Compost Calculator

Background.

A compost calculator has to answer two questions at once, and most only answer the first. The first is geometric: how many cubic yards does it take to cover this area to this depth. The second is agronomic: what is that load actually doing to the soil, in pounds of nitrogen, and is it a soil amendment or an accidental fertiliser application. This page does both, and it is built deliberately around the second because the first is already well served.

Start with the units problem, because it is the reason so many compost orders go wrong. You measure a bed in square feet. You imagine a layer in inches. Your supplier delivers in cubic yards and bills by the ton. Extension bulletins and landscape specifications, meanwhile, write the same instruction as a rate: Penn State's turf publication puts a one-inch layer at roughly 3.1 cubic yards per 1,000 square feet and a two-inch layer at about 6.2. Those are four different ways of saying one thing, and this calculator will take either the depth or the rate and give you all four. Enter a depth and it tells you the rate; enter a rate and it tells you what depth that lands on. The two directions are exact inverses, so you can check one against the other.

Then the part almost nothing else does. Compost is not an inert filler. A cubic yard of screened compost at 50 percent moisture weighs roughly 1,000 pounds according to Oregon State University Extension's compost-analysis guide, half of that is water, and the dry half typically runs between half a percent and three percent nitrogen. Multiply that out over an area of any size and the numbers stop being small. One inch of a 2 percent compost over a single acre carries more than 1,300 pounds of total nitrogen. Most of it is organically bound and will not move this season — Oregon State reports that more than 95 percent of compost nitrogen is organic — but a meaningful fraction mineralises, and how much depends on one number you can read straight off a compost analysis: the carbon-to-nitrogen ratio.

That is the classification this page makes. Oregon State EM 9217's Table 2 gives three bands. A C:N above 20 releases nothing in the first year, and worse, may tie up soil nitrogen and increase your fertiliser requirement. A C:N between 10 and 20 releases about 5 percent of the compost's total nitrogen. A C:N below 10 releases about 10 percent. Enter your ratio and the calculator tells you which band you are in and what that means in pounds. If the answer is zero, that is a real answer and the page says why.

Two scope limits, stated here rather than hidden in an accordion. First, the nitrogen estimates are valid only for stable compost. Oregon State is explicit that some material marketed as compost has not finished composting and that the release figures do not apply to it; if you are applying at a heavy rate, get an analysis. Second, and just as important, nitrogen is rarely the thing that goes wrong. Repeated heavy compost applications build up phosphorus and soluble salts long before they build up excess nitrogen, and neither of those is on this page. A soil test is what tells you when to stop.

This page has no bag count and no bag-price field, and that is on purpose. Counting bags of a surface layer is what the mulch calculator does, and both the mulch calculator and the topsoil calculator say in their own FAQs that the agronomic side is the thing they deliberately leave out. This is that page. If all you want is bags to put in a car boot, use mulch; if you want to know whether a compost programme is feeding the soil or overloading it, stay here.

What is compost calculator?

Compost is organic matter that has been through a controlled decomposition process until it is biologically stable. Oregon State Extension describes composting as "a controlled process for stabilization of organic matter" that returns nutrients and organic matter to soil, improves tilth and water-holding capacity, and — when managed properly — destroys weed seeds and pathogens. The word covers a very wide range of products: yard-trimmings compost, biosolids compost, dairy manure compost, mushroom substrate and bark-heavy landscape blends are all sold as compost and behave differently.

The vocabulary that drives this calculator is bulk density, moisture, total nitrogen, C:N ratio and plant-available nitrogen. Bulk density is pounds per cubic yard, and it is the bridge between the volume you order and the weight a lab report is written about. Moisture is a percentage of wet weight, so a compost at 50 percent moisture is half water by weight. Total nitrogen is reported as a percentage of dry weight, which is why the moisture number matters: the same 2 percent nitrogen means twice as much actual nitrogen in a dry load as in a soaking one, per ton delivered.

The carbon-to-nitrogen ratio is the maturity indicator. Oregon State notes that well-composted material settles at a C:N of 10 to 15, close to soil organic matter itself, while woody composts run above 20. Plant-available nitrogen, usually shortened to PAN, is the fraction of total nitrogen that mineralises into ammonium and nitrate that plants can take up. In most composts the inorganic nitrogen present at the moment of application is under 5 percent of the total; the rest has to be released by soil biology, and only some of it will be in the first season.

Application depths, for reference, come from extension guidance rather than from arithmetic. WSU Whatcom County gives about a quarter inch for lawn topdressing, one to two inches on annual and perennial beds, one to three inches around trees and shrubs, and two to four inches for erosion control. Penn State's turf publication says a one- to two-inch layer should be incorporated four to six inches deep. Maryland Extension's raised-bed page targets 25 to 50 percent organic matter by volume in a made soil. Those are what you type into the depth field; the calculator tells you what they cost in volume, weight and nitrogen.

How to use this calculator.

