Why it's really a volume job
Mulch, soil and gravel are all sold a dozen different ways, by the litre, the bag, the cubic foot, the cubic yard, the bulk bag, but underneath it's one sum: the area you're covering times how deep you want it. Barrow works that out, then the parts people trip on. Loose material settles or compacts once it's spread, so you order a little over the finished depth (gravel most, mulch not at all). It's priced by volume but delivered by weight, so Barrow puts a weight on it, because a cubic metre of gravel is well over a tonne and won't go in the car. And a stack of small bags is often dearer than one bulk load, so if you put in the prices it does that sum too. Densities and bag sizes change from yard to yard, so these are planning figures: confirm the weight and the bag size with your supplier before you order.