Amendment vs. topdressing: the two real uses of compost
Compost does two very different jobs depending on how you use it, and this calculator's mode toggle exists because the math genuinely changes between them. Soil amendment means tilling compost into the existing soil of a new bed, typically a 1 to 2 inch layer worked into the top 4 to 8 inches, to improve structure, drainage, and nutrient content before you plant. Lawn topdressing is the opposite in scale: a thin 0.25 to 0.5 inch layer spread over established turf and left to work down on its own, never tilled in, since there is grass in the way. Treating the two the same, using an amendment-scale depth on a lawn, is the single most common way compost topdressing goes wrong.
Why the mix ratio matters more than the depth alone
For amendment, the number that actually determines your result is not the compost depth by itself, it is the ratio of compost to the total tilled depth: compost depth divided by compost depth plus existing soil depth. A 1 inch layer of compost tilled into 6 inches of native soil gives 1 divided by 7, about 14 percent compost by volume in the finished bed. Till that same inch of compost into only 4 inches of soil instead, and the ratio jumps to 1 divided by 5, 20 percent, because the same amount of compost is now diluted into less total volume. Most references suggest aiming for roughly 15 to 30 percent compost by volume for ordinary garden beds, rising toward 30 to 50 percent for new raised beds built mostly from scratch. Above that upper range, compost starts causing problems of its own: nutrient imbalances, drainage issues, and soil that holds too much water, so more is not automatically better here either.
Why thin lawn topdressing, and what happens if you go too thick
A lawn topdress is meant to work compost down between and around living grass blades, feeding the soil biology and leveling minor low spots without disturbing the turf itself. That only works at a thin depth, 0.25 to 0.5 inch, spread evenly enough that grass blades are still visible through the layer right after application. Go noticeably thicker than that and you risk smothering the grass instead of feeding it, blocking light and air from reaching blades that are now buried under a solid layer rather than lightly dusted.
Why compost is lighter and more variable than topsoil
Compost is typically lighter than topsoil, roughly 800 to 1,200 pounds per cubic yard dry to moist, reaching toward 1,500 pounds only when quite wet, compared to topsoil's roughly 2,050 to 3,375 pound range across the same moisture spread. That difference comes down to composition: compost is largely broken-down organic matter with a lot of air space in its structure, while topsoil carries more mineral content (sand, silt, clay) that packs denser. Compost's finished density also varies more by feedstock, yard waste compost, food-scrap compost, and manure-based compost all settle out somewhat differently, which is why this calculator treats density as an editable, approximate figure rather than a fixed constant, the same honesty approach used for gravel and topsoil density on this site.
Bags vs. bulk, and ordering extra
A standard 1 cubic foot bag of compost means 27 bags per cubic yard; larger 1.5 cubic foot bags need only 18. For a small bed or a light lawn topdress, bags are usually the easier option. Past a couple of cubic yards, bulk delivery by the yard is almost always cheaper and saves the trip to haul bags. As with mulch and topsoil, order about 10 percent extra to cover uneven ground and settling, more if the bed has a lot of curves or the topdress needs to cover an uneven lawn surface.
If the bed also needs a finishing layer of mulch on top once the compost is worked in, the mulch calculator handles that next step with the same area-and-depth approach.