About

Composting fails when the carbon-to-nitrogen ratio (C:N) drifts outside the 25:1 to 30:1 range. Too much carbon (dry leaves, cardboard) and decomposition stalls for months. Too much nitrogen (food scraps, grass clippings) and the pile turns anaerobic, producing hydrogen sulfide and ammonia. Moisture must stay between 40% and 60% by weight. Below that threshold, microbial activity ceases. Above it, oxygen is displaced and putrefaction begins. This calculator uses the dry-weight fractional method: each material contributes carbon and nitrogen proportional to its dry mass, not its wet mass. The tool assumes aerobic, turned-pile composting at ambient temperature. It does not account for vermicomposting kinetics or forced-aeration systems.

Add your materials, specify weights, and the calculator returns the blended C:N ratio, overall moisture percentage, estimated pile volume, and a rough decomposition timeline. Pro tip: most home composters underestimate moisture in "brown" materials. Corrugated cardboard at 8% moisture behaves very differently from rain-soaked leaves at 60%. Weigh inputs after collection, not after drying.

Formulas

The blended carbon-to-nitrogen ratio is computed using the dry-weight fractional method. Each material contributes carbon and nitrogen in proportion to its dry mass, not its wet (as-received) mass.

C:N_blend = n∑i=1 Q_i × C_i × (1 − M_i)n∑i=1 Q_i × N_i × (1 − M_i)

Where Q_i = mass of material i in kg, C_i = carbon fraction (derived from C:N ratio as R_iR_i + 1 where R_i is the C:N ratio of the material), N_i = nitrogen fraction (1R_i + 1), and M_i = moisture content as a decimal fraction.

Overall moisture content of the blend:

M_blend = n∑i=1 Q_i × M_in∑i=1 Q_i × 100%

Estimated pile volume uses bulk density ρ_i in kg/m³:

V_total = n∑i=1 Q_iρ_i

Materials with a C:N ratio of 0:1 (e.g., egg shells, wood ash) contribute negligible organic carbon and nitrogen. They are included in moisture and volume calculations but excluded from the C:N ratio computation to avoid division artifacts.

Reference Data

Material	Type	C:N Ratio	Moisture (%)	Bulk Density (kg/m³)	Notes
Grass Clippings (fresh)	Green	17:1	82	250	High nitrogen; layer thinly to avoid matting
Food Scraps (mixed)	Green	15:1	80	500	Avoid meat and dairy in open piles
Coffee Grounds	Green	20:1	65	590	Near-neutral pH despite acidity of brewed coffee
Chicken Manure	Green	7:1	75	400	Very high N; use sparingly; may contain pathogens
Horse Manure	Green	25:1	70	350	Often mixed with bedding; check for herbicide residue
Seaweed / Kelp	Green	19:1	80	300	Rinse to reduce salt; excellent trace minerals
Vegetable Trimmings	Green	12:1	87	450	Decompose rapidly; chop for faster breakdown
Alfalfa Hay	Green	13:1	12	110	Excellent N source; accelerates slow piles
Clover	Green	16:1	78	200	Nitrogen fixer; good green addition
Dry Leaves	Brown	60:1	38	65	Shred to prevent matting; oak leaves decompose slower
Straw	Brown	80:1	12	40	Provides structure and aeration
Cardboard (corrugated)	Brown	350:1	8	50	Remove tape and glossy print; shred small
Newspaper (shredded)	Brown	175:1	6	80	Soy-based inks are safe; avoid glossy inserts
Sawdust (untreated)	Brown	325:1	25	210	Very high C; use thin layers; never use treated wood
Wood Chips	Brown	400:1	20	350	Best as bulking agent; decomposes over years
Pine Needles	Brown	80:1	30	70	Slightly acidic; use in moderation
Corn Stalks (dry)	Brown	60:1	12	55	Chop to < 10 cm for reasonable decomposition
Dryer Lint (natural fiber)	Brown	100:1	5	30	Only from cotton/linen loads; avoid synthetics
Egg Shells	Brown	0:1	2	800	Calcium source; crush finely; negligible C or N
Paper (office, white)	Brown	170:1	5	100	Shred; avoid heavily printed or waxed paper
Hay (dried grass)	Brown	40:1	15	85	May contain weed seeds; hot composting kills them
Tea Bags (paper)	Green	15:1	70	400	Remove staple; some bags contain polypropylene
Fruit Waste	Green	25:1	85	500	Attracts fruit flies; bury in pile center
Garden Weeds (no seeds)	Green	20:1	75	250	Avoid weeds with mature seeds or rhizomes
Cow Manure	Green	20:1	80	450	Well-aged is safer; fresh may contain E. coli
Human Hair	Green	10:1	8	150	Slow to decompose; spread thinly
Wood Ash	Brown	0:1	2	500	Raises pH; use sparingly; no charcoal ash
Bark (shredded)	Brown	120:1	25	280	Very slow to decompose; best as mulch layer
Cotton Fabric (natural)	Brown	100:1	8	120	Cut into small strips; ensure 100% natural fiber
Peat Moss	Brown	58:1	40	200	Acidic; good water retention; environmental concerns

