How to calculate number of bags of cement in 1 cm3 concrete

Table of contents

1. Introduction

2. Method 1: Total Weight (Density) Method

   • Step 1: Calculate Volume

   • Step 2: Find Cement Weight

   • Step 3: Calculate Cement Bags

3. Method 2: Dry Volume Method

   • Step 1: Understanding Dry and Wet Volume

   • Step 2: Calculate Dry Volume

   • Step 3: Calculate Cement Bags

4. Conclusion

Core concept or idea

In this post you are going to learn how to calculate the number of bags of cement in 1 cubic metre.

Calculating the number of cement bags you need to cast concrete is a key skill for civil engineers and quantity surveyors and also students. Being able to estimate this accurately helps ensure you stick to your budget and avoid unnecessary delays and losses arising from guesswork.

In this guide, you'll learn two different methods to determine the number of cement bags:

1. The total weight (density) method and the dry volume method. I also explore which one is more reliable based on a real example.

The first method to determine how many bags of cement you need when casting concrete is by using the total weight or density of the concrete. To explain this in the simplest way, let’s say you are casting concrete for a slab that is 20 metres long, 8 metres wide, and 0.2 metres thick:

Length: 20 metres

Width: 8 metres

Depth: 0.2 metres (thickness)

V = L x W x H

V = 20 m x 8 m x 0.2 m = 32 cubic metres

volume of the slab is 32 cubic metres

Understanding the Mix Ratio

Considering that we are using a mix ratio of 1:2:4 (1 part cement, 2 parts sand, 4 parts aggregates) to cast concrete for the slab, the ratio is 1 + 2 + 4 = 7.

Step 3: Knowing Concrete Density

The density of plain cast concrete is around 2,200-2,500 kg per cubic metre.

The density can vary slightly based on specific factors, but we’ll use 2,300 kg/m³ for this example.

Step 4: Calculate the Total Weight of the Concrete We find the total weight of the slab by multiplying the volume by the concrete’s density:

Total Weight = Volume X Density Total Weight = 32M³ X 2,300KG/M³ Total Weight = 73,600 kg

Step 5: Weight of Cement in the Mixture

Now we calculate the weight of the cement within the total concrete mixture.

Since you are using 1 part of cement out of the total 7 parts, the equation becomes

Cement Weight = Total Weight / Ratio of cement X Total Ratio

Cement Weight = 73,600 kg/7 X 1

Cement Weight = 10,514 KG

1 bag of cement = 50kg

Step 6: Number of Cement Bags Required

The last step is figuring out how many 50 kg bags of cement you’ll need, and we get this by simply dividing the total weight of the cement by the weight of one bag.

Number of Bags = Total weight of Cement / Weight of One Bag

Number of bags = 10,514 kg / 50 kg = 210.28 Approximately 211 bags of cement

You’ll need roughly 211 bags of cement for this slab using the total weight method.

Method 2: Dry Volume Method

The dry volume method is another way to calculate the number of cement bags needed for concrete. This method focuses on the volume of materials before water is added, and it’s essential to account for how the addition of water affects the mixture’s overall volume.

Step 1: Understanding Dry and Wet Volume Dry volume refers to the volume of materials (cement, sand, and aggregates) before any water is added. When water is introduced, the volume decreases by around 54%. Therefore, dry volume is approximately 1.54 times the wet volume.

Step 2: Calculate the Dry Volume

You already know that the wet volume of your slab is 32 cubic metres.

To find the dry volume: Dry Volume = Wet Volume X 1.54

Dry Volume = 32 CM X 1.54 = 49.28 CM

Step 3: Volume of Cement in the Mix Similar to the total weight method, we’ll now calculate the portion of cement relative to the total dry volume using the 1:2:4 mix ratio.

Cement Volume = Dry Volume / Sum of Ratios X Cement Ratio

Cement Volume = 32 CM/7 X 1 Cement Volume = 4.57 CM

Step 4: Number of Cement Bags Required

To figure out the number of cement bags, divide the total cement volume by the volume of one bag (0.0354 m³ per bag).

Number of Bags = Cement Volume / Volume of One Bag

Number of Bags = 4.57 m³ / 0.0354 m³ Number of Bags = 129 Using the dry volume method, you’ll need around 130 bags of cement for this slab.