Compost Calculator: C:N Ratio and Material Mix
Calculate the carbon-to-nitrogen ratio and moisture level of a compost pile with this free tool. Balance green and brown materials for faster decomposition.
Compost Calculator
Calculate the optimal mix for your compost pile by entering the types and quantities of materials you have available. The calculator will analyze your inputs and provide recommendations for achieving an ideal carbon-to-nitrogen ratio and moisture content.
Material Inputs
Enter material quantities to see compost mix calculations and recommendations.
Composting Tips
- Turn your compost pile regularly to aerate it and speed up decomposition.
- Keep your compost moist but not soggy - it should feel like a wrung-out sponge.
- Chop or shred materials into smaller pieces for faster decomposition.
- Balance green (nitrogen-rich) and brown (carbon-rich) materials for optimal results.
- Avoid adding meat, dairy, or oily foods to your compost as they can attract pests.
Documentation
A compost calculator estimates the carbon-to-nitrogen (C:N) ratio and moisture level of a mix of organic materials, so a gardener can balance "green" and "brown" ingredients for faster, less smelly composting.
What is the carbon-to-nitrogen ratio in compost?
Compost is made by microorganisms that break down organic matter. These microbes need two things from their food: carbon, which supplies energy, and nitrogen, which supplies protein for growth and reproduction. The carbon-to-nitrogen ratio, written as C:N, compares how much carbon a material contains to how much nitrogen it contains.
A C:N ratio of 30:1 means a material has 30 units of carbon for every 1 unit of nitrogen. Every organic material has its own natural C:N ratio. Fresh vegetable scraps are nitrogen-rich, so they have a low ratio. Dry leaves and cardboard are carbon-rich, so they have a high ratio.
Materials with a low C:N ratio (nitrogen-rich) are called "green" materials, even when they are not green in color. Materials with a high C:N ratio (carbon-rich) are called "brown" materials. Mixing greens and browns in the right proportion keeps microbes fed and working efficiently.
Cornell University's composting program gives the ideal starting ratio as around 30:1. At lower ratios, extra nitrogen is lost as ammonia gas, which smells bad. At higher ratios, microbes get too little nitrogen, so the pile stays cool and breaks down slowly. This calculator flags a mix as balanced when its overall C:N ratio falls between 20:1 and 35:1.
How the compost C:N ratio is calculated
The calculator follows the same idea as Cornell's mixture formula, which weighs each ingredient by its dry mass, since water carries no carbon or nitrogen. It works in four steps for each material a person adds:
- Find the dry weight. Raw materials contain water, and water has no carbon or nitrogen. Dry weight = quantity × (1 − moisture content ÷ 100).
- Estimate the carbon weight. Dry organic matter is assumed to be about 45% carbon. Carbon weight = dry weight × 0.45.
- Work out the nitrogen weight. Nitrogen weight = carbon weight ÷ the material's own C:N ratio.
- Add up all materials. The mix's overall ratio = total carbon ÷ total nitrogen.
Example: calculating the C:N ratio of a home compost mix
Suppose someone adds 5 kg of vegetable scraps (C:N ratio 15:1, 80% moisture) and 10 kg of dry leaves (C:N ratio 60:1, 15% moisture).
Vegetable scraps
- Dry weight: 5 × (1 − 0.80) = 1 kg
- Carbon: 1 × 0.45 = 0.45 kg
- Nitrogen: 0.45 ÷ 15 = 0.03 kg
Dry leaves
- Dry weight: 10 × (1 − 0.15) = 8.5 kg
- Carbon: 8.5 × 0.45 = 3.825 kg
- Nitrogen: 3.825 ÷ 60 = 0.06375 kg
Totals
- Total carbon: 0.45 + 3.825 = 4.275 kg
- Total nitrogen: 0.03 + 0.06375 = 0.09375 kg
- Mix C:N ratio: 4.275 ÷ 0.09375 ≈ 45.6:1
A ratio of 45.6:1 is above the calculator's balanced range of 20:1 to 35:1, so the mix has too much carbon. Decomposition will run slower than it would with more nitrogen. Adding more green material, such as extra vegetable scraps, grass clippings, or coffee grounds, or using fewer dry leaves, would bring the ratio down toward the balanced range.
The same mix is also too dry. The scraps hold 5 × 0.80 = 4 kg of water and the leaves hold 10 × 0.15 = 1.5 kg, so the 15 kg mix is 5.5 ÷ 15 ≈ 36.7% water. That is below the calculator's balanced moisture range of 50% to 60%. More green material would raise both the nitrogen and the moisture.
