Personally, I like beds that I can wander through without compressing the soil. Thus I like narrow beds (3-4 feet wide). It makes weeding, mulching, planting and harvesting much easier and more fun.
Maybe a much larger bed in a much larger yard could have foot oaths wandering through it, or at least some kind of stepping stones so that you can get at everything without compressing the soil.
Also, personally, no matter how good the soil that you buy, it can always be improved by being mixed with up to 50% good compost (or even bad compost). Two or three more yards of compost, mixed in as you spread the soil, will get you off to an even better start. But the compost will be digested within a year or two, and then the soil will subside and give you more room for deeper mulch.
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7 cubic yards x 27 cubic feet per cubic yard = 189 cubic feet
If you use the purchased soil 12" deep over the whole bed, you could have a bed that was 189 square feet (47' x 4' or 10' x 19') ... rounding off a little.
Or, say you kill or till the grass and then till some of the new soil into the top 4-6", to create a gradual transition from new soil to old soil. Lay down cardboard or 6 layers of newspaper to discourage grass from re-sprouting.
Then you can expect the plants' roots to take advantage of whatever new soil you give them, then push through the cardboard and keep growing deeper. In that case, you could spread your new topsoil thinner and still have a deeper root zone (after a year or two). Continued addition of compost and mulch will continue to improve the subsoil.
7 cubic yards, spread 6" deep, would give you 378 square feet (almost 100' x 4' or 19' x 20').
Let's see ... with 7 cubic yards and soil "X" inches deep, the size of the bed you can make, in square feet, is:
189 X 12 / X.
(For 18" depth, divide 12 by 18 and get 0.67.
Multiply that by 189 square feet to get 126 square feet.)
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For 10 cubic yards of soil plus compost:
10 x 27 = 270 cubic feet of new soil
With 10 cubic yards and soil "X" inches deep,
the size of the bed you can make, in square feet, is:
270 * 12 / X
