So you have decided to replace some fence posts, or add a walk way, or extend your patio, or dispose of a 1950's style gangster, or do anything else that requires concrete. After you check with your personal trainer, doctor, and have kissed your spine good bye, How exactly do you figure out how much to buy? Do you ask the guy pushing carts at the hardware store? Do you consult a palm reader? Do you just buy a mess of it and then make repeated trips back to the store in a never ending litany of "O.K., 3 more bags, 5 more bags, 12 more bags..."? (Not that that has EVER happened to me...)
Well the answer is surprisingly simple. Much simpler, in fact, than figuring out those dang angles to set your miter saw to to cut crown moldings. I mean come on! What kind of angle is 36.65344579? I can't seem to find that one anywhere on my saw! But I digress into what should be another article. It is even easier than putting together a tricycle at 1am Christmas morning before the kids wake up. The formula is this: Square Footage of the area to be covered x Depth of concrete in inches / 324. This will give you the number of cubic yards of concrete you will need. Now if you are going to have knowledgeable help and are looking at any amount over 1 cubic yard, I recommend you stop here and call in a truck of pre-mixed concrete. You will avoid a bill from your Chiropractor, and it will be a huge time saver. If, on the other hand, you find yourself bereft of suckers, er i mean friends, or are a masochist you will have to journey even deeper into the Land of Math to decide how many bags you will need to purchase.
Take your answer in cubic yards and multiply it by 27. This gives you how many cubic feet you will need. Now take a good long look at yourself in the mirror and decide how much weight you want to lug around at a time. If you have eaten your Wheaties or taken your steroids, you can go with 90# bags of concrete. Take your answer in cubic feet and divide it by .66. This answer is approximately how many 90# sacks you should buy. I always recommend buying a couple extra just in case. If you are more modest in your weight lifting abilities, divide your cubic feet by .45. This will give you the approximate number of 60# bags to buy. Again, I caution you to get a few extra "safety" bags to make sure you don't run short.
Now that you have a solid best guess as to how many bags to buy, it's time to break out the wheelbarrow, warm up the hoe (a garden tool often used to mix concrete, sheesh you people), and call in all your markers to get the job done.