Compost Calculator

Multiply the area in square feet by the depth in feet — inches divided by twelve — then divide by 27 for cubic yards. One cubic yard covers about 324 square feet an inch deep, or 108 square feet at three inches. A 500 square foot bed at two inches needs roughly 3.1 cubic yards. The calculator runs each area separately since depths differ, adds your settling allowance, and converts the total into bags, cubic yards and tons.

It depends on the job. Lawn topdressing goes on at a quarter to a half inch. Established beds usually want an inch or two worked in as an annual top-up. A brand-new bed can take two to three inches tilled into the top eight to ten, which lands around a quarter to a third compost by volume — the usual target for establishment. Surface mulching runs two to three inches, and filling a raised bed from empty is a different job again, better done as a blend of compost with soil rather than pure compost. The calculator sets a typical depth for whichever you pick and shows the resulting blend ratio.

Yes, and it’s a common mistake precisely because compost is seen as harmless. Very high rates raise soluble salt levels, which seedlings and salt-sensitive crops react badly to. Repeated heavy applications also build up soil phosphorus well beyond what plants use, since compost carries far more phosphorus relative to nitrogen than most plants need — an issue for runoff as well as plant health. And a thick unincorporated layer on the surface can go airless and smother roots. The calculator flags rates above roughly a third of the finished layer for exactly this reason.

Not directly — it works differently. Most of the nitrogen in compost is locked in organic form and releases slowly as soil microbes break it down, so only a modest share becomes plant-available in the first year; the calculator uses a typical mineralization assumption to estimate that. What compost does well is build organic matter, improve structure and water holding, and feed soil biology, which pays off over seasons rather than weeks. A bed that gets compost regularly often needs little or no added fertilizer, but a hungry crop in a bed’s first year may still want a nitrogen source alongside it.

Yard-waste compost is the general-purpose workhorse and usually the cheapest by the yard. Composted manure is richer in nitrogen and works well for hungry crops. Mushroom compost is a spent growing substrate, fine for most beds but sometimes higher in salts, so it suits established plants better than seedlings. Leaf mold is low in nutrients but excellent for structure and moisture. Worm castings are potent and priced accordingly — used by the handful, not the yard. Biosolids compost is nutrient-rich and cheap but comes with local rules around food crops. Coarse compost mulch is screened big for surface use. Each preset in the calculator carries its own weight, moisture and nutrient profile.

Bagged is almost always the most expensive per cubic yard and makes sense mainly for small jobs or where a bulk pile has nowhere to sit. Between the two bulk routes, it depends on the price pair and how heavy the compost is. Compost is light — often around half the weight of topsoil per cubic yard — so a ton buys noticeably more volume than it would of soil, and a per-ton price that looks similar to the per-yard price is usually the better deal. Moisture cuts both ways here: buying by weight after heavy rain means paying for water. Enter all three prices with your delivery fee and the calculator compares the delivered totals.