Compost Nitrogen-to-Carbon Ratio Calculator

The ideal C:N ratio for hot composting is 25:1 to 30:1. At this ratio, microorganisms have the perfect balance of energy (carbon) and protein (nitrogen) to multiply rapidly and generate heat. If C:N is too high (>40:1), decomposition slows because nitrogen is limiting. If C:N is too low (<20:1), excess nitrogen is lost as ammonia gas (smelly) and the pile may become anaerobic. At 25:1-30:1, the pile should reach 130-160°F within 24-48 hours. This temperature range kills weed seeds and pathogens while thermophilic bacteria break down materials fastest. Achieving this ratio requires mixing approximately 2-3 parts browns (carbon-rich) to 1 part greens (nitrogen-rich) by volume.

Carbon-rich Browns: dried leaves (50-80:1), straw (80:1), wood chips (400:1), cardboard (350:1), pine needles (80:1), sawdust (500:1), paper (170:1), corn stalks (60:1). Nitrogen-rich Greens: grass clippings (15-20:1), vegetable scraps (15:1), coffee grounds (20:1), seaweed (19:1), horse manure (25:1), chicken manure (7-10:1), alfalfa hay (12:1), blood meal (4:1). Balanced materials: aged manure (20:1), garden trimmings (30:1). The secret to success: vary your inputs. Using only grass clippings (low C:N) creates a slimy anaerobic mess. Using only wood chips (high C:N) takes years to decompose. Mix diverse materials for best results.

Calculate weighted average C:N using the formula: Total C = Σ(Weight₁ × C%₁), Total N = Σ(Weight₁ × N%₁), C:N Ratio = Total C / Total N. Example: Mix 10 lbs dried leaves (C:N 60:1, 48% C, 0.8% N) with 5 lbs grass clippings (C:N 18:1, 42% C, 2.3% N). Leaves: 10 × 0.48 = 4.8 lbs C, 10 × 0.008 = 0.08 lbs N. Grass: 5 × 0.42 = 2.1 lbs C, 5 × 0.023 = 0.115 lbs N. Total C = 6.9 lbs, Total N = 0.195 lbs, C:N = 6.9/0.195 = 35:1. Adjust by adding more greens or browns to reach 25:1-30:1. For quick estimation by volume: 3 parts browns to 1 part greens = ~30:1. By weight: roughly equal parts browns and greens.

Too much carbon (>40:1): pile won't heat up, decomposition takes 6-12+ months, materials look unchanged, fungi dominate over bacteria, pile may dry out because carbon absorbs moisture. Solution: add high-nitrogen materials (grass clippings, manure, blood meal, alfalfa). Too much nitrogen (<20:1): ammonia smell (like cat urine), pile becomes slimy and anaerobic, nitrogen is lost to the atmosphere as NH₃ (wasting fertilizer value), attracts pests (flies, rodents). Solution: add high-carbon materials (dried leaves, straw, cardboard, sawdust). Correct imbalance immediately to save the batch. A properly balanced pile should smell earthy, not putrid or like ammonia. Temperature is the best indicator: 130-160°F = happy microbes.