Compost Calculator

Background.

An inch of compost sounds like very little. Spread over a single acre it is 3,630 cubic feet — just over 134 cubic yards, or about 67 tons on a truck scale at ordinary moisture — and if the lab report says 2% nitrogen, that thin layer carries more than 1,300 lb of it. Most of that nitrogen is bound up in organic matter and will not reach a plant this season. Some of it will, and how much depends largely on one figure printed on the compost analysis: the carbon-to-nitrogen ratio.

On a garden scale the numbers shrink but the conversions are just as awkward. Beds are measured in square feet, layers are imagined in inches, bulk compost is sold by the cubic yard and sometimes weighed by the ton, and extension bulletins and landscape specifications write the same instruction as a rate. Penn State Extension's turf guidance, for instance, puts a one-inch layer at about 3.1 cubic yards per 1,000 sq ft and a two-inch layer at about 6.2. You can start from either a depth or a rate here; the other comes back alongside cubic feet, a per-acre rate and the cost of the load.

The weight and nitrogen figures follow from three numbers on a compost analysis — bulk density, moisture and total nitrogen — and the first-season release comes from Oregon State University Extension's guide EM 9217, which sorts composts into three bands by C:N. Below 10, about a tenth of the nitrogen becomes available in the first year. From 10 to 20, about a twentieth. Above 20, none, and a woody compost in that band can briefly borrow nitrogen from the soil rather than add to it.

What is compost calculator?

A compost application can be described three ways that mean the same thing: a layer depth in inches, a volume per unit of area (cubic yards per 1,000 sq ft, or per acre), and a weight per unit of area. Moving between them needs two properties of the material. Bulk density is the weight of a cubic yard as delivered; Oregon State puts screened compost at 50% moisture near 1,000 lb, and very wet compost can exceed 1,500. Moisture is the percentage of that wet weight that is water, so what is left after subtracting it is the dry matter. Laboratories report nutrients as a percentage of dry matter, which is why total nitrogen in pounds is dry weight × the nitrogen percentage. Plant-available nitrogen, or PAN, is the part of that total that soil microbes convert to ammonium and nitrate within the first growing season. The C:N ratio is the maturity signal that predicts it: Oregon State describes well-finished compost as settling at 10 to 15, close to soil organic matter, while woody composts run above 20.

How to use this calculator.

1. 01Choose whether you are working from a layer depth or from a published rate in cubic yards per 1,000 sq ft. Gardeners usually think in inches; specifications and extension bulletins usually give a rate.
2. 02Enter the area in square feet, splitting an irregular bed into rectangles and adding them. A full acre is 43,560 sq ft.
3. 03Enter the depth or the rate. For reference, WSU Whatcom County Extension suggests about 1/4 in to topdress a lawn, 1–2 in on annual and perennial beds, 1–3 in around trees and shrubs and 2–4 in for erosion control.
4. 04Enter bulk density, moisture and total nitrogen from the supplier's compost analysis. Without one, 1,000 lb per cubic yard, 50% moisture and 1.5% nitrogen (the median in Oregon State's compost survey) are sensible placeholders, but get real figures before a large order.
5. 05Enter the C:N ratio from the same analysis — it is the only input that changes the first-season nitrogen estimate — and the delivered price per cubic yard, or 0 for compost from your own heap.
6. 06Read the volume to order, then the note beside the results, which names the nitrogen band you landed in and repeats the condition that the estimate holds only for stable, finished compost.

The formula.

From a depth, volume in cubic feet is area × depth ÷ 12, and cubic yards is that ÷ 27. From a rate, cubic yards is rate × area ÷ 1,000 and cubic feet is cubic yards × 27; the depth that rate produces is cubic feet ÷ area × 12. Both directions then restate the application as cubic yards per 1,000 sq ft (volume × 1,000 ÷ area) and per acre (volume × 43,560 ÷ area). The 27 cubic feet per cubic yard and 43,560 sq ft per acre are the exact definitions in NIST Handbook 44. One inch over 1,000 sq ft is therefore 1,000 ÷ 12 ÷ 27 = 3.0864 cubic yards, which is where the extension figure of 3.1 comes from. Weight follows: wet pounds = cubic yards × bulk density, and wet tons is that ÷ 2,000. Dry weight = wet weight × (100 − moisture %) ÷ 100. Total nitrogen = dry pounds × nitrogen % ÷ 100. First-season PAN = total nitrogen × a percentage from Oregon State EM 9217 Table 2: 10% when C:N is below 10, 5% from 10 to 20 inclusive, 0% above 20. A ratio of exactly 10 or exactly 20 therefore sits in the 5% band. The band is a step function, so the nitrogen estimate jumps rather than slides as C:N crosses 10 or 20; a lab result of 9.8 and one of 10.2 differ by half in predicted release even though the composts are much alike. The volume assumes an even layer with nothing added for spillage, uneven ground or compost settling as it is raked out, so round up to the next quantity your supplier sells.

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.