Compost C:N Ratio Calculator

Why the Carbon to Nitrogen (C:N) Ratio Matters for Compost

Composting turns kitchen scraps, yard waste, and other organic materials into a dark, crumbly soil amendment. Microorganisms drive this process, and like any living community, they need the right food balance to thrive. In composting, that balance is described by the carbon to nitrogen ratio, usually written as C:N.

Carbon is mainly the energy source for compost microbes. Nitrogen is needed to build proteins, enzymes, and new microbial cells. When there is an appropriate mix of both, microbes work quickly and efficiently, generating heat as they break down organic matter. When the balance is off, piles can become slow, smelly, or compacted.

For most home composting situations, a C:N ratio of about 30:1 by weight is considered ideal. This means that for every 1 part nitrogen, you want about 30 parts carbon in the overall mix of materials. Your compost will still decompose outside this range, but performance and odors are more likely to become a problem.

Greens vs. Browns: What They Mean

To make the C:N concept more practical, composters often divide materials into two simple categories:

• Greens – nitrogen-rich materials that are usually moist and break down quickly.
• Browns – carbon-rich materials that are usually dry and slower to decompose.

Typical green materials include:

• Fresh grass clippings
• Vegetable and fruit scraps
• Coffee grounds and tea leaves
• Fresh manure from herbivores (e.g., chickens, cows, rabbits)
• Green plant prunings and weeds that have not gone to seed

Common brown materials include:

• Dry leaves
• Straw and hay
• Wood chips and sawdust
• Shredded cardboard and paper
• Corn stalks and other dried plant stems

In general, greens add nitrogen and moisture, while browns add carbon and structure. Achieving a good compost C:N ratio involves mixing appropriate amounts of these two groups, not aiming for perfection with every individual ingredient.

How the Compost C:N Ratio Is Calculated

The C:N ratio describes the amount of carbon compared with the amount of nitrogen in your compost mix. In simple terms:

C:N ratio = (total carbon) ÷ (total nitrogen)

Written more formally, the ratio is:

where M is the mass of the material, C is the fraction of carbon by weight, and N is the fraction of nitrogen by weight. The mass term cancels if you look at a single uniform material, but when you blend different components, you sum their carbon and nitrogen contributions before forming the ratio.

Our calculator simplifies this for home composters by assuming:

• Browns (carbon-rich materials) contain about 50% carbon by weight.
• Greens (nitrogen-rich materials) contain about 25% carbon and a higher proportion of nitrogen.

Using your inputs for total browns and total greens, it estimates total carbon and total nitrogen, then divides carbon by nitrogen to give a single C:N ratio for the whole mix. You can use kilograms, pounds, or any other consistent unit of weight, as long as both inputs use the same unit.

How to Use the C:N Ratio Calculator

To get a useful estimate from the calculator, follow these steps:

1. Gather your brown materials (such as dry leaves, straw, or shredded cardboard) and weigh them, or estimate their weight.
2. Gather your green materials (such as kitchen scraps, fresh grass, or manure) and do the same.
3. Enter the total weight of browns into the carbon-rich material field.
4. Enter the total weight of greens into the nitrogen-rich material field.
5. Submit the form to see the estimated C:N ratio for your compost mix.

The result might be shown as something like 28:1 or 40:1. This number is dimensionless – it just expresses the proportion of carbon to nitrogen.

Interpreting Your Compost C:N Ratio Result

Once you have a calculated C:N ratio, you can compare it to the commonly recommended target of about 30:1. Use these general guidelines:

• Around 25:1 to 35:1 – This is close to ideal for most backyard composting. Piles in this range tend to heat up, decompose steadily, and produce finished compost in a few months under good conditions.
• Higher than 35:1 (carbon-heavy, too many browns) – The pile may decompose slowly, remain cool, and look dry or fibrous. Microbes do not have enough nitrogen to reproduce rapidly, so activity is limited.
• Lower than 25:1 (nitrogen-heavy, too many greens) – The pile can become wet, compacted, and smelly. Excess nitrogen may be lost as ammonia gas, and anaerobic (low-oxygen) conditions can develop.

Treat your first calculation as a starting point, not a final verdict. You can adjust your mix and re-calculate as you add more materials over time.

