Methodology

Composting Calculator Data Sources & Methodology

Every number on compost.tools traces to a citable source. This page lists all primary sources, known disagreements between authorities, and explains how we resolve them.

Primary-source citations Disagreement table Formula notes

Last reviewed: April 2026

Data Hierarchy

When sources conflict, we follow this preference order:

University cooperative extension publications — Peer-reviewed, regionally tested, written for practical application. Primary sources: Cornell, NC State, Oregon State, Penn State, Nebraska, Clemson, UAF, and UMD.

USDA and EPA datasets — NRCS National Engineering Handbook Part 637, EPA WARM model, USDA AMS organic compost guidance.

On-Farm Composting Handbook (NRAES-54) — The field standard since 1992, edited by Robert Rynk at Cornell. Still the most comprehensive single reference for composting data.

CREF Appendix B — The US Composting Council Research & Education Foundation maintains an updated feedstock database that incorporates newer data than NRAES-54.

Peer-reviewed literature — Used to fill gaps not covered by extension sources.

Known Source Disagreements

The composting literature contains genuine disagreements between authoritative sources. We do not silently pick a value — we surface the disagreement and explain our choice.

Topic	Source A	Value A	Source B	Value B	What compost.tools uses	Rationale
Optimal composting moisture	Cornell Waste Management Institute; most extension	50–60%	OSU EM-9217 (active phase)	55–65%	40–65% acceptable; 50–60% target shown	Wider range is more defensible for varied pile conditions. OSU's 55–65% noted as active thermophilic phase guidance.
Worm consumption rate	UMD Extension	1× body weight/day (ideal)	UAF, Clemson, OSU EM-9034	0.5× body weight/day	0.5× as practical default; 1× shown as ideal-conditions maximum	Majority of extension sources. UMD's 1× is a theoretical maximum under optimal conditions (60–75°F, fully established bin, ideal feed).
Methane GWP multiplier	EPA WARM v16 (IPCC AR4 2007)	GWP-100 = 25	IPCC AR6 (2021)	GWP-100 = 27.9; GWP-20 = 81	WARM value (25) as auditable standard; AR6 20-yr (81) shown as note	EPA WARM is the regulatory/auditing standard in the US. AR6 values shown transparently so users can understand the higher-impact scenario.
Lawn topdressing max depth	Penn State Extension	½" max for surface-only	Some commercial sources	Up to 1"	¼"–½" recommended; warning shown beyond ½"	Penn State's peer-reviewed guidance. Depths above ½" can smother grass without prior aeration.
Container compost percentage	NC State Extension AG-593	15–40%	UMD container media guidance	Up to 100% for some applications	30% default; 5–50% range; warn above 40%	Higher percentages require mature, low-salt, stable compost. NC State's 15–40% is appropriate for typical bagged composts. UMD's 100% applies to professionally-made finished compost.
C:N optimal starting ratio	Cornell Waste Management Institute	~30:1	NC State AG-593	25:1–35:1 as target range	25–35:1 as target range; color code: green 25–35, amber 20–25 or 35–50, red otherwise	A range is more defensible than a single point. Piles from 25–35:1 decompose actively; outside this range shows diminishing efficiency, not failure.

