This series of concrete calculators has been developed to help you plan your project. You can calculate how much concrete, cement, sand, mortar or render you will need. For each calculation you will have to know the dimensions of your object. The calculator will perform the sums for you - where relevant it will add a small percentage for the wastage that inevitably happens when mixing & using cement-based products, and then it will round the result up to the nearest whole 20kg bag.

http://www.cementaustralia.com.au/wps/wcm/connect/website/packaged-products/home/calculators/