How to Adjust a C:N Ratio That Is Too High or Too Low

Based on your calculator result, you can change your compost recipe:

If your ratio is too high (very carbon-heavy)

Signs include slow decomposition, a cool pile, and very dry, woody material. The calculator may show values like 40:1, 50:1, or higher. To bring the ratio closer to 30:1:

• Add more greens, such as fresh grass clippings, kitchen scraps, or coffee grounds.
• Reduce very woody materials like large amounts of sawdust or wood chips, or mix them with plenty of greens.
• Check moisture: add water if the pile is dry and crumbly.

If your ratio is too low (very nitrogen-heavy)

Signs include strong ammonia smells, a dense or soggy texture, and a pile that mats down. The calculator may show values like 15:1 or 20:1. To move closer to 30:1:

• Add more browns, such as dry leaves, straw, shredded paper, or cardboard.
• Layer wet greens with dry browns to improve airflow and absorb excess moisture.
• Turn the pile to introduce oxygen and mix high-nitrogen pockets with carbon-rich material.

Each time you modify the mix significantly, you can re-estimate the total weight of greens and browns and use the calculator again to see how your C:N ratio has changed.

Typical C:N Ratios of Common Compost Materials

Every compost ingredient has its own approximate C:N ratio. These values vary with moisture content, plant species, and handling, but they give useful ballpark numbers when planning your mix. The table below compares common materials:

Use these values as reference points. If your compost recipe uses unusually high-carbon ingredients like sawdust, you will typically need a larger proportion of greens to stay near the 30:1 goal.

Worked Example: Estimating a Compost C:N Mix

Suppose you are building a new compost pile with the following materials:

• 20 kg of dry leaves
• 10 kg of fresh grass clippings
• 5 kg of vegetable scraps

First, group them into browns and greens:

• Browns: 20 kg of dry leaves
• Greens: 10 kg grass + 5 kg scraps = 15 kg total greens

You could enter 20 as the carbon-rich (browns) value and 15 as the nitrogen-rich (greens) value in the calculator. Behind the scenes, the tool uses its assumed average carbon and nitrogen contents to estimate the overall ratio, but you can also do a simplified hand estimate using the typical C:N ratios:

• Dry leaves at about 60:1
• Grass clippings at about 17:1
• Vegetable scraps at about 20:1

Without going into full laboratory-level detail, you can already see that the browns in this mix are quite carbon-rich, while the greens are moderate. The calculator will usually report a ratio somewhere near the ideal range. If the output happens to be higher than 35:1, you would know to add more greens (perhaps more grass clippings) and then re-calculate.

Assumptions and Limitations of This Calculator

Because real-world compost materials vary widely, any simple calculator must rely on assumptions. Being aware of these helps you interpret results realistically:

• Average composition only. The tool uses typical carbon and nitrogen percentages for broad categories (browns vs. greens). Individual materials like coffee grounds, straw, or manure can differ from these averages depending on source and moisture.
• Weight-based estimates. The ratio is calculated by weight, not volume. A full bucket of dry leaves weighs far less than a full bucket of wet grass. Whenever possible, estimate or measure by weight, and always keep units consistent between inputs.
• Moisture content is not directly modeled. Wet materials contain more water and less actual dry matter per kilogram. The calculator does not adjust for this explicitly, so very wet materials may make the numbers look more precise than they are.
• Mixing and airflow are critical. A theoretically perfect 30:1 mix can still perform poorly if the pile is never turned, is too small, or is compacted and starved of oxygen.
• Backyard guidance, not lab analysis. The output is intended to guide home composting decisions, not replace soil tests, professional nutrient management plans, or laboratory feedstock analysis.

Treat the calculated C:N ratio as a directional indicator. If your compost seems slow, smelly, too wet, or too dry, adjust based on both the number you see and the physical condition of the pile.

Other Factors That Affect Composting Success

Even with a good C:N ratio, other conditions influence how well your compost performs:

• Moisture: Aim for a feel similar to a wrung-out sponge. Too dry and microbes slow down; too wet and the pile can go anaerobic.
• Oxygen: Turning the pile or using a system that allows airflow keeps aerobic microbes active and prevents odors.
• Particle size: Shredding or chopping materials increases surface area and speeds decomposition, especially for tough browns like leaves and straw.
• Pile size and insulation: Larger piles hold heat better and may reach higher temperatures, which helps break down materials faster and can kill weed seeds and pathogens.

Combining a reasonable C:N ratio with these good composting practices will usually give you reliable, high-quality compost without overly complicated calculations.