Article — Compost C:N Ratio Calculator
Compost calculator: mix browns and greens for a hot pile
A balanced compost pile starts at a carbon-to-nitrogen ratio between 25:1 and 30:1. Microbes consume roughly 25 carbon atoms per nitrogen atom they incorporate. Below 20:1 the pile reeks of ammonia as excess nitrogen escapes; above 35:1 the pile stalls because microbes run out of nitrogen for new cell construction. This compost calculator uses Cornell-published C:N values to mix any combination of browns and greens.
Composting is the controlled aerobic decomposition of organic material by bacteria, fungi, and actinomycetes. Done right, it turns kitchen scraps, leaves, and grass into a dark, crumbly soil amendment in 8 to 16 weeks. Done wrong, the pile smells, attracts pests, or simply sits cold for months.
What is the compost C:N ratio?
Every organic material has a fixed carbon-to-nitrogen ratio reflecting its composition. Dry leaves average 50:1 — high in lignin and cellulose, low in protein. Fresh grass clippings are 15:1 — low in lignin, high in protein. Microbes need both elements: carbon as an energy source (burned in respiration), nitrogen for amino acids and DNA.
The Cornell Waste Management Institute summarizes the research: piles starting between 25:1 and 30:1 reach the hottest internal temperatures fastest and finish in the shortest time. Below 25:1, decomposition is fast but smelly because nitrogen volatilizes as ammonia. Above 30:1, decomposition slows progressively — at 50:1 a pile may take a year to finish.
The compost C:N ratio drops as decomposition proceeds — microbes respire carbon as CO2 while retaining nitrogen in their cells. A pile that starts at 30:1 finishes at 10:1 to 15:1, the same range as healthy garden soil. That convergence is one sign the compost is mature and ready to use.
Browns vs. greens for compost
Composters classify materials as browns (high carbon) or greens (high nitrogen). The names refer to chemistry, not color — coffee grounds are dark brown but count as green because they are nitrogen-rich (20:1). Manure can be any color and still classifies as green. The volume-based rule of thumb is 3 parts brown to 1 part green; the more accurate mass-based ratio depends on which specific browns and greens you use.
Sawdust 400:1 brownCardboard / paper 170:1 brownStraw / hay 80:1 brownDry leaves 50:1 brownKitchen scraps 15–20:1 greenCow/horse manure 20:1 greenFresh grass clippings 15:1 greenPoultry manure 10:1 ultra-greenBuilding a balanced compost pile
Build the compost pile in alternating 5 to 10 cm layers — a thin layer of browns, then greens, then water lightly, then repeat. The layered structure mixes once you turn the pile a week later, and the alternation prevents anaerobic pockets in matted grass or wet kitchen scraps. Aim for a finished pile at least 1 cubic meter (1.3 cubic yards) — anything smaller loses heat too fast and never reaches the 55 to 60°C thermophilic phase.
Particle size matters more than most home composters realize. Shred leaves, chop kitchen scraps to under 5 cm, and tear cardboard into small pieces. Smaller particles expose more surface area to microbial attack and dramatically speed decomposition. A pile of whole oak leaves takes a year; the same leaves shredded by a lawnmower compost in two months.
Compost moisture and oxygen
A compost pile needs 50 to 60 percent moisture by mass — the texture of a wrung-out sponge. Below 40 percent, microbes go dormant. Above 70 percent, water displaces oxygen and the pile turns anaerobic, smelling of sulfur and ammonia.
Oxygen drives the aerobic decomposition that produces heat. Microbes consume oxygen as fast as they consume carbon, so a pile depletes its internal oxygen within a day or two of building. Turning the pile every 1 to 2 weeks re-oxygenates the core. Compost tumblers, bins with passive air vents, or simple pitchfork turning all work. Aerobic piles emit only CO2 and water vapor; anaerobic piles emit methane, hydrogen sulfide, and ammonia — the source of bad compost smell.
Home compost piles rarely get hot enough to kill pathogens in animal products. Meat, fish, dairy, and oily food attract rodents, raccoons, and flies. They also smell heavily as they decompose. Use a sealed in-vessel system (Bokashi, electric composter) or municipal organics collection for those items.
Compost temperature and timing
A working hot compost pile climbs to 55 to 65°C within 3 to 7 days of building. The thermophilic bacteria active at those temperatures are the fastest decomposers and the only ones that reliably kill weed seeds and pathogens. Above 65°C the microbiome dies back and the pile self-limits. Below 40°C, decomposition slows to a crawl. A long-stem compost thermometer pushed to the pile core gives the cleanest reading.
Total composting time depends on method. Hot composting with regular turning finishes in 8 to 16 weeks. Cool composting (just piling materials and waiting) takes 6 to 12 months. Commercial in-vessel composting at 60 to 70°C with forced aeration finishes in 3 to 4 weeks. After active decomposition, a curing phase of 4 to 8 weeks stabilizes the compost into a dark, sweet-smelling soil amendment.
Save autumn leaves in a separate bin and use them as your brown source year-round. They are free, abundant, and at 50:1 C:N they balance the high-N kitchen scraps that accumulate weekly. A 1-cubic-meter pile of leaves stored dry will balance two summers of typical household compost.
Fixing a failed compost pile
Smelly compost almost always means too much nitrogen or anaerobic conditions. Ammonia smell means C:N below 20 — add browns immediately and turn the pile. Rotten-egg or sulfur smell means anaerobic — turn the pile to add oxygen and mix in dry browns to absorb excess moisture. Both fixes work within 24 to 48 hours.
A cold pile stalls for one of four reasons: too much carbon (add greens), too small (rebuild to at least 1 cubic meter), too dry (water to wrung-sponge texture), or too compacted (turn for oxygen). A balanced pile of correct size and moisture should reach 55°C within a week or the chemistry is off.
Using finished compost in the garden
Finished compost is dark brown, crumbly, sweet-smelling, with no recognizable original materials. The volume reduces by 60 to 70 percent and mass by 40 to 50 percent from the starting material. A 100 kg compost mix yields about 50 kg of finished product, or 0.4 cubic meters of compost from a 1 cubic meter starting pile.
Compost supplies organic matter, slow-release nutrients (typical analysis: 1-1-1 NPK), and microbial inoculant to garden soil. Apply 2 to 5 cm as topdressing or work into the top 15 cm before planting. Mature compost can also brew compost tea for foliar application. Unfinished compost should not be applied directly to growing plants because continuing decomposition pulls nitrogen out of soil and starves the crop.
- Target C:N = 25:1 to 30:1 starting mix
- Browns = leaves 50:1, straw 80:1, cardboard 170:1
- Greens = grass 15:1, kitchen 20:1, manure 20:1
- Moisture = 50–60% (wrung-sponge feel)
- Pile size = at least 1 m³ for heat retention
- Hot pile temp = 55–65°C (kills weed seeds)
- Time to finish = 8–16 weeks hot, 6–12 months cold
- Yield = 40–60% of starting mass as finished compost