The intended size of shed is 4m x 2m, using single layer of blocks. Height will be 2m. The external dimension of foundation trench are 4.2m and 2.2m to give a width of 30cm. The trench is 70cm deep. I intend to put about 4 inches of hardcore at the bottom. I have lots of it to get rid of from garden. The concrete will not go all way up to the top.

Q1.

Is the 10cm allowance at top correct?

Q2.

Based on above, my calculations of the concrete required are:

[(2.2 x 0.3 x 2 sides) + (3.6 x 0.3 x 2 sides) = [1.32 + 2.16] x 0.5 (concrete thickness allowing for first brick layer and hardcore) = 1.08 m3

Are these correct?

Parking is an issue, so getting pre-mixed concrete is going to be difficult. I will have to mix my own. I have looked online for the calculators and cant make sense of them.

Q3.

How many jumbo bags of ballast do I need?

Q4.

How many bags of cement do I need?

Thanks in advance.