Compost C:N Ratio Calculator

How the Compost C:N Ratio Calculator Works

This calculator estimates the overall carbon-to-nitrogen ratio of your compost pile based on the ingredients you add. Each organic material has a known C:N ratio — greens like kitchen scraps and grass clippings are nitrogen-rich with low ratios, while browns like dry leaves and cardboard are carbon-rich with high ratios. Enter the volume of each ingredient in buckets or gallons and the tool computes the weighted average C:N ratio instantly. All calculations run in your browser with nothing sent to any server.

What Is the Ideal C:N Ratio for Compost?

The ideal carbon-to-nitrogen ratio for active composting is between 25:1 and 30:1. At this balance, microorganisms break down organic matter efficiently, generating heat that kills weed seeds and pathogens. A ratio below 20:1 means too much nitrogen — the pile will smell like ammonia and lose valuable nitrogen as gas. A ratio above 35:1 means too much carbon — decomposition slows dramatically and can take months or years. Most gardeners aim for roughly equal volumes of greens and browns, which naturally falls near the 25 to 30:1 sweet spot because browns are much denser in carbon per unit volume.

Greens vs Browns for Composting

Greens are nitrogen-rich materials that are usually moist and fresh. Kitchen scraps, coffee grounds, fresh grass clippings, fruit and vegetable waste, fresh manure, and seaweed are all greens with C:N ratios between 15:1 and 25:1. Browns are carbon-rich materials that are typically dry and woody. Dry leaves, straw, cardboard, newspaper, wood chips, sawdust, pine needles, and corn stalks have C:N ratios between 60:1 and 400:1. A well-balanced compost pile layers these two types to achieve the optimal ratio for fast decomposition.

How to Fix an Imbalanced Compost Pile

If your compost smells bad or is slimy, it has too much nitrogen. Add browns like shredded cardboard, dry leaves, or straw to absorb moisture and raise the C:N ratio. If your pile is dry and not decomposing, it has too much carbon. Add greens like kitchen scraps or fresh grass clippings and water the pile to lower the ratio. Turn the pile every one to two weeks to introduce oxygen, which speeds up decomposition regardless of the ratio. Shredding or chopping materials into smaller pieces also accelerates breakdown by increasing surface area for microorganisms.

Tips for Faster Composting

Maintain a pile size of at least 3 feet by 3 feet by 3 feet to retain heat. Keep moisture levels similar to a wrung-out sponge — about 40 to 60 percent. Turn the pile weekly to aerate it and redistribute heat. Chop large materials into pieces under 2 inches for faster decomposition. Avoid adding meat, dairy, oils, or diseased plants as they attract pests and create odors. With proper C:N balance, moisture, and aeration, a hot compost pile can produce finished compost in 4 to 8 weeks. Cold composting with less maintenance takes 6 to 12 months.