Bricks, Masonry, Asphalt and Concrete - estimating needed concrete

what is the formula for determining needed cubic yards of concrete for a 5 inch slab 30X50 ft.? DRD

One way is to multiply by feet to get cubic feet. Than divide by 27 (the number of cubic feet in one cubic yard) to get your answer.

To convert your only dimension not in feet, divide by 12: 5 divided by 12 = 0.4167 feet.

0.4167' x 30' x 50' = 625.05 cubic feet

625.05 / 27 = 23.15 cubic yards

Also, on a pour that large you are going to have some waste. Some concrete will fall out over the forms, the grade may be a bit deep here and there, etc. It's also not unheard of that the concrete company brings you a bit less than you called for. It's always a good idea to thoroughly check your grade to make sure it is 5 inches everywhere, and then order a little extra concrete as well. In this case I would round the yardage up to 24 yards. Spending around $100 on an extra yard (insurance against running short) is a lot cheaper than the time and money wasted on a call back. If you run short by say, 3 wheelbarrows, you will have to order an extra yard anyway, and deal with the hassle and added expense of them sending out a short load.
Another way to order it is to call for 18 plus. This means that they will send out the first 2 trucks (9 yards each=18yards) and then when you are finished pouring them out, you re-measure and figure how much more you need. Maybe your grade was shallower than 5 inches and you only need one more yard. In this case, they don't charge a haul charge for the last yard because they already knew you'd be calling for more. Conversely, maybe you find that you needed 26 yards total so you call for another 8 yards on your plus. That way you don't run short on your last truck by mistakenly ordering only 6.
I know this is long winded, but it's hard to explain. The gist is that if you happen to run short, it is a huge pain in the butt. Ordering extra is relatively painless.

Pecos

Pecos told ya how to do it. And if you are unsure, take the measurements and tell them to the supplier. They do the math all the time and order an extra 1/2 yard or so. (you can get a good idea if you are running deep or shallow by where the first trucks ran) :D

also it is easier to finish smaller loads. dont know if you plan on ordering them back to back. Have a form ready for your stopping points. If you got a little extra after your stopping point, you can just spread it out in the grade if you got another truck coming. Dont send it back.