Ordering the correct amount of material for any given build phase, should be a preliminary consideration for any Self-Builder. If you order too much you will either have to try and sell it on at a loss or throw it away. Order too little and you will incur part load charges that can be more than the cost of the material in the first place. You can either work this out on paper, adding the correct allowance for uneven trench bottoms and waste or consider using one of the far more accurate building material quantity calculators.
Structural concrete volumes for foundations and floor slabs seems to be the area that gives people the most problem. Starting with the foundations, accurate measurement is crucial to ensure you have enough concrete to fill the trench to the correct level. Failing to do this will lead to problems with the construction of the footings. If your trench fill concrete is too high or too low, you will loose “gauge” on the brick and blockwork below DPC. The easiest way to calculate the top of the structural concrete fill level, is to work gauge back down from DPC level. Standard gauge is increments of 75 mm and you should work to a tolerance of + or – 5 mm.
Lets assume that you have a depth from DPC to the bottom of the trench of 1015 mm. Your drawing shows that the structural concrete depth should be 450 mm. this would leave a footing depth of 565 mm which would not work to gauge. You should increase the structural concrete depth to 490 mm in order to solve this problem.
TIP: Once you have established the level for the top of the structural foundation concrete, transfer this around each trench using a site level. An old fashioned way of doing this is to push a 6 “nail into the trench side at the correct level leaving 50% of the nail protruding. concrete.
When ordering your concrete, allowances should be made for uneven trench bottoms and waste. If you are planning on building any garden walls or shed bases, it would be a good idea to get the foundations dug prior to ordering your first load of concrete. If you've done calculations correctly, you will have a few shovel fulls left over. This surplus can then be utilized in these areas.