How Many Bags of Concrete Per m3? (+ Concrete Calculator)

Are you planning a DIY project that involves laying concrete? Whether you’re setting fence posts or pouring a driveway, figuring out how much concrete you need is an important step.

Fortunately, determining the number of concrete bags needed for a project is a simple task that can be achieved with a bit of knowledge and some straightforward calculations.

In this article, we’ll provide a step-by-step guide on how to calculate the amount of concrete you need per cubic metre, including a handy concrete calculator to simplify your calculations.

Be sure to read through to the end, where we cover some important tips and common concrete-mixing mistakes to avoid.

How much does a bag of concrete weigh?

Most concrete bags are 20 kg or 10 kg.

Concrete mix is a mixture of cement, water, and aggregates. These products simply need water added in line with the instructions, and to be mixed slowly until you have a workable consistency.

How many bags of concrete per m3?

You will require approximately 100 – 110 x 20kg Concrete Mix bags per m3 of concrete you need.

The guidance varies slightly depending on the brand. For example, looking at the popular concrete mix available at Bunnings, we see the following:

  • Bastion 20kg Concrete Mix: 100 x 20kg Bags per cubic metre
  • Australian Builders 20kg Concrete Mix: 100 x 20kg Bags per cubic metre
  • Dingo Fast Set High Strength Concrete: 110 x 20kg Bags per cubic metre

These guidelines are based on you using the recommended amount of water in your mix. The guidance is usually 2.3 – 2.4 litres per 20kg bag.

For the rest of the article, we err on the side of caution and assume 110 x 20kg bags per cubic metre for our calculations.

So, for example, if you require a 2m x 2m slab with a depth of 50mm, your calculation would be as follows:

2 x 2 x 0.05 = 0.2m3

0.2 x 110 = 22 bags of 20kg Concrete mix.

Concrete volume calculator

Use our concrete calculator below to calculate how many bags of concrete you need for your project.

Note that while it is pre-set with the assumption of 110 20kg bags per cubic metre, you can override this based on the instructions from the concrete manufacturer you purchase from.

How many m3 are in a bag of concrete?

Now that we know how many bags are in one cubic metre, we can do the reverse calculation: knowing how many metres cubed are in a bag.

Let’s continue with the example using a 20kg bag of concrete.

If 110 x 20kg bags = 1m3


1 x 20kg bag = 0.01m3

So, if for example, you had a project requiring 1m x 0.2m with a depth of 50mm, you should be able to get away with one bag of concrete (as 1 x 0.2 x 0.05 = 0.01).

How many bags of concrete do you need for a 10×10 slab?

The calculation for the metres squared is to multiply the width (10 metres) by the length (10 metres).

This equals 100 square metres.

Now multiply by 0.1 (assuming a 10cm thick concrete slab) for the thickness in metres, which gives 10 cubic metres.

As discussed above, one cubic metre requires approximately 110 x 20kg Concrete Mix bags.

Therefore in order to fill 10 cubic metres, you will need 1,100 bags of concrete (10 x 110).

How many bags of concrete do you need for a 4×4 slab?

For a 4×4 slab, we can repeat the calculation above with the new measurements:

4 x 4 x 0.1 (assuming a depth of 10cm) = 1.6m3

One cubic metre requires approximately 110 x 20kg Concrete Mix bags.

Therefore, you will need 176 bags of concrete (1.6 x 110).

Tips for mixing concrete

Mixing concrete is an important part of many DIY projects, but it can be tricky if you’re not sure what you’re doing. Here are some tips to help you get it right:

  • Use the right tools: To mix concrete, you’ll need a large mixing container (like a wheelbarrow) and a shovel or hoe for mixing. You may also want to use a power mixer if you have a big project. Be sure to wear gloves and protective eyewear.
  • Add water slowly: It’s important to add water slowly to your mix so that you don’t end up with concrete that’s too dry or too wet. Add water in small increments and mix thoroughly before adding more.
  • Mix thoroughly: To get the best results, it’s important to mix your concrete thoroughly. Mix for at least 5 minutes or until the mixture is smooth and even.
  • Don’t overmix: Overmixing can cause your concrete to lose strength, so stop mixing as soon as it’s smooth and even.
  • Follow the instructions: Finally, be sure to follow the instructions on your concrete mix package, including the recommended amount of water and mixing time.

Types of concrete:

Concrete comes in many different types, each with its own unique properties and uses. Here are some of the most common types:

  • Ready-mix concrete: This is the most common type of concrete used for DIY, and is available in pre-mixed bags that just need water added. It’s great for most general-purpose projects.
  • Self-levelling concrete: This type of concrete is designed to level itself out, making it ideal for projects where a flat surface is important, such as garage floors or basement floors.
  • Decorative concrete: Decorative concrete is used to create interesting and unique finishes, such as stamped or stained concrete. It’s great for adding a personal touch to your project.
  • High-strength concrete: This type of concrete is designed to withstand heavy loads, making it ideal for projects like bridges or high-rise buildings.

Cost of concrete:

The cost of concrete can vary widely depending on a number of factors, including the type of concrete, the size of the project, and what brand you choose.

Bags of concrete mix typically cost around $5 to $15 per 20 kg bag, but this can vary depending on the brand and where you purchase it.

Common mistakes when laying concrete:

Pouring concrete can be a complex process that requires careful attention to detail. Even the smallest mistake can have significant consequences on the quality and durability of the final product.

To help you avoid these common pitfalls, we’ve listed some of the most common mistakes people make when mixing and pouring concrete.

  1. Insufficient site preparation: A poorly prepared site can lead to an uneven surface, causing settling to occur. This is why it’s essential to ensure the site is compacted.
  2. Incorrect concrete mix: Getting the mix wrong can render your concrete useless, and different situations call for different concrete mixes. Take extra care to get the mix right.
  3. Not preparing for the weather: Bad weather conditions can have adverse effects on concrete. Protect your setting concrete from rain by checking the forecast beforehand and using waterproof coverings.
  4. Inadequate finishing work: After pouring the concrete, finishing work is necessary to avoid problems later on. Screed the concrete to get it level, edge it to prevent rough and sharp edges, and float it to remove any marks from the edging process.
  5. Premature removal of forms: Avoid removing the forms too early. It’s important to exercise some restraint and wait at least 24-48 hours before removing them to prevent undoing all of your hard work.

By avoiding these common mistakes, your concrete will not only look great, but it will also last for a long time. With proper preparation and attention to detail, you can achieve a durable and high-quality finished product that will stand the test of time.

Photo of author

Steve Kropp

Based in Melbourne, Steve's passion is vegetable gardening, and he’s been writing about it for almost 5 years. He also loves all things DIY and is always looking for a new project. When not working on his own garden projects or blogging, Steve enjoys spending time with his family, cooking meals with produce harvested from his garden, and coaching his son’s footy team.


Leave a Comment