From bed size to cubic yards
Landscaping materials are measured by volume, not area. Multiply the length and width of your bed to get the surface area, then multiply by the depth to turn that area into a volume. Dividing by 27 converts cubic feet into the cubic yards used for bulk orders and deliveries.
Ordering the right amount
Round up so a partial bag or short delivery does not leave a thin patch, and order a little extra to top up beds as mulch breaks down over the season. For large areas, compare the per-yard cost of bulk delivery against bagged product, since bulk is usually cheaper once you pass a dozen bags.