 cubic yards. Hence, by dividing the volume by the 80-lb bag yield, estimated as 0.02197, we get almost 56.2 bags, rounding up to 57.

How many bags of concrete do I need for a 12×12 slab 4 inches thick?

The 12×12 slab with 4 inch thickness requires 162 40-lb bags, 108 60-lb bags, or 81 80-lb premixed bags. Briefly, these values are calculated according to a yardage of 1.777 and the aforementioned yields or coverage per sack.

How many bags of concrete do I need for a sonotube?

In order to calculate how many bags of concrete you need for a sonotube, you should first calculate the volume of the sonotube. The following table summarizes the answer.

How many bags of concrete do I need for a fence post hole 12 inches in diameter and 2.5 feet deep?

Briefly, you need 7 40-lb bags, 5 60-lb bags, or 4 80-lb bags of concrete to fill the post hole. Here is how to calculate the required number of bags:
First, you need to calculate the volume of the post hole.
Volume = 1.96344 cubic feet or 0.07271 cubic yards.
The second step is to divide the volume by the yield.
For example, for a 60lb bag, 0.07271 / 0.01648 = 4.412, rounding up to 5 60lb bags.

Or, you can use the above table to easily determine the number of bags you need for the post hole, which results in the following:
– 7 40lb bags.
– 5 60lb bags.
– 4 80lb bags.

However, you should also consider the volume of the post embedded in the hole, and subtract it from the volume of the post hole. For example, if the post is 4 x 4 inches , then its embedded volume is 0.0102881 cubic yards. Hence, the required volume of concrete is 0.07168119. With rounding up, this is equivalent to 7 40-lb or 4 60-lb bags. 

How many bags of concrete in a yard?

To determine how many concrete bags per yard, you simply divide one cubic yard by the yield of the bag.

• How many 40lb bags of concrete in a yard?

There are 91 40lb bags in one yard.
The yield of a 40-lb premix bag = 0.01099.
Hence, the number of 40lb bags per yard = 1/0.01099 = 91.

• How many 60lb bags of concrete in a yard?

There are 61 60lb bags per yard.
Where, the yield of a 60-lb sack = 0.01648.
Thereby, the number of 60lb bags per yard = 1/0.01648 = 60.7.

• How many 80lb bags of concrete in a yard?

There are 46 80lb bags in a yard.
In fact, the yield of an 80-lb bag = 0.02197.
Hence, the number of 80lb bags in a yard = 1/0.02197 = 45.5.

How many cubic feet in a bag of concrete?

Determining how many cubic feet per a concrete bag means that you are determining its yield. However, the yield given in previous examples is in cubic yards. Hence, in order to convert cubic yards to cubic feet, you should multiply by 27.
Note, each linear yard converts to 3 feet, thereby a cubic yard convert to 27 yards (3x3x3 = 27).

• How many cubic feet in a 40lb bag of concrete?

There are 0.29673 cubic feet in a 40lb bag. This value is simply calculated by multiplying 0.01099 x 27. Hence, there are 3.37 40lb bags of concrete per cubic feet.

• How many cubic feet in a 60lb bag of concrete?

There are 0.44496 cubic feet in a 60lb bag. Hence, there are about 2.25 40lb bags per cubic feet.

• How many cubic feet in an 80lb bag of concrete?

There are 0.59319 cubic feet in an 80lb bag. Thereby, there are about 1.68 80lb bags in a cubic foot.

How many bags of concrete are on a pallet?

Typically, there are 80 40-lb bags, 56 60-lb bags, or 42 80-lb bags of concrete on a pallet.

Metric Questions

How many ready mixed concrete bags do I need for a 3×6 slab 10 cm thick?

In order to calculate the required number of ready mixed concrete bags for a slab of 3m x 3m x 10cm, you need to determine the volume first.
Volume = 3 x 6 x 10/100 = 1.8 cubic metres.
Then, you need to divide the volume by the yield of the premix sack. For example, a 20 kg bag has a yield of 0.0089285 cubic metres, as mentioned previously in a table.
Hence, 1.8 / 0.0089285 = 202 bags of premix.
In brief, for a 3m x 6m x 12cm, you need 202 20kg, 162 25kg, 135 30kg, or 101 40kg premix bags.   

How many 25kg premixed concrete bags do I need for a 2×5 slab 12 cm thick?

In brief, a 2m x 5m x 12cm slab requires 108 25kg premixed concrete bags. Where, the volume of the slab is 1.2 cubic meters, and the yield of a 25kg of a premixed bag is 0.0111111 cubic meters.
Hence, the number of required bags is 1.2 / 0.0111111 = 108.

How many ready mixed concrete bags do I need for post holes?

In order to calculate how many ready mixed concrete bags of you need for a post hole, you should first calculate its volume. The following table includes a general summary.

This table is helpful in determining the volume or number of bags for the post hole. However, you should consider the volume of the embedded part of the post itself, and subtract it from the given volume in the table, as explained in the following example. 

How many ready mixed concrete bags do I need for a fence post hole 40 cm in diameter and 80 cm deep?

In brief, you need 12 20-kg bags, 9 25-kg bags, 8 30-kg bags, or 6 40-kg bags for the post hole. You can either calculate the volume of the post hole in cubic meters, and then divide it by the yield of the premix bag, or simply refer to the table above.

However, you should also consider the size of the post. For example, let us consider that the fence post is 12 cm square shaped. Hence, its embedded volume equals 0.01152 m3. Thereby, the actual volume of concrete required is 0.089008 m3, which is equivalent to 10 20-kg, 9 25-kg, 7 30-kg, or 5 40-kg bags.

How many ready mixed concrete bags per cubic metre?

You can calculate how many ready mixed bags per 1m3 by simply dividing 1m3 by the yield of the bag in m3.

• How many 20kg bags of concrete in a cubic metre?

There are 112 20kg bags in one cubic metre.
The yield of a 20-kg premix bag = 0.0089285.
Hence, the number of 20kg bags per yard = 1/0.0089285 = 112 bags.

• How many 25kg bags of concrete in a cubic metre?

There are 90 25kg bags per cubic metre.
Where, the yield of a 25-kg premix bag = 0.0111111.
Thereby, the number of 25kg bags per cubic meter = 1/0.0111111 = 90 bags.

• How many 30kg bags of concrete in 1m3?

There are 75 30kg premix bags in 1m3.
In fact, the yield of a 30-kg bag = 0.0133333.
Hence, the number of 30kg bags in 1m3 = 1/0.0133333 = 75 bags.

• How many 40kg bags of concrete in a cubic meter?

There are 56 40kg bags of premix in a cubic meter.
Where, the yield of a 40-kg concrete bag is 0.0178571.
Thus, the number of 40kg bags in a cubic meter = 1/0.0178571 = 56 bags.