Compost C:N Ratio Calculator
Enter the materials going into your compost pile to calculate its carbon-to-nitrogen ratio and see whether it falls in the ideal 25-40:1 range.
Enter how many 5-gallon buckets of each material you’re adding. Leave any at 0 to skip them.
Browns (carbon-rich)
Greens (nitrogen-rich)
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Balance check
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Mix breakdown
Target range
The ideal compost pile has a carbon-to-nitrogen ratio of 25-40:1 by weight. Too much nitrogen (a low ratio, under 25:1) makes a slimy, ammonia-smelling pile; too much carbon (a high ratio, over 40:1) breaks down slowly and stays cold. For example, 4 buckets of dry leaves, 2 buckets of grass clippings, and 1 bucket of food scraps works out to about 19.9:1 — too nitrogen-heavy, and it needs more brown material to balance.
C:N ratios of common compost materials
These are approximate ratios based on typical carbon and nitrogen content by weight for each material.
| Material | Type | Approx. C:N ratio | Notes |
|---|---|---|---|
| Dry leaves | Brown | 50:1 | Widely available; shred for faster breakdown |
| Straw | Brown | 77:1 | Bulky, adds good aeration to dense piles |
| Shredded cardboard | Brown | 450:1 | Very high carbon; use in moderation, remove tape and labels |
| Wood chips / sawdust | Brown | 500:1 | Extremely slow to break down; best for pathways or thin layers |
| Fresh grass clippings | Green | 18:1 | Packs down easily; mix with browns to avoid matting |
| Fruit & veg scraps | Green | 16:1 | High moisture; bury under browns to deter pests |
| Coffee grounds | Green | 22.5:1 | Acts as a green despite brown appearance; fine texture aids mixing |
| Aged manure | Green | 16:1 | Rich in nitrogen; use well-aged manure to avoid burning plants |
How to calculate your compost pile's C:N ratio
- List each material and quantity. Measure inputs in 5-gallon buckets — a practical unit for home compost piles.
- Convert volume to weight. Multiply buckets by 0.67 cubic feet per bucket, then by that material’s bulk density in lb per cubic foot.
- Calculate carbon and nitrogen weight per material. Multiply each material’s weight by its percent carbon and percent nitrogen by weight.
- Sum carbon and nitrogen separately. Add up total carbon pounds and total nitrogen pounds across every material in the pile.
- Divide total carbon by total nitrogen. The result is your pile’s C:N ratio — compare it to the 25-40:1 ideal range.
Worked example
Using the calculator’s default mix: 4 buckets of dry leaves, 2 buckets of fresh grass clippings, and 1 bucket of fruit & veg scraps.
- Dry leaves weight (4 × 0.67 cf × 6 lb/cf)16.08 lb
- Grass clippings weight (2 × 0.67 cf × 35 lb/cf)46.9 lb
- Food scraps weight (1 × 0.67 cf × 40 lb/cf)26.8 lb
- Total carbon (sum of each material's weight × %C)39.87 lb
- Total nitrogen (sum of each material's weight × %N)2.00 lb
- C:N ratio (39.87 ÷ 2.00)≈ 19.9 : 1
- ResultToo nitrogen-heavy — add more browns (leaves, straw, or cardboard)
Signs your pile is out of balance
The way your pile looks, smells, and heats up tells you whether it needs more browns or greens.
- Ammonia smellToo much nitrogen (ratio under 25:1) — add dry browns like leaves, straw, or shredded cardboard
- Pile won't heat upToo much carbon (ratio over 40:1) or too dry — add greens and water to jump-start decomposition
- Slimy, matted, wet clumpsNitrogen-rich material packed too densely without air — add coarse browns and turn the pile
- Very slow breakdown after several weeksExcess carbon, especially woody material — shred materials smaller and mix in more greens
- Fruit flies or pests around the pileExposed food scraps — bury greens under a layer of browns
Common mistakes
- Estimating by volume instead of weight.C:N ratios are based on weight, not volume — a bucket of wood chips weighs far less than a bucket of food scraps at the same volume.
- Treating coffee grounds as a brown.Coffee grounds look brown but behave like a green material, with a relatively low C:N ratio around 22.5:1.
- Adding too much cardboard or wood chips at once.Their C:N ratio is 450-500:1 — a small amount shifts the whole pile toward excess carbon quickly.
- Skipping moisture content.This calculator estimates dry-weight ratios; wet materials like fresh grass carry extra water weight that dilutes nutrient density.
- Ignoring particle size.Large, uncut material breaks down slowly regardless of ratio — shred or chop browns and greens for faster composting.
- Assuming all manure is the same.Nitrogen content varies widely by animal — poultry manure is far more nitrogen-rich than aged horse or cow manure.
What this calculator assumes
Adjust your result if your project differs from these.
- Materials are measured in standard 5-gallon buckets (0.67 cubic feet each).
- Weight is calculated from typical bulk density figures for each material — actual density varies with moisture and how loosely material is packed.
- Carbon and nitrogen percentages are dry-weight averages from published composting references, not lab-tested values for your specific material.
- The ideal 25-40:1 range assumes standard hot composting; cold or vermicomposting methods tolerate a wider range.
- Ratio is calculated only from the materials and quantities you enter — it does not account for materials already decomposing in an existing pile.
- Moisture, aeration, and pile size also affect composting speed and are not captured by the C:N ratio alone.
Common questions
What is the ideal C:N ratio for compost?
25-40:1 by weight. Ratios in this range give microbes enough nitrogen to grow while providing enough carbon for energy, without excess of either.
What happens if my C:N ratio is too low?
Too much nitrogen relative to carbon causes a slimy, compacted pile with an ammonia smell, and nitrogen is lost to the air as gas.
What happens if my C:N ratio is too high?
Too much carbon relative to nitrogen slows decomposition — the pile stays cool and takes much longer to break down.
Are coffee grounds a brown or a green?
A green, despite the brown color. Coffee grounds have a C:N ratio around 22.5:1, similar to fresh grass clippings.
How do I raise my compost pile's C:N ratio?
Add more brown materials — dry leaves, straw, shredded cardboard, or wood chips — to increase carbon relative to nitrogen.
How do I lower my compost pile's C:N ratio?
Add more green materials — fresh grass clippings, food scraps, or aged manure — to increase nitrogen relative to carbon.
Why does this calculator use buckets instead of pounds?
5-gallon buckets are a practical, consistent unit for measuring compost inputs at home without a scale.
Does grass clippings alone make good compost?
No — grass clippings alone have a C:N ratio around 18:1, too nitrogen-heavy on their own. Mix with browns like dry leaves.
How much carbon and nitrogen is in dry leaves?
Dry leaves are roughly 50% carbon and 1% nitrogen by weight, giving a C:N ratio around 50:1.
Why do wood chips take so long to compost?
Wood chips have a very high C:N ratio, around 500:1 — far more carbon than nitrogen, so microbes break them down very slowly without added nitrogen.
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Compost C:N Ratio Calculator Estimate
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Mix
| Browns | — |
| Greens | — |
| C:N ratio | — |
| Assessment | — |
Notes
Estimate generated by materialcalculate.com. Uses published approximate C:N ratios and bulk densities per material.