Compost and Topsoil Calculator
for Pre-approved or Custom Amendment Rates
Before using this calculator tool, please read the booklet – Achieving the Post-construction Soil Standard Download PDF 800 K.
This calculator may be used 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.
For soil treatment options that involve adding compost to the soil, permit applicants may choose to use the pre-approved amendment rate or a custom amendment rate. To calculate a custom amendment rate, you will need to enter information from laboratory tests of the site soil and specified compost products. On many sites, a custom rate will be lower than the pre-approved rate. Soil and compost testing must be performed by an accredited soil testing laboratory. Sampling must be conducted by an independent engineer or landscape professional. Tested sample must be a composite of samples taken from at least 10 locations distributed throughout the landscape area to be treated, following standard sampling protocols (available form most soil laboratoies). Organic matter tests should be by the Loss on Ignition method.
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.
The results of these calculations must be entered on the Soil Management Plan Download PDF 900 K, and a print-out of the calculation results should be attached to the Plan. See Achieving the Post-construction Soil Standard Download PDF 800 K for more information. Department of Local Services, Permitting Division reviewers will also use the calculator to verify the Soil Management Plan information. Please note that there are separate calculation sheets for turf and planting beds.