Concrete is generally mixed with cement, coarse aggregate (gravel), fine aggregate (sand) and water. Sand and aggregate help reduce the cost of concrete, and also limit the amount of shrinkage that occurs as the concrete cures. Choice of aggregates depend on the thickness and use for the final product.
A starting point for concrete made with Portland cement is (by volume) about 26 percent sand and about 41 percent gravel. Mix this with 11 percent cement and 16 percent water. This makes a good general-purpose concrete for pouring foundations, pediments and other structures. Percents are by volumes of materials.
For a quick job where you don't calculate the ratios exactly, you can mix one part cement with two parts sand and three parts gravel. Add a small amount of water and mix, repeat adding water until the concrete is a uniform consistency. Add as little water as possible to make the concrete liquid enough to pour and manipulate without being runny.
Very fine sand requires the use of more cement in the concrete mix. Sand is graded by sieve size, and good sand should have less than 10 percent pass through a number 100 sieve. Very coarse sand makes the concrete less workable and may yield a coarser surface finish.
Gravel size should be considered in relation to the thickness of the slab that is going to be poured. The size of the individual aggregates should be less than one-third of the thickness of the slab. If too much aggregate is used, the concrete may have air pockets or voids that will weaken the structure and possibly form surface imperfections. Gravel in concrete that has too much water will settle to the bottom of the mold, with finer particles forced toward the top.
Special projects like countertops and patching can be made with finer sand and no aggregate. Follow manufacturer's instructions for sand and gravel mixing ratios for special projects requiring a smoother final finish.