The Compost and Topsoil Calculator below is for use by permit applicants to determine the compost and/or topsoil needs for site development projects to meet the post-construction soil standard required by King County's Clearing and Grading regulations KCC 16.82. This information must be included on the Soil Management Plan Download Microsoft Word document 900 K form required as part of the permitting process. Steps for preparing a Soil Management Plan are found on pp. 8-9 in the Achieving the Post-construction Soil Standard brochure Download PDF 1 MB.

The calculator can calculate:

• a custom compost amendment rate (i.e., number of inches of compost to be layered over the site surface and mixed in)
• quantity of compost (in cubic yards) needed
• quantity of site soil (in cubic yards) to be stockpiled, and/or
• quantity of imported topsoil (in cubic yards) needed.

Follow these four steps to determine compost or topsoil needs and provide results as required by the permitting process:

Step 1: Determine which soil management option(s) are needed for each area marked on project site map by reading Achieving the Post-construction Soil Standard brochure Download PDF 1 MB (pp. 4-6).

Step 2: Choose Turf or Planting beds and determine Amendment rate of compost needed:

• For soil treatment options that involve adding compost to the soil, permit applicants must choose to use a pre-approved amendment rate of application or a custom amendment rate.
• On many sites, a custom amendment rate will be lower than the pre-approved rate. To calculate a custom amendment rate, you will need to enter information from laboratory tests* of the site soil and specified compost products.