How the calculation works

Every material is part water and part dry matter, and the carbon and nitrogen are in the dry matter. For each material on your list the calculator works out:

Dry matter = weight × (100 − moisture %) ÷ 100.
Nitrogen = dry matter × nitrogen % ÷ 100.
Carbon = nitrogen × the material’s C:N ratio (Cornell: %C = %N × C:N).

It adds these up for the whole pile, then works out the mix:

Mix C:N = total carbon ÷ total nitrogen.
Mix moisture = total water ÷ total weight × 100.

This is the mass-balance formula the Cornell Waste Management Institute gives for the C:N ratio and moisture of a compost mixture.

To reach your target, the calculator solves the same formula for one unknown weight: amount to add = (target × total nitrogen − total carbon) ÷ (dry fraction × nitrogen fraction × (C:N of the material − target)). Cornell gives this solution for a second or third material; the same algebra works however many materials are already in the pile. If the answer comes out negative, the material is on the wrong side of the target: you can’t lower the ratio with something browner than the target, or raise it with something greener. A material whose own C:N equals the target can’t get the mix there either, however much you add.

Why you can’t average the ratios

A C:N ratio is carbon per unit of nitrogen, so the ratio of a mix depends on how much nitrogen each material brings, and that depends on how wet it is and how much nitrogen its dry matter holds. Averaging the ratios treats every material as if it brought the same amount of nitrogen. Sometimes the answer comes out close anyway: equal weights of grass clippings (17:1) and fallen leaves (54:1) average 35.5:1, against a real 34.6:1. But equal weights of grass clippings and shredded cardboard (563:1) average 290:1, while the real mix is about 88:1.

Worked example

This is the example the calculator opens with: 10 lb of grass clippings and 10 lb of fallen leaves, using the NRAES averages (grass: 82% moisture, 3.4% nitrogen, 17:1; leaves: 38% moisture, 0.9% nitrogen, 54:1).

Dry matter: grass 10 × 0.18 = 1.8 lb; leaves 10 × 0.62 = 6.2 lb.
Nitrogen: 1.8 × 3.4% = 0.0612 lb; 6.2 × 0.9% = 0.0558 lb; total 0.117 lb.
Carbon: 0.0612 × 17 = 1.0404 lb; 0.0558 × 54 = 3.0132 lb; total 4.0536 lb.
Mix C:N = 4.0536 ÷ 0.117 = 34.6:1: inside NRCS’s 20:1–40:1 range, and a little browner than Cornell’s 30:1.
Moisture = (8.2 + 3.8 lb of water) ÷ 20 lb = 60%, the top of Cornell’s 50–60%.

To reach exactly 30:1 you would add (30 × 0.117 − 4.0536) ÷ (0.18 × 0.034 × (17 − 30)) = 6.8 lb more grass. But the new 26.8 lb pile would be 65.6% moisture, wetter than the 65% limit NRCS gives. Following NRCS and Cornell, you would leave the mix at 34.6:1, because a slightly high C:N only slows composting down. In metric the same mix is 5 kg of each, and the top-up would be 3.4 kg of grass.

Checking against Cornell’s example

Cornell’s own example uses 10 kg of grass clippings at 77% moisture with 2.4% nitrogen and 45% carbon (so 18.75:1), and leaves at 35% moisture with 0.75% nitrogen and 50% carbon (66.67:1). To try it, switch to metric, change the first material to “Custom material” with the grass figures and 10 kg, remove the second material, choose “Custom material” as the material to add with the leaf figures, and keep the 30:1 target. The calculator gives Cornell’s answer of 3.5 kg of leaves, and shows the catch Cornell points out: the mix would be over 66% moisture (66.2%).

