Compost C:N Ratio Calculator

Compost C:N Ratio Explained

Composting is a microbial process that converts organic material into stable humus. Microbes need carbon for energy and nitrogen to build proteins. The C:N ratio of a compost pile is the weighted balance of carbon-rich “browns” (dry leaves, straw, wood chips) and nitrogen-rich “greens” (grass clippings, food scraps, manure). When the ratio is too high, decomposition slows because nitrogen is limiting. When the ratio is too low, excess nitrogen can produce odors and ammonia losses. Most compost systems perform well around 25:1 to 30:1, which offers a balanced environment for microbial growth and heat production.

This calculator uses a weighted average. Each material contributes carbon and nitrogen based on its mass and its typical nutrient percentages. The totals are summed across all materials, and the ratio is computed as total carbon divided by total nitrogen. Presets give common values for typical compost inputs, but you can customize them if you have lab data. Because moisture affects total mass, the calculator is most accurate when you enter masses on an as-used basis.

A compost pile is a dynamic system: as materials break down, the ratio changes. Use this tool when building a pile or adding large new inputs. The output advice is a guideline—if the ratio is high, add greens or fresh manure; if it is low, add browns like shredded leaves or straw. Combine that guidance with moisture management and aeration for best results. This approach supports backyard composting, homesteading, and sustainable agriculture practices.

C:N values vary by moisture content and decomposition stage. Fresh grass clippings can be much wetter than dried leaves, so equal weights do not mean equal dry-matter carbon. If you compost large quantities of one material, consider using dry-matter estimates or volume-based adjustments. The calculator’s weighted average is a helpful planning tool, but your senses are also data: if the pile smells sour or stays cool, the mix likely needs correction. Adjust gradually and recheck as you add new materials throughout the season.

Formula

For each material \(i\), compute \(C_i = m_i \cdot c_i\) and \(N_i = m_i \cdot n_i\). Totals are \(C_{tot} = \sum C_i\) and \(N_{tot} = \sum N_i\). The ratio is \(\text{C:N} = C_{tot} / N_{tot}\).

Example Calculation

If you add 10 lb of dry leaves (50% C, 0.8% N) and 5 lb of grass clippings (45% C, 3% N), total carbon is 7.25 lb and total nitrogen is 0.235 lb. The resulting ratio is about 31:1, which is close to the ideal range for active composting.

FAQs

What is the ideal compost C:N ratio?

Many compost systems target 25:1 to 30:1 for fast decomposition.

What are greens and browns?

Greens are nitrogen-rich materials; browns are carbon-rich materials.

Do I need exact percentages?

No. Typical values provide a good estimate, especially for backyard piles.

Does moisture affect the ratio?

Moisture affects decomposition speed but not the chemical C:N ratio.

Are results private?

Yes, everything runs locally in your browser.

How it works

This calculator computes weighted carbon and nitrogen totals and reports the resulting ratio. All computation runs client-side for privacy and speed.

Disclaimer

This calculator provides estimates based on typical C and N percentages. Compost inputs vary; adjust with observation and local guidance.