Measure the area you will actually cover
Sketch the bed and measure its inside edges. Subtract paths, slabs, and other spaces you will leave uncovered. For an irregular outline, divide the bed into rectangles or other simple shapes and add their areas without counting overlaps twice.
For this example, use a 20 × 10 ft bed and a chosen 3 inch layer. That is an arithmetic example, not a recommended installation depth for every rock product. Ask your supplier or installer what depth suits the product and use, and account for any existing material.
Calculate cubic yards and an allowance
The example needs 20 × 10 × 3 ÷ 324 = 1.8519 cubic yards before extra. The denominator combines 12 inches per foot and 27 cubic feet per cubic yard. In metric, multiply area in square meters by depth in centimeters and divide by 100 to get cubic meters.
An optional 10% purchasing allowance changes 1.8519 to 2.0370 cubic yards. Keep the unrounded result for pricing and display it as approximately 2.04 cubic yards. The extra percentage is your planning choice; it does not independently predict compaction or correct inaccurate measurements.
Convert to tons with the actual rock density
The volume with extra is 55 cubic feet. If the supplier specifies 100 lb per cubic foot, the estimated weight is 5,500 lb, or 2.75 US short tons. Those numbers depend on that illustrative density. Do not assume lava rock, river rock, and decomposed granite all share it.
Ask whether the density describes loose material as delivered or a compacted layer. Also confirm the unit on a quote: one US short ton is 2,000 lb, while a metric tonne is 1,000 kg.
Compare orderable quantities and total costs
Using hypothetical prices, 2.0370 cubic yards at $60 per cubic yard plus $75 delivery totals $197.22 before tax or labor. If the yard only sells half-yard increments, an order of 2.5 cubic yards would instead total $225 with the same delivery fee. These are comparison examples, not current supplier prices.
Enter your own volume, density, bag size, and prices in the rock cost calculator. Compare the options rather than adding them together: bulk by yard, bulk by weight, and bagged purchases are alternative ways of buying the same material. Confirm unloading access and the final quote before ordering.
Calculate volume from your measurements, use the supplier’s density for weight, and price the actual quantity the supplier will sell.
Worked examples and sample prices are illustrative, not current supplier quotes. These articles explain quantity arithmetic, not structural or horticultural design. See methodology and limits.