Targets: what the sources say

Source	Starting C:N	Moisture
Cornell Waste Management Institute	About 30:1	50–60%, what “most literature” recommends; its operator fact sheet says 40–60%
USDA NRCS	20:1 to 40:1 for rapid composting	Generally 40–65%; ideally 60% once the ingredients are mixed
Nebraska Extension	About 30:1	40–60%

The calculator aims for your target (30:1 unless you change it) and judges the mix against these figures: below 20:1 is too much green and above 40:1 too much brown; below 40% moisture is too dry, 50–60% is ideal and above 65% is too wet. Between 40% and 50%, and between 60% and 65%, the sources disagree, so the calculator calls those “on the dry side” and “on the wet side”.

Too much green (low C:N): Cornell says nitrogen is supplied in excess and lost as ammonia gas, causing bad odors. Nebraska Extension says such a pile heats up fast, then quickly cools down and smells of ammonia.
Too much brown (high C:N): there isn’t enough nitrogen for the microbes, so the pile stays relatively cool and breaks down slowly (Cornell).
Too wet: water fills the spaces air needs and the pile can turn anaerobic (Cornell). Cornell also warns that grass clippings may feel dry but hold a lot of water, which is released as they break down and turns them slimy.
Too dry: decomposition slows down (Cornell). NRCS says microbial activity stops altogether below 15% moisture.

The material values used

The presets come from the table of typical raw-material characteristics in the NRAES On-Farm Composting Handbook (Appendix A, Table A.1), which Cornell publishes and NRCS adapts as Table 2A-1. The calculator uses the table’s average where it gives one, otherwise its typical value (for mixed food scraps, the middle of the typical ranges); ranges are in parentheses. The last column shows the C:N from Cornell’s home-composting table, which comes from a different publication, so you can see how much the figures vary.

Material	Table row (figure used)	Nitrogen (% of dry weight)	C:N (… : 1)	Moisture (% of wet weight)	Cornell home table C:N
Grass clippings	Grass clippings: average	3.4 (2–6)	17 (9–25)	82	15–25:1
Vegetable scraps	Vegetable produce: typical value	2.7	19	87	15–20:1
Fruit scraps	Fruit wastes: average	1.4 (0.9–2.6)	40 (20–49)	80 (62–88)	–
Mixed food scraps	Garbage (food waste): midpoint of the typical ranges; typical moisture	2.4 (1.9–2.9)	15 (14–16)	69	–
Cow manure	Cattle: average	2.4 (1.5–4.2)	19 (11–30)	81 (67–87)	5–25:1
Horse manure	Horse, general: average	1.6 (1.4–2.3)	30 (22–50)	72 (59–79)	5–25:1
Chicken manure (no bedding)	Laying hens: average	8 (4–10)	6 (3–10)	69 (62–75)	5–25:1
Chicken litter (manure and bedding)	Broiler litter: average (the table notes the C:N is estimated from ash or volatile solids data)	2.7 (1.6–3.9)	14 (12–15)	37 (22–46)	5–25:1
Sheep manure	Sheep: average	2.7 (1.3–3.9)	16 (13–20)	69 (60–75)	5–25:1
Fallen leaves	Leaves: average	0.9 (0.5–1.3)	54 (40–80)	38	30–80:1
Straw	Straw, general: average	0.7 (0.3–1.1)	80 (48–150)	12 (4–27)	40–100:1
Shredded cardboard or newspaper	Corrugated cardboard: typical value	0.1	563	8	560:1

Cornell’s home table gives a single 5–25:1 range for all manure.
Sheep manure is only in the NRCS copy of the table.
Shredded newspaper uses the cardboard figures. Cornell’s home table lists “newspaper or corrugated cardboard” together at 560:1, and cardboard’s 0.1% nitrogen, 563:1 and 8% moisture all fall inside the NRAES ranges for newsprint (0.06–0.14% nitrogen, 398–852:1 and 3–8% moisture).
Real materials vary. NRCS says table values should only be used for rough estimates and recommends testing your own materials, and Nebraska Extension points out that brown clippings from a poorly kept lawn hold far less nitrogen than lush clippings from a fertilized one. If you have better figures, use a custom material.

