Compost coverage explained: depth, cubic feet and bags
How area and depth set the volume of compost you need, why 27 cubic feet make a cubic yard, and how to choose between bags and bulk.
Area times depth is the whole formula
Every coverage estimate comes down to one idea: volume is the area you are covering multiplied by how thick a layer you spread. The only catch is units. Area is measured in square feet but depth is easiest to think about in inches, so the depth has to be converted to feet first by dividing by 12. That turns a 2 inch layer into 0.1667 feet, and a 100 square foot bed then needs 100 times 0.1667, or 16.6667 cubic feet. Measure the area accurately and the rest is arithmetic the tool handles for you.
Converting cubic feet to cubic yards
Bagged amendment is priced in cubic feet, but bulk suppliers quote cubic yards, and the two are easy to confuse. A cubic yard is a cube three feet on each side, so it holds 3 times 3 times 3, which is 27 cubic feet. To move between them you divide cubic feet by 27 for yards or multiply yards by 27 for feet. The 16.6667 cubic foot bed above is only 0.6173 of a cubic yard, which shows why small jobs rarely justify a bulk delivery.
Bags versus bulk delivery
The break-even point between bags and bulk depends on how much you need and what delivery costs in your area. Bags are convenient for small beds, easy to carry and store, and let you buy exactly what a single project needs. Bulk is cheaper per cubic yard once you are covering large areas or a whole lawn, but it usually carries a delivery fee and needs somewhere to pile the material. The calculator gives both figures so you can compare a shelf of bags against a yard of bulk at a glance, and it rounds bags up because you cannot buy a fraction of one.
Choosing a top-dressing depth
Depth is where people most often over or under estimate. For lawns a light quarter to half inch layer is enough to feed the surface without smothering the grass, and going deeper wastes material and can bury the crowns. For garden beds a one to three inch layer worked into the top few inches gives roots plenty to reach. Because volume scales directly with depth, halving the depth halves the compost you buy, so it pays to pick a realistic depth before you order.