Soil Amendment Calculator
Whether you are adding compost to a new raised bed, applying lime to raise soil pH, or top-dressing an established lawn, knowing the exact volume and weight of amendment required prevents over-application and saves money. This calculator multiplies your garden area by the application depth to give cubic feet and cubic yards, then converts to approximate weight using a configurable bulk density. The default bulk density of 800 lb per cubic yard represents mature finished compost. Adjust for your specific product using the bag label or supplier data.
Soil amendment volume formula
Volume (cu ft) = Area (sq ft) x Depth (in) / 12
Volume (cu yd) = Volume (cu ft) / 27
Weight (lb) = Volume (cu yd) x Bulk density (lb/cu yd)
Divide by 27 to convert cubic feet to cubic yards. Most landscape suppliers sell bulk compost and topsoil by the cubic yard.
Typical bulk density reference values
- Finished compost: 700 to 1,100 lb per cu yd (use 800 as default).
- Topsoil: 1,800 to 2,200 lb per cu yd.
- Peat moss: 400 to 600 lb per cu yd.
- Perlite: 200 to 250 lb per cu yd.
- Ground limestone: 1,350 to 1,600 lb per cu yd.
Soil amendment calculator: frequently asked questions
How do I calculate how much compost to add?
Determine the target compost depth and multiply by garden area. For example, to add 3 inches of compost to 200 sq ft: 200 x (3/12) = 50 cubic feet. Divide by 27 to convert to cubic yards (1.85 cubic yards). One standard wheelbarrow holds about 3 cubic feet.
How much garden lime do I need to raise soil pH?
Lime rates depend on current pH, target pH, and soil texture. A typical recommendation is 5 to 10 lb of ground limestone per 100 sq ft to raise pH by one unit in sandy soil, and 8 to 15 lb per 100 sq ft in clay soil. Always base amendments on a soil test from a USDA-certified lab or Cooperative Extension service.
What is the typical bulk density of compost?
Finished compost has a bulk density of 800 to 1,100 kg per cubic meter (50 to 68 lb per cubic foot). For conversion from volume to weight, the calculator uses 800 lb per cubic yard (roughly 30 lb per cubic foot) as a conservative default for mature compost.
How deep should I incorporate a soil amendment?
For new beds, incorporate amendments to the full depth of the root zone: 6 to 8 inches for vegetables, 4 to 6 inches for lawns. For established gardens, top-dress with 1 to 2 inches and let earthworms and rain incorporate it. Deep incorporation is done once; top-dressing is done annually.
Does lime need to be incorporated into the soil?
Lime works best when incorporated into the top 6 to 8 inches. Surface application is acceptable for established lawns (because tilling would damage roots) but takes longer to change pH. USDA and Extension guidelines recommend soil testing 3 to 6 months after liming to assess effectiveness.
Official sources
- USDA NRCS Soil Health: nrcs.usda.gov.
- USDA National Institute of Food and Agriculture: nifa.usda.gov.
Reviewed by the CalculatorHub team, edited by James Graham, 15 June 2026. See our methodology.