Potting Soil Calculator
Peat-based mixes are light and sold by volume, so cubic feet is the number that matters.
The method
volume = area × depth; weight = volume × density
Depth in inches, volume in yards
This is where the arithmetic usually goes wrong. A cubic yard is 27 cubic feet, so spread 4 inches deep it covers 81 square feet — not 27. Divide the depth in inches by 12 before multiplying, or work in a single unit throughout.
Order by volume when you can and treat the weight as a cross-check. Loose densities vary by about 10% between quarries and rise sharply when the material is wet, so a ton of gravel is a less definite quantity than a cubic yard of it.
Add a compaction allowance for anything that will be rolled or tamped. Crusher run and road base typically lose 20–25% of their loose volume once compacted.