Primary Sources

Rynk, R. (Ed.). (1992). On-Farm Composting Handbook (NRAES-54). Northeast Regional Agricultural Engineering Service, Cornell University. Available via Cornell eCommons. — The primary reference for C:N ratios, bulk density, and moisture content used throughout the feedstock database.
US Composting Council Research & Education Foundation (CREF). (2021). Appendix B: Typical Characteristics of Common Feedstocks. compostfoundation.org/B/. — Updated feedstock database; primary supplement to NRAES-54 for materials not covered in the 1992 edition.
Cornell Waste Management Institute. Compost Chemistry and Microbiology. compost.css.cornell.edu. — Formula reference for C:N blending calculations and moisture management principles.
NC State Extension. (2025, rev.). AG-593: Large-Scale Organic Materials Composting. content.ces.ncsu.edu. — C:N target ranges, moisture guidance, and garden application rates.
OSU Extension. (2025). EM-9217: Interpreting Compost Analyses. extension.oregonstate.edu. — Moisture content guidance (active phase 55–65%); PAN and nutrient analysis framework for compost value calculator.
Penn State Extension. (1997). Using Composts to Improve Turf Performance. extension.psu.edu. — Topdressing depth limits (½" maximum without aeration); turf application rates.
Nebraska Extension. (2013). G2222: Garden Compost. University of Nebraska–Lincoln. extensionpubs.unl.edu. — Raised bed compost percentages; garden incorporation rates; 50% maximum recommendation.
USDA NRCS. (2010). National Engineering Handbook Part 637 Chapter 2: Composting. USDA NRCS. — Manure feedstock data; large-scale composting parameters.
USDA AMS. 5021: Compost and Vermicompost in Organic Crop Production. ams.usda.gov. — Organic compost and vermicompost requirements cited in pile temperature calculator.
EPA. (2023). Waste Reduction Model (WARM) v16. epa.gov/warm. — GHG emission factors used in CO₂ savings calculator: food waste net avoided ≈ 0.72 MTCO₂e/short ton; yard trimmings ≈ 0.03 MTCO₂e/short ton.
EPA. (2018). National Overview: Facts and Figures on Materials, Wastes and Recycling. epa.gov. — Per-capita food waste and yard waste baseline estimates.
EPA. (1993). Appendix B to 40 CFR Part 503: Pathogen Treatment Processes. law.cornell.edu. — PFRP temperature and time requirements: 131°F (55°C) for 3 days (static pile) or 15 days with 5 turnings (windrow).
Clemson Extension. HGIC 1607: Worm Composting. hgic.clemson.edu. — Bin sizing and feeding rate guidance; 0.5× body weight/day practical limit.
OSU Extension. EM-9034: Composting with Worms. extension.oregonstate.edu. — Population growth rates; doubling time range 60–90 days; bin construction and management guidance.
University of Maryland Extension. Indoor Worm Composting or Vermicomposting. extension.umd.edu. — 1× body weight/day under ideal conditions (noted as maximum, not typical).
University of Maryland Extension. Growing Media (Potting Soil) for Containers. extension.umd.edu. — Container media examples ranging from 50:50 blends to 100% compost for some crops.
UAF Cooperative Extension. Composting with Worms. uaf.edu/ces. — Bin sizing rule-of-thumb and home-bin reproduction guidance.

Formula References

C:N Ratio Blending (Model A — dry-mass weighted harmonic mean):

D_i = Volume_i × BulkDensity_i × (1 − Moisture_i)
R_mix = (Σ D_i) / (Σ D_i / R_i)
Moisture_mix = Σ(Volume_i × BulkDensity_i × Moisture_i) / Σ(Volume_i × BulkDensity_i)

Source: Cornell Waste Management Institute. The harmonic mean is mathematically correct for blending ratios. Simple arithmetic averaging of C:N ratios is a common error that produces incorrect results.

Moisture Content (oven-dry method):

MC% = ((Wet_Weight − Dry_Weight) / Wet_Weight) × 100

Water to Add:

W_add = W_pile × (m_target − m_current) / (1 − m_target)

Where m values are moisture fractions (0–1). Source: NRAES-54 Chapter 4.

Limitations

All C:N ratios, moisture contents, and bulk densities in the feedstock database are representative ranges from published literature. Actual values vary substantially based on:

Species, growth stage, and fertilization history (for plant materials)
Feed type and bedding ratio (for manures)
Moisture content at time of measurement (bulk density is especially moisture-sensitive)
Regional and seasonal variation

For commercial or regulated composting, obtain a laboratory analysis of your specific feedstocks. Contact your state university's cooperative extension service for subsidized soil and compost testing programs.