I’m building a large raised bed planter. Its 25 feet long and 18 inches high, BUT its 16 feet wide at one end and gradually narrows down to 4 feet at the other end. I need to order top soil for it by the square yard.

Is there anyone out there that would know the mathematical formula to answer this question?

Find the area of the trapezoid formed by the planter walls

Multiply the result by the depth of the planter to find the total volume

If this isn’t a homework problem, you’ll want to add some extra for compaction; 10 percent or so ought to do. Also, you’ll want to allow for drainage, so fill the bottom with crushed stone, covered with sand or coarse gravel.

When you get your cubic yardage carefully calculated, the soil will be precisely measured by the cubic-yard bucket of a payloader tractor. There’s a bit of a fudge factor in that.