Compost Carbon-to-Nitrogen (C:N) Ratio Calculator
Calculate a compost C:N ratio from browns, greens, weights, and moisture, then see balance warnings, repair amounts, and dry-matter charts.Compost Mix Check
Current result
| Metric | Value | Copy |
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| Blend insight {{ index + 1 }} | {{ line }} | |
| Balance suggestion | {{ balanceSuggestion.explanation }} |
| Target | Helper | Wet add | Dry add | Projected ratio | Projected moisture | Note | Copy |
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| Material | C:N | Wet weight | Dry weight | Water | Carbon | Nitrogen | Copy |
|---|---|---|---|---|---|---|---|
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{{ row.materialClass }} • {{ formatPercent(row.moisturePercent, 0) }} moisture • Dry basis
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Introduction
A compost recipe can look balanced by volume and still behave badly once the pile starts working. Leaves compress, grass turns wet, cardboard may stay woody for weeks, and food scraps bring water as well as nitrogen. The carbon-to-nitrogen ratio, usually written as C:N, gives a mass-based way to compare those materials before they are mixed.
Composting is an aerobic biological process. Microorganisms use carbon-rich material as an energy source and nitrogen-rich material for growth. Water keeps the biology active, while air space prevents the pile from sliding into anaerobic odor problems. C:N is only one part of that system, but it is one of the easiest parts to estimate while a repair is still practical.
| Material role | Common examples | What it usually changes |
|---|---|---|
| Carbon-rich browns | Dry leaves, straw, cardboard, sawdust, wood chips | Raise C:N, add structure, and help offset wet greens. |
| Nitrogen-rich greens | Grass clippings, food scraps, coffee grounds, fresh manures | Lower C:N and feed microbial growth, but often add moisture. |
| Bridge materials | Some manures, mixed yard waste, partly aged plant material | Move the ratio less sharply and can make corrections easier to manage. |
The useful number is based on dry matter, not bucket count. A 20 lb pail of wet grass may contain far less dry material than a 20 lb box of dry leaves. That is why moisture changes the effect of a feedstock. After water is removed from the calculation, the remaining dry matter is split into carbon and nitrogen according to that material's C:N estimate.
Active piles often start around 25:1 to 35:1, with hot, regularly turned piles commonly managed nearer 25:1 to 30:1. Those ranges are planning bands rather than guarantees. Wood chips and sawdust can hold carbon that breaks down slowly, a fine wet mix can lose air space, and a pile that looks good by C:N can still stall if it is dry, cold, compacted, contaminated, or too small to hold heat.
The ratio is most useful when it prevents a large mistake. It can show that a green-heavy pile needs browns, that a woody batch needs nitrogen, or that moisture is the harder problem. It cannot prove finished compost maturity, detect herbicide residues, measure pathogens, or replace a real pile check for smell, structure, temperature, and moisture by hand.
How to Use This Tool:
Enter one row for each feedstock so the calculator can convert each material's fresh weight into dry matter before combining the pile totals.
- Choose Imperial (lb) or Metric (kg). Material weights convert when the measurement system changes, so recheck rounded values if you switch units after entering a recipe.
- Set Target ratio minimum and Target ratio maximum, or pick a Target preset in Advanced. The active-pile preset uses 25:1 to 35:1; the hot-compost preset uses 25:1 to 30:1.
- Leave Account for moisture enabled for fresh as-handled materials. Turn it off only when every weight already represents dry matter.
Moisture entries are disabled when dry-basis mode is selected, and the moisture result changes to dry basis only.
- Add material rows, choose the closest preset, then edit C:N ratio, Moisture, and Weight when your batch is wetter, drier, older, woodier, or mixed with bedding.
- Open Advanced when you need a different moisture window or a specific Brown repair helper and Green repair helper. Those helpers decide which material appears in the repair plan.
- Fix validation errors before reading the result. Target maximum must exceed target minimum, moisture targets must stay from 10% to 90% with high above low, material weights and C:N ratios must be positive, and material moisture must stay below 100%.
