This calculator was built by Serge (MSc) using government standards (AHAM, ASHRAE, EPA, ISO). My academic background covers Plant Biochemistry, Environmental Biology, Biogeochemistry, and Ecotoxicology. This is an estimation tool for educational purposes, not professional advice, medical diagnosis, or assessment.

Compost C:N Ratio Calculator

Compost Carbon:Nitrogen Ratio Calculator

Composting works when you get the mix right. Too much “brown” (carbon-rich materials like leaves and straw) and decomposition crawls. Too much “green” (nitrogen-rich materials like grass and food scraps) and your pile smells and gets slimy.

I studied biogeochemistry. That taught me that soil biology depends on carbon and nitrogen balance. Microbes that break down compost need the right ratio of both to thrive.

This calculator helps you estimate the C:N ratio of your pile and tells you whether you need more brown material or more green to hit the sweet spot.

Calculate Your Compost C:N Ratio

Brown Materials (Carbon-Rich) Leaves, straw, cardboard, shredded paper, wood chips, sawdust. Estimate total weight in kg. Green Materials (Nitrogen-Rich) Grass clippings, food scraps, manure, coffee grounds, fresh plant matter. Estimate total weight in kg.

How This Works

Compost breaks down through microbial activity. Microbes need carbon (energy source) and nitrogen (protein building blocks).

Brown materials are carbon-rich. Leaves and straw have C:N ratios of 40:1 to 80:1 or higher. Green materials are nitrogen-rich. Grass clippings and food scraps have C:N ratios of 15:1 to 25:1 or lower.

The ideal compost pile sits around 25:1 to 30:1 C:N ratio. At this ratio, microbes have enough carbon for energy and enough nitrogen for growth. Decomposition is fast and efficient.

Understanding Your Result

The calculator estimates your pile’s C:N ratio based on the weights you entered:

Above 35:1. Too much carbon. Your pile breaks down slowly. Microbes are starving for nitrogen. Add more green material or nitrogen-rich amendments like grass clippings, manure, or food scraps.

25 to 35:1. Sweet spot. This ratio favours active decomposition. Your pile should heat up and break down in 4 to 6 weeks. This is what you want.

Below 25:1. Too much nitrogen. Your pile may smell like ammonia and become slimy or anaerobic. Add more brown material like leaves or cardboard. Also ensure good aeration.

Important Limitations

This is an estimation based on average C:N values. Real C:N ratios vary by material type, age, and moisture content.

✗ Does not account for exact C:N of your specific materials
✗ Does not account for moisture content
✗ Does not account for aeration frequency
✗ Does not account for ambient temperature
✗ Does not predict time to finished compost

Use this calculator as a starting guide. Monitor your pile. If it smells or is too wet, add brown material. If it is not heating or breaking down, add green material and ensure good aeration.

Scientific Basis

Based on:

USDA Composting Guidelines. The United States Department of Agriculture provides recommendations for compost carbon:nitrogen ratios. Research shows 25:1 to 30:1 is optimal for aerobic decomposition.
Learn from USDA composting guidance →

ISO 14855 Biodegradability Testing. International standards for how organic materials decompose and the role of C:N ratio in decomposition rate.
Learn about ISO biodegradability standards →