Compost C:N Ratio Calculator: Balance Greens and Browns

Greens, browns, and scientific C:N are related but not identical

Backyard advice often says to use about 2:1 or 3:1 browns-to-greens by volume. That is a useful starting point because dry leaves, straw, paper, and wood add bulky carbon, while grass clippings, food scraps, coffee grounds, and many manures add more nitrogen and moisture. The scientific C:N target is different: it is carbon mass divided by nitrogen mass, usually planned near 25:1 to 30:1 for an active starting mix.

Use rough volume ratios when you are layering a small bin. Use this calculator when the pile smells, stays cool, contains unusually wet materials, or needs a more specific "how much should I add?" answer.

Compost C:N ratio explained

Compost microbes need carbon for energy and nitrogen for proteins and cell growth. A low ratio means excess nitrogen: the pile may become wet, slimy, anaerobic, or ammonia-smelling. A high ratio means excess carbon: the pile may stay cool and break down slowly. Most active starting mixes work best near 25:1 to 30:1, but texture, air, particle size, and moisture still matter.

In simple mode, the calculator treats your entered material weights as the basis for a quick weighted average. In advanced mode, it first removes the moisture percentage from each as-used weight so wet grass, food scraps, and manure can be compared more fairly with dry leaves, straw, cardboard, and sawdust.

Formula

Simple mode computes C_i = m_i x c_i and N_i = m_i x n_i. Advanced mode uses dry matter first: d_i = m_i x (1 - moisture_i/100), then C_i = d_i x c_i and N_i = d_i x n_i. The final ratio is total carbon divided by total nitrogen.

The correction solver uses the same carbon and nitrogen contribution for the selected material and solves (C + xC_add) / (N + xN_add) = target ratio for the added mass x.

Material C:N lookup and presets

Preset C and N percentages are editable planning assumptions. The visible C:N value shows the source value or typical range used to set the preset.

Worked examples

Sources and assumptions

Presets are representative values, not lab guarantees. Compost materials vary by species, age, storage, bedding, rain exposure, and decomposition stage. Adjust C, N, and moisture when you have local or lab data.

• Cornell Composting: Compost Chemistry for the 30:1 planning target, high-carbon/high-nitrogen examples, odors, slow decomposition, and finished compost range.
• Cornell Composting: C/N Ratio for the mixture formula using mass, carbon percent, nitrogen percent, and moisture percent.
• Cornell Waste Management Institute: On-Farm Composting Handbook Appendix A for representative C:N, nitrogen, and moisture values for feedstocks.

FAQs

What is the ideal compost C:N ratio?

A common starting target is about 25:1 to 30:1. The calculator defaults to 30:1 because it is a practical target for active composting.

Is compost C:N ratio measured by weight or volume?

Scientific C:N is measured by weight: carbon mass divided by nitrogen mass. Volume rules such as 2:1 or 3:1 browns-to-greens are rough backyard shortcuts.

What happens if C:N is too high?

The pile is carbon-rich and nitrogen-limited. It may stay cool, dry, and slow. Add greens such as grass clippings, food scraps, coffee grounds, or manure.

What happens if C:N is too low?

The pile is nitrogen-rich. It may turn wet or slimy and may smell like ammonia. Add dry browns such as leaves, straw, shredded cardboard, or wood chips.

How much brown material should I add to food scraps?

Enter the food scraps, choose dry leaves, straw, cardboard, or another brown as the correction material, and calculate. Wet food scraps often need more dry brown material than a small scoop by volume suggests.

Are coffee grounds green or brown?

Coffee grounds are usually treated as a green because their C:N value is near nitrogen-rich materials, even though they look brown.

Are eggshells green or brown?

Eggshells are mostly mineral calcium carbonate. They can be composted in small crushed amounts, but they do not meaningfully fix a carbon or nitrogen imbalance.

Should I include moisture?

Use advanced moisture-adjusted mode when you weigh wet materials such as grass, food scraps, or manure. Simple mode is fine for quick backyard estimates with mixed, approximate amounts.

What C:N ratio is finished compost?

Finished compost is often around 10:1 to 15:1 because carbon is lost as carbon dioxide while the pile decomposes.

Why does my pile smell?

Common causes are too much nitrogen, too much water, compaction, or not enough oxygen. Add dry browns, mix in bulky material, and turn or loosen the pile.

Are results private?

Yes, everything runs locally in your browser.

How it works

This calculator computes weighted carbon and nitrogen totals, reports the resulting ratio, diagnoses likely symptoms, and solves for a selected correction material. All computation runs client-side for privacy and speed.

Troubleshooting quick guide

Disclaimer

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