A material at 100% moisture has no dry matter, so it cannot supply carbon or nitrogen in this calculation.
- Read Blend Check first. Use Repair Plan for low-edge, midpoint, and high-edge additions, then check Material Ledger to confirm which row supplies most of the dry matter.
Interpreting Results:
Start with Final C:N ratio, Classification, Blend moisture, and Moisture status. The ratio and moisture checks answer different questions. A pile can be balanced by C:N and still too dry to heat, or moist enough but too nitrogen-heavy to stay pleasant.
| Result cue | Boundary | What to check next |
|---|---|---|
| Nitrogen heavy | Final C:N ratio is less than the target minimum. | Review brown-helper additions and make sure projected moisture does not fall below the selected window. |
| Balanced | Final C:N ratio is greater than or equal to the minimum and less than or equal to the maximum. | Verify Blend moisture, Largest dry matter share, and material estimates before assuming the pile is ready. |
| Carbon heavy | Final C:N ratio is greater than the target maximum. | Review green-helper additions and check whether a wet green repair would create a moisture problem. |
| Too dry | Blend moisture is less than the moisture target minimum. | Use the moisture correction and check whether dry browns dominate the dry mass. |
| Too wet | Blend moisture is greater than the moisture target maximum. | Compare dry brown additions with real pile structure, drainage, and air space. |
Do not overread one decimal place. Feedstock C:N and moisture values are estimates unless measured. The useful decision is usually direction and scale: whether to add browns, add greens, add water, remove water, or reduce a dominant row before mixing.
Technical Details:
Compost C:N is a mass ratio. Each material contributes a wet weight, a moisture percentage, and a material C:N estimate. Moisture accounting removes water from the wet weight. The dry matter that remains is divided into carbon and nitrogen shares according to the material ratio, then all carbon and nitrogen shares are summed for the blend.
Dry matter explains why equal scale weights can move the pile very differently. At 70% moisture, 20 lb of material contributes 6 lb of dry matter. At 10% moisture, 20 lb contributes 18 lb of dry matter. If both rows use the same C:N number, the drier row has three times the influence because three times as much dry matter enters the carbon and nitrogen totals.
Formula Core:
The governing equations remove water, split dry matter by material C:N, and divide total carbon by total nitrogen.
| Symbol | Meaning | Unit or form |
|---|---|---|
| W | Entered material weight | lb or kg, converted to a common mass unit |
| m | Material moisture as water fraction | 70% is used as 0.70 |
| D | Dry matter after water is removed | Same mass unit as weight |
| R | Material C:N ratio | Parts carbon per 1 part nitrogen |
| C and N | Carbon and nitrogen mass assigned from dry matter | Same mass unit as dry matter |
For 20 lb of dry leaves at 10% moisture and 60:1 C:N, dry matter is 18 lb. Carbon is 18 × 60 / 61, or about 17.70 lb, and nitrogen is 18 / 61, or about 0.30 lb. Those values are added to the carbon and nitrogen from every other material before the final blend ratio is computed.
Repair Math:
Repair rows solve for the dry mass of one selected helper material needed to reach a target ratio. The dry addition is converted back to wet weight by dividing by the helper material's dry fraction.
The denominator must move the blend in the needed direction. A carbon-rich helper can raise a low ratio, while a nitrogen-rich helper can lower a high ratio. If the helper cannot reach the selected target, the repair row is blocked rather than showing a misleading weight.
| Boundary | Rule | Effect |
|---|---|---|
| Target ratio band | Minimum and maximum must be greater than 0, and maximum must exceed minimum. | Controls Nitrogen heavy, Balanced, and Carbon heavy. |
| Moisture window | When moisture is enabled, low and high targets must stay from 10% to 90%, with high above low. | Controls Too dry, In moisture window, and Too wet. |
| Material class | 35:1 or higher is Carbon-rich; 20:1 or lower is Nitrogen-rich; values between are Bridge material. | Labels material rows for scanning but does not change the arithmetic. |
| Display precision | Ratios usually display to one decimal; percentages usually display to one decimal; weights use fewer decimals as values grow. | Small display differences are rounding, not laboratory certainty. |
Blend moisture is water mass divided by wet mass. When the mix is dry, the moisture correction estimates water needed to move toward the middle of the selected window. When it is wet, the correction estimates the excess water or the dry bulking mass needed to offset it.
