How the amount is worked out
Volume is length × width × depth. Depth is entered in inches and divided by 12 to get feet, so the result is in cubic feet. Divide by 27 to get cubic yards, the unit most landscape suppliers sell by. Weight is cubic yards multiplied by the density you enter, in tons per cubic yard. Bags are the cubic feet divided by half a cubic foot, a common bag size at home centres, rounded up to whole bags.
If you enter a price, choose whether it is per ton or per cubic yard, and the calculator multiplies it by the matching amount. Delivery fees and minimum loads are not included, so add them yourself.
Choosing the density
The calculator starts at 1.4 tons per cubic yard. That is a rule of thumb, not a fixed property of gravel. Crushed stone, pea gravel, river rock and recycled concrete all weigh different amounts, and the same stone weighs more wet than dry. Suppliers that sell by the ton usually know the figure for each product they stock, so ask for it and type it in. The volume does not change when you change the density; only the tons do.
Choosing the depth
The default depth is 3 inches, a common starting point for a decorative layer, a garden path or a top dressing over existing stone. Thinner layers tend to wear through to the soil underneath. Driveways, parking pads and bases under pavers, sheds or slabs are normally thicker and built in compacted layers, often with a coarser stone at the bottom. For those jobs, run the calculator once per layer with its own depth and density, and follow the plan or your contractor for the depths.
Compaction and extra
Loose gravel settles once it is spread and driven on or tamped, so the finished layer ends up thinner than the loose depth. Many people order a little more than the bare volume to allow for this and for uneven ground. The calculator gives the bare volume; add whatever margin suits your job before you order.
Odd shapes
Split the area into rectangles and add the results. For a round area, the area is about 0.785 × diameter², so you can enter the diameter as the length and 0.785 × diameter as the width.