Give the measurements in metres, also the thickness. A slab of ten by six at twenty centimetres is 10 times 6 times 0.20, so twelve cubic metres. Getting that comma wrong is how a mixer drives back half full, or worse, how you run out in the middle of a pour.
The 2400 kilos per cubic metre is usual for ordinary concrete, but it is a property of the mix and the field is open. The extra percentage is yours to set: the tool does not decide how much spare you want on a lorry.
This tool does not choose a concrete class and it does not work out your reinforcement. How much steel goes in and where belongs to the engineer and to the standard that applies where you build. Fill in the kilos per cubic metre from your own drawing if you want the weight added up.
