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.
This calculator flags a mix as balanced when its overall C:N ratio falls between 20:1 and 35:1. Ratios below 20:1 tend to release excess nitrogen as ammonia, which smells bad. Ratios above 35:1 give microbes too little nitrogen, so decomposition slows down.
How the compost C:N ratio is calculated
The calculator works out the ratio 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.
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 | 175:1 | 8% |
| Cardboard | Brown | 350:1 | 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.
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. A moisture level between 40% and 60% is treated as balanced, roughly the dampness of a sponge that has been wrung out.
Below 40%, microbes slow down or go dormant. Above 60%, water pushes air out of the pile, and the pile can turn anaerobic, meaning it breaks down without oxygen. Anaerobic piles decompose slowly and often smell of rotten eggs.
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. A common starting guideline is roughly one part green material to two or three parts brown material by volume, then adjusted 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? Many composting guides suggest an ideal range of 25:1 to 30:1. This calculator marks any ratio from 20:1 to 35:1 as balanced, since decomposition still proceeds well across that wider range.
How long does compost take to finish? It depends on the materials, pile size, how often it is turned, and the weather. An actively managed, well-balanced pile can finish in three to six months. A pile left alone can take a year or more.
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 larger pile, at least one meter in each direction, holds heat better than a small one.
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, "Compost Chemistry," Cornell University.
- Rynk, Robert, ed. "On-Farm Composting Handbook," Northeast Regional Agricultural Engineering Service, 1992.
- United States Environmental Protection Agency, "Types of Composting and Understanding the Process."