he form in which you buy concrete depends on the size and nature of your job. Concrete is normally measured in cubic yards. To determine how much you?ll need, figure the volume inside your forms in cubic feet and divide by 27 (the number of cubic feet in a cubic yard). For example, a 4-in.-thick slab that covers 90 sq. ft. takes up 30 cu. ft., or just over 1 cubic yard. Projects using up to about a cubic yard can be handled with a portable cement mixer that you can rent. The proportions of cement, sand, gravel and water can vary depending on the use of the concrete. For example, thin work?between 2 in. and 4 in. thick?will require more cement, whereas a higher-mass pour can afford to use more aggregate. An average 1:2:3 mix contains one part cement, two parts sand and three parts gravel. To make 1 cubic yard of concrete, you?d need seven 94-pound bags of cement, about 1/2 cubic yard of sand and just over 3/4 cubic yard of gravel. The amount of water you use depends on how wet the sand is. If it?s already moist, you?ll need about 4-1/2 gal. per bag of cement.