Practical tips

Get the moisture right first. If the mix is too wet, add a drier material; Cornell suggests partly decomposed leaves or wood chips. If it is too dry, add water: the calculator shows roughly how much brings the mix to 50%. Cornell’s rule of thumb is that dry leaves need about 20 US gallons of water per cubic yard to start with (about 99 litres per cubic metre).
Some carbon is slow. Cornell notes that newspaper, straw and corn stalks break down slowly because some of their carbon is locked up in lignin or resistant cellulose, so a higher starting C:N can be planned for them. Sawdust’s carbon is more available than wood chips’ because of its larger surface area.
The ratio falls as the pile composts. Cornell says it drops from about 30:1 to 10–15:1 in finished compost, because microbes give off about two-thirds of the carbon they use as carbon dioxide.
Manure, meat and dairy. Nebraska Extension says not to add manure from dogs, cats, pigs or any other type of livestock if the compost will be used in a vegetable garden, because bacteria and parasites that infect these animals could survive composting and be eaten with the produce. It also lists meat, fish, bones, dairy products, oils and grease as materials to keep out of the pile, and says meat, bones, grease and dairy products can attract rodents and other wildlife.

Common questions

Why can’t I just average the C:N ratios of my materials?: Because each material brings a different amount of nitrogen. The ratio of a mix is its total carbon divided by its total nitrogen, so the materials carrying the most nitrogen pull the mix toward their own ratio. Mix 10 lb of grass clippings (17:1) with 10 lb of shredded cardboard (563:1): the average of the two ratios is 290:1, but the real mix is about 88:1, because the wet grass still holds more than six times as much nitrogen as the dry cardboard.
Can I measure in buckets instead of weighing?: Not reliably, which is why this calculator works by weight. The same volume can weigh very different amounts. In the NRAES table (NRCS Table 2A-1), a cubic yard of loose, dry leaves weighs 100–300 lb (about 59–178 kg per cubic metre), but compacted, moist leaves weigh 400–500 lb (about 237–297 kg/m³). Grass clippings weigh 300–400 lb loose and 500–800 lb compacted (about 178–237 and 297–475 kg/m³). Nebraska Extension also says balancing greens and browns works best by weight rather than volume. To weigh a bucketful, stand on a bathroom scale holding it and subtract your own weight; then you can count buckets of that material.
Where are coffee grounds, wood chips and sawdust?: The tables used here don’t give all three figures for them. The NRCS table lists coffee grounds at 20:1, as does Cornell’s home-composting table, but gives no nitrogen or moisture. It gives hardwood chips 560:1 and softwood chips 641:1, both with 0.09% nitrogen, but no moisture. For sawdust its averages are 442:1, 0.24% nitrogen and 39% moisture, but the table is compiled from many references and these figures don’t fit together: 0.24% nitrogen at 442:1 would mean 106% carbon, which is impossible. To add wood chips, choose “Custom material” and enter the C:N and nitrogen above with the moisture of your own chips (see the next question). For coffee grounds or sawdust you would need your own nitrogen figure, such as a lab test.
How do I find the moisture of my own material?: Weigh a sample, dry it until its weight stops dropping (NRCS’s test method treats a sample as dry once its weight stays the same between two drying stages), then weigh it again. Moisture % = (wet weight − dry weight) ÷ wet weight × 100. In Cornell’s example, 10 g of grass clippings dried down to 2.3 g, so they were 77% water. For a quick check, Nebraska Extension describes the squeeze test (Cornell gives the same test for leaves): the material should feel damp, with only a drop or two of liquid coming out when you squeeze a handful tightly.
Which matters more, the C:N ratio or the moisture?: Moisture. NRCS says that if you can’t get both right, set the mix up for the right moisture first, then balance the C:N on the high side of the 20:1–40:1 range: a high C:N only slows the process, but a low one can cause odors and lose nitrogen as ammonia. Cornell reaches the same conclusion in its worked example and says it is usually best to err on the side of a high C:N ratio. If the mix is dry, though, Cornell says to get the C:N ratio right and add water as needed.

Sources

Last checked and updated: November 1, 2026.