Compost Ratio Calculator

The Carbon-to-Nitrogen Ratio

Every organic material has a carbon-to-nitrogen (C:N) ratio. Microorganisms that decompose compost need both carbon (for energy) and nitrogen (for protein synthesis). The ideal C:N ratio for active composting is 25:1 to 30:1.

Green vs Brown Materials

Green materials (nitrogen-rich) include fresh grass clippings, food scraps, coffee grounds, and fresh manure. They typically have C:N ratios below 30:1 and provide the nitrogen that fuels microbial activity.

Brown materials (carbon-rich) include dry leaves, straw, cardboard, and wood chips. They have higher C:N ratios (often 50:1 to 500:1) and provide the carbon that gives microbes energy while adding structure and airflow to the pile.

Calculation Method

The calculator computes a weighted average of C:N ratios based on the amount of each material. Each material contributes carbon and nitrogen proportional to its quantity and known C:N ratio.

Volume vs Weight

Weight-based calculations are more accurate because materials vary greatly in density. However, volume-based estimates are convenient for most home composters. The calculator adjusts density factors when using volume mode to provide a closer approximation.

What If the Ratio Is Off?

• Too high (> 30:1) — Decomposition slows. Add more green materials (grass, food scraps, manure).
• Too low (< 20:1) — Pile may smell and become anaerobic. Add more brown materials (leaves, straw, cardboard).
• Ideal (25–30:1) — Fast decomposition with minimal odor. Maintain moisture and aeration.