Settlement is real, but it is not a fixed percentage
Loose soil can reduce in volume as particles rearrange, water fills pores and normal traffic compresses the material. The amount depends on soil texture, moisture, handling, placement and the load applied. A universal “always add 20%” rule can therefore be as misleading as adding no allowance at all.
Separate three different uncertainties
- Measurement allowance covers imperfect dimensions and uneven ground.
- Settlement allowance covers expected volume reduction after placement.
- Supplier rounding converts the estimate into available bags, buckets or load increments.
Combining these invisibly can inflate the order. Our calculators show the allowance as a separate input and preserve the base volume in the result.
Use a smaller allowance when
The area is regular, dimensions are verified, depth is controlled, the supplier sells small increments and a second purchase would be easy. Use a more cautious allowance when the site is irregular, material will settle significantly or a second delivery would be disproportionately expensive.
Compaction is not always desirable
Soil needs pore space for water and air. Heavy mechanical compaction can damage structure and restrict roots. USDA NRCS identifies bulk density as an indicator of compaction and notes that it affects infiltration, rooting depth, water capacity, porosity and aeration. Placement should follow the project specification rather than treating maximum density as the goal.