How close to calculate concrete order

I plan to do the following. 10 x 22 slab in front of garage 220 sq ft. 6 x 12 shed floor 72 sq ft.
TOTAL = 292 sq feet.
All will be 4 inches thick. That means I get 81 sq feet per yard.
I planned to order 3.5 yards, but that's only 283.5 sq ft.
However, the shed is 6 ft on the outside, so the inside is a little less, plus it has several posts that will mean a little less. So, I guess I can say I really need about 70 sq ft.
With this in mind, I am short by 6.5 sq ft.
But, I plan to cover much of the ground with flat and small rocks that I saved when I did built the shed from the post holes and other rocks I keep. The shed floor will particularly get the rocks, because the driveway will get some wire mesh, whereas the shed wont.
I am estimating that with the rocks I should be darn close. I intend to make the end of my driveway form adjustable so if I am a little short, or have extra concrete I can adjust to suit what I get. (It dont have to be exactly 22 feet long).
Am I figuring this right, or should I order another 1/4 yard (it can be ordered in 1/4 yard increments).
I should also mention that I may not have the soil leveled so it's precisely 4 inches thick. Some spots may be a little more, others a little less.
What would you do?
(I dont want a lot of waste either).
Thank U
LM
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On Sat, 08 Nov 2008 19:45:02 -0600, snipped-for-privacy@invalid.com wrote:

Form up an apron in front of the shed with the last board left out (farthermost from the shed). Order 4 yards and use the excess on the apron, setting the final form board right before you run out.
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Yep.
Better a little over than under
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wrote in message

True, you will find that you always spill some, plus the area may have a few slight dips that eat up concrete. The apron idea is good. Also look for a supplier who has trucks that mix it on the spot and meter the output, this way you only pay for what you use, and you will never run short nor be over.
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It is always good to form up some sidewalks, pads, or other things. But things that are not crucial, merely convenient. Put the extra concrete in there, rather than having them take it back. Sometimes, they make a mistake and send more than you order, and you get the benefit of extra sidewalks and pads and such. It's a common practice to have extra forms, and if you don't have enough concrete, just yank them, and no harm no foul.
Steve
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On Sat, 08 Nov 2008 19:45:02 -0600, snipped-for-privacy@invalid.com wrote Re How close to calculate concrete order:

I note that you didn't account for concrete sticking to the chute and/or wheel barrow as it is delivered.
I would order an extra 1/2 CuYd. The problem of waste is trivial compared to the problem or shortage. When you have fresh poured concrete, you don't have a lot of time to correct errors.
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snipped-for-privacy@invalid.com wrote:
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Last time I ordered concrete I got a price on 4 yds. Before it came I decided to do a bit more and called and asked for 5. The 5 cost $10 more than the 4 because there was no longer a short load fee.
Personally I would never try to come close- forms break, spillage happens, math fails. . .
Weight the difference between whatever 1/2 yard costs- and how screwed you are if you are 1/4 yard short.

There's no such thing as waste. I knock together some forms to make patio blocks/steps 1foot by 2 foot from 2x4s. Screw them together and dump the waste in them They must come in handy because I don't have any more lying around.
[i just read fretwell's suggestion on the apron- way better idea for you] Order 4 yards.
Jim
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