1. 01Choose what you already know. Pick the depth basis if you are thinking in inches of compost, or the rate basis if a specification or bulletin has given you cubic yards per 1,000 square feet.
2. 02Enter the area receiving compost, in square feet. Break irregular beds into rectangles and add them. One acre is 43,560 square feet.
3. 03Enter either the layer depth or the rate, whichever basis you picked. The other one comes back as an output, so you can sanity-check your number against Penn State's 3.1 cubic yards per 1,000 square feet per inch.
4. 04Enter the bulk density. If you have no figure, Oregon State's rule of thumb for screened compost at 50 percent moisture is 1,000 pounds per cubic yard; ask your supplier for the real number before a large order.
5. 05Enter the moisture and total nitrogen from a compost analysis. If you have no analysis, Oregon State's proficiency-programme median total nitrogen is 1.5 percent of dry weight.
6. 06Enter the carbon-to-nitrogen ratio. This is the single input that decides the first-season nitrogen figure, so it is worth getting from a lab rather than guessing.
7. 07Add the bulk price per cubic yard, or 0 if the compost is coming off your own heap.
8. 08Read the note beside the result. It names the nitrogen band you landed in, repeats the stability caveat that limits it, and gives extension topdressing depths for comparison.

The formula.

In depth basis, volume in cubic feet is area in square feet multiplied by depth in inches and divided by 12; cubic yards is that divided by 27. In rate basis the calculator starts from the other end: cubic yards is the rate multiplied by the area and divided by 1,000, cubic feet is that multiplied by 27, and the layer depth falls out as cubic feet divided by area, multiplied by 12. The two bases are exact inverses — run a depth through, feed the resulting rate back in, and you get the same volume and the same depth.

The restated rates are simple scalings: cubic yards per 1,000 square feet is volume multiplied by 1,000 and divided by area, and cubic yards per acre is volume multiplied by 43,560 and divided by area. Both come from NIST's general tables, where 27 cubic feet make a cubic yard and 43,560 square feet make an acre.

Weight is where the compost analysis enters. Wet weight in pounds is cubic yards multiplied by bulk density in pounds per cubic yard, and wet tons is that divided by 2,000. Dry weight is wet weight multiplied by (100 minus the moisture percentage) divided by 100. Total nitrogen in pounds is dry weight multiplied by the nitrogen percentage and divided by 100 — dry weight, not wet, because that is the basis laboratories report on.

First-season plant-available nitrogen is total nitrogen multiplied by a percentage taken from Oregon State EM 9217's Table 2, chosen by the carbon-to-nitrogen ratio. Below 10 the figure is 10 percent; from 10 up to and including 20 it is 5 percent; above 20 it is zero. The band is selected from the raw ratio you typed, not from a rounded or displayed version of it, so a C:N of 9.996 stays in the 10 percent band even though it would print as 10.0.

Nothing is rounded before the final answer. There is no bag count on this page, so there is nothing to round up: every figure is carried at full precision through the chain and rounded once on the way out.

A worked example.

Frequently asked questions.

References& sources.

1. [1]Oregon State University Extension Service, EM 9217, "Interpreting compost analyses", Sullivan, D.M., Bary, A.I., Miller, R. and Brewer, L. Published October 2018, reviewed 2023. Source of the bulk density rule of thumb (screened compost at 50% moisture is about 1,000 lb/cu yd, very wet compost over 1,500), the moisture and dry-weight basis convention, Table 1 (median total nitrogen 1.5% dry weight by combustion, median C:N 10.4), Table 2 (first-year plant-available nitrogen bands: above C:N 20 → 0%, 10 to 20 → 5%, below 10 → 10%), the statement that PAN estimates are valid only for stable composts, and the worked acre example reproduced in the example on this page. Retrieved 2026-07-31; open access.
2. [2]Penn State Extension, "Using Composts to Improve Turf Performance", Landschoot, P., updated 1 January 1997. Source of the application rates used as the independent check — a one-inch layer is approximately 3.1 cubic yards per 1,000 sq ft and a two-inch layer about 6.2 — of the 4 to 6 inch incorporation depth, and of the 0.5 to 3.0 percent nitrogen guideline range for compost. Retrieved 2026-07-31; open access.
3. [3]Washington State University Extension, Whatcom County, "Using Compost As A Soil Amendment". Source of the reference topdressing depths (about 1/4 inch on lawns, 1–2 inches on annuals and perennials, 1–3 inches around trees and shrubs, 2–4 inches for erosion control and 3–4 inches on slopes), the 6 inch incorporation depth, and the corroborating statement that a one-inch layer over 100 sq ft is 8 cubic feet. Retrieved 2026-07-31; open access. The page carries no named author or publication date, which is recorded here rather than invented.
4. [4]National Institute of Standards and Technology, Handbook 44 – 2026, Appendix C, "General Tables of Units of Measurement". Units of Volume: 27 cubic feet = 1 cubic yard, 1,728 cubic inches = 1 cubic foot. Units of Area: 1 acre = 43,560 square feet. Source of every unit conversion on this page. Retrieved 2026-07-31; open access PDF, downloaded and text-extracted.
5. [5]University of Maryland Extension, "Soil to Fill Raised Beds", updated 6 November 2024. Source of the 25%–50% organic matter by volume target for a made soil and of the compost-to-topsoil blend ratios of 1:2 and 1:1 referenced in the definitions section. Retrieved 2026-07-31; open access.
6. [6]University of Minnesota Extension, "Interpreting your compost report". Corroborating source for the dry-weight reporting basis of compost nutrient analyses and for the role of moisture content in converting a laboratory result into a field application. Retrieved 2026-07-31; open access.