Home Depot, Lowe's, and Southern States sell Quikrete 80-pound concrete mix in-store, and Amazon.com sells it online. Quikrete.com offers a quantity calculator and tips for using the product.
Quikrete is a blend of cement, sand and gravel in a ready-to-use mix for patios, steps, sidewalks and floors as well as for setting posts. To use Quikrete, first dump the mix into a tub or wheelbarrow and scoop a hollow spot in the center. Add about 3 quarts of water to each 80-pound bag. Add two-thirds of the water and start mixing with a hoe while gradually adding enough water to make it resemble oatmeal.