Frequently Asked Questions

Ammonia off-gassing occurs when nitrogen volatilizes as NH₃. This happens even at correct C:N ratios if the pile is too wet (above 65% moisture), too compacted (anaerobic pockets), or if a nitrogen-rich layer is exposed on top. The calculator assumes homogeneous mixing. In practice, you must physically blend materials, not just layer them. Turn the pile and add dry browns to the surface.

The time estimate assumes materials are chopped to roughly 5-10 cm pieces. Whole branches or un-shredded cardboard can triple decomposition time because microbes work on surface area, not volume. Shredding a material from 20 cm to 2 cm increases surface area by approximately 10×, proportionally accelerating colonization. If your inputs are un-shredded, multiply the estimated time by 1.5-3×.

Not directly. Vermicomposting operates at a narrower C:N range of 20:1 to 25:1 and requires much lower temperatures (15-25 °C). The moisture target is higher at 70-85%. This calculator models aerobic thermophilic composting (hot piles reaching 55-65 °C). Worm bins also cannot tolerate citrus, onions, or high-salt inputs that this calculator does not flag.

Decomposition slows dramatically. Microorganisms require nitrogen to build proteins. When carbon is excessive, microbes cycle through available nitrogen slowly, and the pile may take 12-24 months instead of 2-4 months. The pile will not heat up enough to kill weed seeds or pathogens (requires sustained 55 °C for 3+ days). Add nitrogen-rich greens like grass clippings (C:N 17:1) or alfalfa (C:N 13:1) until the blend drops below 35:1.

No. This calculator computes the initial blend ratio at the time of mixing. During composting, carbon is lost as CO₂ (roughly 50-70% of initial carbon) and nitrogen can be lost as NH₃ or N₂O (5-20% in well-managed piles). The C:N ratio naturally decreases over time as carbon is respired faster than nitrogen is lost. Finished compost typically has a C:N of 10:1 to 15:1 regardless of starting ratio.

The volume uses average bulk density values for loose, uncompacted materials. Actual volume depends on compaction, moisture absorption, and how materials interlock. Freshly piled materials may occupy 20-40% more volume than the estimate due to air gaps. After 2-3 weeks of decomposition and settling, volume typically decreases by 30-50%. Use the estimate for initial bin sizing, not final compost yield.

Measure it yourself if precision matters. The calculator uses published average moisture values, but actual moisture varies widely. Grass clippings range from 60% (sun-dried) to 85% (fresh-cut after rain). The squeeze test is a practical field method: grab a handful and squeeze firmly. Ideal moisture produces 1-2 drops of water. If water streams out, the pile is too wet. If no moisture appears, it is too dry.