Advanced Tips:
- Use the active-pile target when feedstock estimates are rough; use the hot-compost target when you are turning often and want a tighter starting band.
- Edit preset moisture before trusting a repair amount. Fresh grass after rain and dry leaves stored under cover can be far from their default values.
- Check Dry Matter Map when the repair seems extreme. A single woody row can dominate the dry matter even if its wet weight looks moderate.
- Use Carbon & Nitrogen to compare what a material contributes chemically, not just how much it weighs.
- If Repair Plan suggests adding more material than the existing pile, reduce the dominant feedstock or split the batch instead of forcing one large correction.
- Copy the JSON only after removing sensitive custom labels if the recipe names a private site, customer, or waste stream.
Worked Examples:
These examples use the default imperial weights and moisture accounting so the ratio, moisture, and repair cues can be checked together.
Balanced ratio, dry pile
A starter mix with 20 lb dry leaves at 60:1 and 10% moisture, 12 lb fresh grass at 17:1 and 70% moisture, and 12 lb kitchen scraps at 15:1 and 70% moisture lands at about 34.0:1. That is Balanced in a 25:1 to 35:1 band, but Blend moisture is about 42.7%, so the moisture result points toward adding water rather than changing the C:N recipe.
Wet green-heavy batch
A batch with 6 lb dry leaves, 20 lb fresh grass, 20 lb kitchen scraps, and 8 lb aged chicken manure lands near 17.7:1. The Classification becomes Nitrogen heavy and Blend moisture is about 61.1%, just above the default moisture window. The brown-helper repair may fix C:N, but the projected moisture must be checked before accepting the addition.
Woody mix with a huge green repair
A dry mix of 15 lb sawdust, 18 lb straw, 12 lb shredded cardboard, and 6 lb fresh grass can land above 113:1. If the green-helper repair calls for a very large grass addition, inspect Material Ledger and reduce the woody dry mass. Trying to fix a large carbon load with one wet green can create a moisture and handling problem.
FAQ:
Is C:N based on weight or volume?
This calculator uses weight. Each material's entered weight is converted to dry matter, then dry matter is split into carbon and nitrogen by that material's C:N ratio.
Should I leave Account for moisture turned on?
Yes for fresh leaves, grass, scraps, manure, and other as-handled feedstocks. Turn it off only when every entered weight has already been converted to dry matter.
Why can a balanced ratio still need work?
Classification checks only the selected C:N band. Check Blend moisture, Moisture status, pile structure, and the largest dry matter contributor before treating the recipe as ready.
Why did a repair row show such a large addition?
The helper material may be wet, mild, or far from the target direction. Try another brown or green helper, or adjust the original material weights when the suggested addition is larger than the pile.
What causes the target or material error messages?
Common causes are a target maximum that is not above the minimum, a moisture window outside 10% to 90%, a material weight of 0, a nonpositive C:N ratio, or material moisture at 100% or higher.
Are my material rows private?
The calculation runs in the browser. After you edit the recipe, a shared page address can include entered material values, so do not share the URL if names or weights are sensitive.
Glossary:
- C:N ratio
- Parts carbon per one part nitrogen by mass.
- Feedstock
- A material added to compost, such as leaves, grass, food scraps, manure, straw, or cardboard.
- Dry matter
- The portion of a material left after its water is removed from the entered weight.
- Moisture window
- The selected acceptable range for water as a percent of total wet mass.
- Brown helper
- A carbon-rich preset used to size a repair when the blend needs more carbon.
- Green helper
- A nitrogen-rich preset used to size a repair when the blend needs more nitrogen.
References:
- Composting, U.S. Environmental Protection Agency.
- C/N Ratio, Cornell Composting.
- Compost Chemistry, Cornell Composting.
- Large-Scale Organic Materials Composting, NC State Extension Publications.