Almost every bulk material is sold by the cubic yard, and almost every area you measure is in feet with a depth in inches. There is one formula that goes straight between them.
A cube three feet on every side. That is 27 cubic feet, and it is a genuinely large amount of material: roughly a small pickup bed heaped, or a pile about waist high and the width of a car.
Having that picture matters, because the most common mistake here is not arithmetic. It is ordering a number that sounds reasonable on a screen and turns out to be a driveway-filling mountain when it arrives.
You will be measuring an area in feet and a depth in inches. Converting the depth to feet first, multiplying, then dividing by 27 works, but it is three steps and two chances to slip.
Instead, use the divisor that does it in one:
324 is 27 cubic feet times 12 inches. It converts the depth from inches and the volume to yards in a single step, so there is nothing to get wrong in between.
A bed 20 feet by 8 feet at 3 inches deep: 20 times 8 times 3 is 480, divided by 324 is 1.48 cubic yards. Order one and a half.
If you are working the other way round, from a known depth to an area, these are worth remembering because they come up constantly.
One cubic yard covers approximately:
The 3 inch figure is the one to keep in your head, because 2 to 3 inches is the standard depth for mulch and decorative stone. Roughly 100 square feet per yard at 3 inches is close enough for a quick estimate standing in the garden.
Length and width are measured. Depth is chosen, and choosing badly costs more than mismeasuring.
Doubling the depth doubles the material and doubles the cost, and on the same area that is a far bigger swing than any measuring error. Getting a 200 square foot bed slightly wrong might change your order by a tenth of a yard. Ordering it at 4 inches instead of 2 changes it by nearly two full yards.
So decide the depth deliberately and for a reason. Mulch depth and gravel depth both have specific recommendations, and they are different from each other.
Note that weight is a separate question and varies enormously by material. A cubic yard of mulch might weigh a few hundred pounds while a cubic yard of wet gravel is well over a ton. If delivery access or vehicle capacity is a constraint, ask the supplier for the weight rather than assuming, because the volume tells you nothing about it.
Bulk materials compact. Mulch settles as it breaks down, gravel beds down under traffic, and soil drops noticeably after the first heavy rain.
Adding around 5 to 10 percent covers settlement, uneven ground and spillage during spreading. Uneven ground is the underrated one: a bed that looks flat rarely is, and the low spots absorb material without showing it.
Ordering slightly long is cheap. A second delivery of half a yard usually costs more in delivery charge than the material itself is worth. Measuring the area in the first place is covered here.
Not sure where to begin? The tools we point most people at first are gathered in our main picks.