It's important to order enough concrete for your project. If you don't, you'll waste time (and money, if you're hiring someone else to pour the concrete), may incur a short load charge from your ready mix supplier and risk a cold joint where one pour ended and another pour begins. You can use online concrete calculators to work out how much you need, but it's a good idea to double check this with your own calculation. Don't rely on the building plans for this; measure depths and widths as they are on your site.
Concrete for Slabs
The rule of thumb for slabs is to add 1/4 inch to the thickness of your slab (assuming the job is evenly graded to the right depth, and the grade is well compacted). If you have odd-shaped slabs, turn them into rectangles for the purposes of your calculation.
Say you have a 2 feet by 2 feet slab with a depth of four inches, which is the normal depth for a concrete slab. To calculate the volume of concrete you need in cubic yards, you would multiply the length (2 feet) by the width (2 feet) by the depth (4 inches). Divide the answer by 12 to get everything into feet; the answer is 1.3 cubic feet.
Concrete for Round Holes
The formula is different for round holes. You would multiply radius by radius by depth by pi (3.14). Remember, the radius is half the width of the hole. For example, if you have a hole that is 24 inches deep and 8 inches wide, the calculation is 4 x 4 x 24 x 3.14. The answer is 1,206 inches, but you must convert this from cubic inches to cubic feet. There are 12 cubic inches in 1 cubic foot, so divide the answer by 12 x 12 x 12 (or 1,728). The final answer is 0.7 cubic foot.
Number of Bags Required
Your bag of concrete tells you how many cubic feet the contents will fill, known as the yield. For example, the yield on an 80-pound bag of Sakrete, which is premixed cement, sand and gravel, is about 0.6 cubic feet. To work out how many bags you need, divide the volume of your project by the yield. For example, if your volume is 1.3, you divide 1.3 by 0.6. The answer is 2.2 bags.
Margin of Safety
To make sure you don't order too little concrete, add a margin of safety. If your order is 1 to 5 cubic yards, order 0.5 to 1 cubic yards extra. If your order is 6 to 10 cubic yards, order 1 cubic yard extra. If your order is 11 to 20 cubic yards, order 1 to 1.5 cubic yards extra.