Why 27
A yard is 3 feet. A cubic yard is a cube 3 feet long, 3 feet wide, and 3 feet deep: 3 times 3 times 3 equals 27 cubic feet. That is the whole story. Landscape suppliers quote bulk soil in cubic yards because it is the unit delivery trucks are sized around, while bags are labeled in cubic feet because that unit reads better on a store shelf.
The conversion runs both ways. To go from cubic feet to cubic yards, divide by 27. To go from cubic yards to cubic feet, multiply by 27. Most topsoil math starts with a cubic-feet measurement from your tape measure and ends with a cubic-yard number for the supplier.
Cubic feet first: the measurement step
Measure length and width in feet and depth in inches, then convert depth by dividing by 12. A bed that is 10 feet by 10 feet at 3 inches deep is 10 times 10 times 3 divided by 12, which is 25 cubic feet. Then 25 divided by 27 is 0.93 cubic yards, so one yard covers it with room to spare.
For triangular beds, multiply length times width divided by 2 times depth. For round beds, use 3.14 times radius squared times depth. The topsoil calculator handles the rectangular version directly, but the 27 shortcut lets you sanity-check any shape on paper.
Bag math: how many bags make a yard
The two common bag sizes map cleanly to the yard. One-cubic-foot bags: 27 divided by 1 is exactly 27 bags per yard. Forty-pound bags, which hold about 0.75 cubic feet: 27 divided by 0.75 is 36 bags per yard. Half a yard is 13.5 cubic feet, so 18 forty-pound bags or 14 of the one-cubic-foot bags.
Two-cubic-foot bags: 27 divided by 2 is 13.5, so 14 bags per yard. This is the bag math suppliers use to tell you when bulk is cheaper. Multiply the price of one bag by the bags-per-yard number and compare it to the bulk price per yard. At a 3.50 dollar bag price, a yard of one-cubic-foot bags costs 94.50 dollars, while bulk topsoil often sells for 30 to 60 dollars a yard before delivery.
Coverage: what one yard actually covers
Coverage depends on depth, and the numbers are exact fractions of 27. At 1 inch deep (1/12 of a foot), one yard covers 27 divided by 1/12, which is 324 square feet. At 2 inches, it covers 162 square feet. At 3 inches, 108 square feet. At 6 inches, 54 square feet. These are the four numbers worth memorizing if you do this often.
For a typical example, spreading one yard at 2 inches deep across a garden gives you a 162 square foot area, roughly a 12 by 13.5 foot bed. If your bed is bigger, scale up: a 324 square foot bed at 2 inches needs 2 yards.
Worked example: the whole chain at once
Take a 15 by 12 foot bed with 6 inches of topsoil. First cubic feet: 15 times 12 times 6 divided by 12 equals 90 cubic feet. Cubic yards: 90 divided by 27 is 3.33 yards. Bag equivalent in forty-pound bags: 90 divided by 0.75 is 120 bags. Weight at 2,000 pounds per yard: 3.33 times 2,000 is 6,660 pounds, about 3.3 tons. One chain of multiplication and division takes you from tape measure to truckload, bag count, and weight.
Cubic yards to tons: the weight conversion
Suppliers sometimes price soil by the ton instead of the yard, and trucks are limited by weight, not volume. A cubic yard of dry screened topsoil weighs roughly 2,000 pounds, one ton. Wet topsoil is heavier, closer to 2,400 pounds a yard, and dense clay topsoil can push 2,600.
Convert by multiplying yards by the per-yard weight. The 15 by 12 foot bed from the earlier example needed 3.33 cubic yards, which is about 3.3 tons dry or roughly 4 tons wet. A supplier quoting 55 dollars a ton is quoting about 55 dollars a yard for dry soil, but for wet soil a ton is less volume, so the yard price is effectively higher. Always convert to a common unit before comparing quotes.
Weight also limits how much you can haul yourself. A pickup truck rated for a half-ton payload can carry about one yard of dry topsoil before exceeding its rating. A full yard of wet topsoil at 2,400 pounds is over the limit on most half-tons. For self-pickup, take half-yard loads and make two trips.
What to watch out for
Suppliers sell by the loose yard, and soil compacts in transit. A yard dumped from a truck looks bigger than a yard after you spread and water it. This is why the 5 to 10 percent settling allowance matters: it absorbs the gap between the loose volume you bought and the compacted volume that stays put.
Also confirm whether the supplier measures in loose or compacted yards when comparing quotes. Two suppliers quoting 40 dollars a yard are not offering the same deal if one fills the loader loosely and the other compacts it. Ask for the fill method, and ask whether the price includes delivery and a minimum order size.