Mixing concrete and getting the right concrete mix is vital for durable and long lasting foundations and along with our concrete volume calculator found below, we can help you establish exactly what you need for your project.
What is Concrete
Concrete is a mixture of cement, sand (fine aggregate), small stone or gravel (coarse aggregate) and water. It has many applications, from fence posts to motorway bases and, because of this, there are many different concrete mix ratios that the constituents can be mixed to with each resulting in a different strength.
Concrete Mix Ratio for a C20 Mix
This project will concentrate on mixing concrete for a general purpose mix that’s suitable for garden paths, fence posts, and shallow retaining wall foundations. It is a medium strength mix and is known as a C20 mix. This means it will attain a strength equivalent to withstanding a compression of 20 Newton’s per square mm after 28 days. This mix is not suitable for house foundations. For these please read our foundations project and ensure all the criteria are met, including the trench sizes and ground suitability.
Unless specified otherwise by an architect or structural engineer, a C15P concrete mix can be used which is comprised of:
- 1 part cement
- 2 parts sand or very fine aggregate
- 6 parts coarse aggregate
The calculator below will help you work out exactly what you will need for your project. (This calculator is for a C20 mix)
You will see that your answer will be given in cubic meters. Input your dimensions in meters…I.E. 500mm = 0.50meters
Concrete Mix Ratio Calculator
Enter your dimensions to calculate the volume of concrete needed for your project, then enter this total into the second calculator for the materials necessary.
Please remember that water volumes are approximate and care should be taken when adding this. Also please remember that cement can damage your skin, so wear the appropriate safety equipment and clothing when using it.
How to Mix Concrete
A bag of ready mixed aggregate can be bought in most builders merchants or it can be bought loose and delivered to you by lorry. Buying it loose is generally cheaper and, if you have the room to "drop", it is a more practical option. Check out your local builders merchants for some fantastic deals on bulk sand, cement and aggregates.
In most areas of the country, this mix of aggregates (sharp/grit sand and small stones or gravel) is called "ballast", in the West Country it is most often called ½inch or 10mm to dust. This describes the sieved state of the stone as it comes out of the quarry crushers.
Cement can be bought (mostly in 25kg bags, although some stores still sell 50kg bags) from all builders merchants. It is crushed limestone, blended with other raw materials (sometimes shale and or sand) ground into a powder and then heated in a kiln. This process produces a cement clinker which is mixed with Gypsum and ground further to produce the cement.
Water is a very important part of the mix and the volume of water used can dictate the strength of the finished mix. On site or in ready mixed concrete yards a "slump test" is used to test the water content of the concrete.
When mixing concrete, getting the correct amount of water in your mix is essential. Too little or too much will render the mix unusable.
Testing Cement Mix
A cone made of steel is used for this test. The cone is 300mm high, with a top opening of 100mm diameter and a bottom opening of 200mm diameter. The mixed concrete is placed into the cone through the top, a bar is used to compact the concrete and remove air voids, within the cone. The cone is then lifted clear. By laying the bar on top of the cone, it is possible to measure how far the concrete "slumps". A slump of approx 50mm is acceptable for C20 concrete.
Concrete Mix Ratio for a C20P Mix
The proportions of materials for a C20P (P = Portland cement) concrete mix are:
- 1 part cement
- 2 parts fine aggregate (sand)
- 4 parts coarse aggregate
If you are using premixed ballast, then:
- 6 parts pre mixed ballast
- 1 part cement
In an ideal world, where everything is delivered dry, a water to cement ratio of approximately 0.55 should be used. For example, if you require 25kg of cement in your mix, then 25 x 0.55 = 13.75kg (liters) of water. This is the maximum amount of water that should be used.
Most of the time however, the sand and other materials will be damp, or even wringing wet, and care has to be taken to ensure your concrete mix does not become too sloppy. It should be able to support itself, almost fully, in a heap on the shovel.
If you are only needing a small amount e.g. to concrete a small fence post or repair an existing concrete path it makes no sense to mix up a great load. To these ends, your local DIY shed should stock a range of ready mixed DIY concretes ideal for this job.
Concrete Mix Ratio for a C35P Mix
When mixing concrete for a very strong mix, normally labeled C35P, the mix is:
- 1 part cement
- 1 part stone
- 2 parts sand
Or if you are using a ready mixed aggregate then:
- 1 part cement
- 3 parts ready mixed aggregate
When mixing concrete you will find that, while the ballast will heap on your shovel, the cement will slide off. This will give you incorrect proportions when mixing – 1 part of cement should be equal to 1 part ballast.
It is better to measure your quantities using a bucket to ensure correct proportions. If the exact same concrete mix ratios are required repeatedly, a builders bucket should be used to maintain consistancy.
In the video below the mix is being used for a sub base and a slight variation in the mix will not matter a great deal.
Concrete Mix By Hand
Mixing concrete by hand is a bad idea unless you only have a tiny amount to do.
Only very rarely will you be able to get the cement evenly distributed throughout the mix and this leads to weak concrete.
Using Cement Mixers
A cement mixer can be hired from all tool hire shops, and if you have a lot to do it often pays to buy your own. Once your project is at an end you will always have the option of selling the mixer which is what most self-builders do.
To ensure a well mixed batch we suggest you add your ingredients to the mixer in the following order. 75% of the water followed by 50% of the aggregates. Add all of the cement and then the rest of the aggregates and the remainder of the water.
When mixing cement, the key to success is using the correct concrete mix ratio for the job that you are doing, whether it’s for general purpose use e.g. a fence post or whether you are going for something a little stronger, just make sure you’ve got your quantities right. Hacking it all up again is not a nice job!
All project content written and produced by Mike Edwards