I have a question, I need to concrete small area, 0.95m x 0.90m x 0.2m - overall the volume is 0.17 m3 of concrete. The calculator here suggests I need 63 Kg of cement and 0.377 tonnes of ballast (which I assume is 377Kg) - this is just over 15 x 25Kg bags. When I look at the hole and lay some bags of ballast alongside it seems completely implausible that all those materials are going to fit inside the area to be concreted. A 'Major Bag' of Ballast from Tarmac measures 1m x 35cm by 5cm (the weight is unspecified except that the retailer states approx £0.10p per Kg and at £2.30 a bag that is 23Kg. That volume of one bag equates to 0.0175 m3. Multiply that by 16 and you've got 0.28 m3 - in ballast alone - this is way more than the 0.17 m3 of concrete I need. Is there some magic at play here, am I missing something?