I am ordering some compost. I am having trouble figuring how many yards. My neighbor and i have roughly the same size yards, 70'x50'. I am topdressing my yard, and beds. My size includes my beds, maybe a touch more but not too much bigger than 70'x50'. My neighbor is completely redoing his lawn and will be tilling in the compost. I don't know which tiller he will use, bu I imagine 3" minimum depth. What should I get? 6 yards?