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Welcome to the Bunnings Workshop community @mitc69. It's terrific to have you join us, and many thanks for your question about creating a concrete slab.
A slab of that size would be a push to pour by hand. By the time you'd have mixed the bags and got them down, the area that you began with would be beginning to set. You'd need at least three friends: two on mixing and another to assist you with spreading. You're really at the point of something that's just not possible by hand. You could do it with an expansion gap down the middle. Mix and pour one half one day and the other the next.
On the maths, that is two cubic metres of concrete or 231 x 20kg bags of concrete. If you're following a typical concrete mix ratio of 1:2:4 (cement:sand:aggregate) then you'd need approximately 35 x bags of cement, 69 x bags of sand and 138 x bags of aggregate. It's going to require around 350L of water.
I strongly encourage you to consider having a slab of this size pumped or divided into manageable sections.
Here's a helpful guide: How to pour a concrete slab.
Please let me know if you have any questions.
Mitchell
Hi Mitchell
thanks for the maths, save me time to test and trail... I did get a quote but it was rather too high in that regard 😕
Unlike brissy I can't seem to find any concrete truck that ready to pump or is willing to come out to the farms hence the diy
And I'm planing to do section by section at this rate 
and again much appreciated
Just some updates, not perfect but with some self level will be able to tile it later
massive 13x10x14mm slab once completed, then another 20x10 for the machines
first slab took about 13hrs solo, with good ozitio mixer !
and again thanks
Hello @MarioTedone
Welcome to the Bunnings Workshop community. It's sensational to have you join us, and thanks for sharing your question about how many concrete bags you'll need for your concrete base.
To calculate how many 20kg bags of concrete you will need for your base, we start by determining the total volume of concrete required. The base dimensions are 4 meters long, 1 meter wide, and 120mm (0.12 meters) deep. To find the volume, multiply the length by the width and the depth: 4 × 1 × 0.12 = 0.48m cubic meter. This means the total volume of concrete required is 0.48 cubic meters.
Next, we need to determine how much volume a single 20kg bag of pre-mixed concrete can cover. Typically, a 20kg bag covers about 0.01 cubic meters of volume when mixed. This figure can vary slightly depending on the specific product, so it's worth checking the coverage information provided by the manufacturer of the concrete mix you’re using. Finally, we calculate the number of bags needed by dividing the total volume required by the volume covered by each bag. In this case, 0.48 ÷ 0.01 = 48.
Therefore, you will need approximately 48 bags of 20kg concrete to complete your base. It’s a good idea to purchase a couple of extra bags to account for any spillage, uneven levelling, or other unforeseen needs during the project.
However, if you your bag is 80kg you'll need to determine the coverage of an 80kg bag. Typically, an 80kg bag of pre-mixed concrete covers about 0.04 cubic meters when mixed. Dividing the total volume by the coverage per bag, 0.48 ÷ 0.04, shows that you’ll need 12 bags of 80kg concrete. To ensure you have enough for the project, it’s advisable to purchase one or two extra bags to allow for other unforeseen needs.
Please remember to wear personal protection such as gloves, goggles and a mask when working with concrete.
If you have any other questions we can help with, please let us know.
Eric
Hi @gailando
I am going to assume a general purpose cement GP mixture for slabs for a shed floor or path and you will doing 70 mm - 100mm thickness ,
Cement 1 sand 2 Aggregate 3
Estimated 12 bags (70mm thick) you may need up to 3 bags more to finish the job. If you over order bunnings will refund fresh un damaged bags
Bastion 20kg General Purpose Cement I/N: 0760330 $8.59
I hope this is some SOLID advice.