Common compost materials and their properties
The table below lists the values this calculator uses for each material.
| Material | Category | C:N ratio | Moisture content |
|---|---|---|---|
| Vegetable scraps | Green | 15:1 | 80% |
| Fruit scraps | Green | 20:1 | 80% |
| Coffee grounds | Green | 20:1 | 80% |
| Grass clippings | Green | 20:1 | 80% |
| Animal manure | Green | 15:1 | 80% |
| Dry leaves | Brown | 60:1 | 15% |
| Straw | Brown | 80:1 | 12% |
| Newspaper | Brown | 560:1 | 8% |
| Cardboard | Brown | 560:1 | 8% |
| Wood chips | Brown | 400:1 | 20% |
These are typical values for each material category. The true ratio of any real batch of leaves or scraps can vary depending on plant type, age, and how it was stored. Published tables show wide ranges: Cornell lists autumn leaves at 30–80:1, straw at 40–100:1, wood chips or sawdust at 100–500:1, and newspaper or corrugated cardboard at 560:1. The calculator uses Cornell's 560:1 for both newspaper and cardboard.
Moisture content in compost
Microbes also need water to survive. The calculator adds up the water each material carries and reports it as a percentage of the total mix weight. The calculator treats a moisture level between 50% and 60% as balanced, the range Cornell considers optimum for composting. That is about as damp as a wrung-out sponge.
Below about 30%, bacteria slow down. Above about 65%, water pushes air out of the pile, and the pile can turn anaerobic, meaning it breaks down without oxygen. Anaerobic piles decompose slowly and can give off the rotten-egg smell of hydrogen sulfide.
Green materials vs. brown materials
Green materials (nitrogen-rich, low C:N ratio): vegetable and fruit scraps, fresh grass clippings, coffee grounds, and manure from plant-eating animals.
Brown materials (carbon-rich, high C:N ratio): dry leaves, straw, cardboard, newspaper, and wood chips.
A pile built mostly from one category rarely works well. The US Environmental Protection Agency suggests adding at least two to three times the volume of browns to the volume of greens, then adjusting using the actual C:N ratio.
How to use this compost calculator
- Select a material from the list.
- Enter its quantity in kilograms.
- Select "Add material" to include more materials, or "Remove" to take one out.
- Read the results: the C:N ratio, moisture content, total weight, total volume, and the percentage of green versus brown material.
- Follow the recommendations shown to adjust the mix.
Troubleshooting a compost pile
| Problem | Likely cause | Fix |
|---|---|---|
| Bad smell | Too much nitrogen, too wet, or not enough air | Add brown material and turn the pile |
| Very slow breakdown | Too much carbon, too dry, or cold weather | Add green material and water, or insulate the pile |
| Pests | Exposed food scraps or meat and dairy in the pile | Bury scraps under brown material; keep meat and dairy out |
| Pile is too dry | Not enough water or too much brown material | Add water and green material |
| Pile is too wet | Too much water or too much green material | Add brown material and turn the pile |
Frequently asked questions
What is a good C:N ratio for compost? Cornell's composting program gives about 30:1 as the ideal starting point. This calculator marks any ratio from 20:1 to 35:1 as balanced.
How long does compost take to finish? It depends on the materials, pile size, how often it is turned, and the weather. A well-balanced pile that is turned regularly finishes much sooner than a pile that is left alone.
Why does my compost smell bad? A bad smell usually means the pile has too much nitrogen, too much water, or too little air. Mixing in dry, carbon-rich material and turning the pile usually fixes it.
Can I compost in winter? Yes, though cold slows the microbes down. A small pile loses heat faster than a large one, so a bigger or insulated pile stays active longer in cold weather.
What if my material is not on the list? Pick the closest match by category and moisture level. Dry, woody materials behave like straw or wood chips; wet, soft materials behave like vegetable scraps.
References
- Cornell Waste Management Institute, Cornell University. Compost Chemistry. Cornell Composting: Compost Chemistry — roles of carbon and nitrogen; ideal C/N around 30:1; ammonia loss at low ratios; typical C/N ranges; hydrogen sulfide odor.
- Cornell Waste Management Institute, Cornell University. C/N Ratio (Tom Richard and Nancy Trautmann). Cornell Composting: C/N ratio calculation — mixture C/N formula using wet mass, moisture, carbon and nitrogen of each ingredient.
- Cornell Waste Management Institute, Cornell University. Compost Physics. Cornell Composting: Compost Physics — 50–60% moisture optimum; below 30% inhibits bacteria; above 65% causes anaerobic pockets; wrung-out sponge test.
- US Environmental Protection Agency. Composting At Home. EPA: Composting At Home — at least two to three times the volume of browns to greens; wrung-out sponge moisture.