Compost Carbon-to-Nitrogen (C:N) Ratio Calculator

{{ badge.label }}: {{ badge.value }}

Target profile:

Profiles set both the C:N band and moisture window; manual edits switch the profile to Custom.

Target C:N band:

Enter the accepted low and high edges as parts carbon per one part nitrogen.

to : 1

Account for moisture:

Water is removed before each material contributes to the ratio-only dry-matter proxy.

Moisture window:

The profile defaults to 50%–60%; use a measured site target when available.

to %

Materials:

Open a row to edit its feedstock preset, C:N estimate, moisture, and scale weight.

Feedstock preset:

Material name:

C:N estimate:

: 1

Moisture:

Fresh weight:

Brown repair helper:

Green repair helper:

Result weight display:

Compost blend ratio, moisture, mass, and planning proxy summary
Measure	Value	Planning note	Copy
{{ row.label }}	{{ row.value }}	{{ row.note }}

Compost helper additions for the selected target band
Target	Helper	Fresh add	Dry add	Projected C:N	Projected moisture	Status	Copy
{{ row.target }}	{{ row.helper }}	{{ formatMass(row.wet_add_kg) }}	{{ formatMass(row.dry_add_kg) }}	{{ formatRatio(row.projected_ratio) }}	{{ formatPercent(row.projected_moisture_pct) }}	{{ row.status_label }}

Per-material dry matter and ratio-proxy contribution ledger
Material	Role	C:N	Fresh	Dry	Water	C proxy	N proxy	Copy
{{ row.label }}	{{ row.role_label }}	{{ formatRatio(row.ratio) }}	{{ formatMass(row.wet_kg) }}	{{ formatMass(row.dry_kg) }}	{{ formatMass(row.water_kg) }}	{{ formatMass(row.carbon_proxy_kg) }}	{{ formatMass(row.nitrogen_proxy_kg) }}

Compost microbes use carbon as an energy source and nitrogen to build proteins and new cells. The carbon-to-nitrogen ratio, written C:N, is a practical way to plan the starting mix. A ratio near 30:1 is a common starting point, but the useful range depends on feedstock availability, particle size, aeration, moisture, and the composting method.

A low C:N ratio means nitrogen is abundant relative to carbon. The pile may heat quickly, but excess nitrogen can be lost as ammonia and create odor. A high ratio can leave microbes short of nitrogen and slow decomposition. Those labels describe the blend, not a fixed rule that every pile must reach immediately.

Browns such as dry leaves, straw, cardboard, and woody material tend to have higher C:N ratios.
Greens such as grass clippings, food scraps, coffee grounds, and manure tend to have lower ratios.
Moisture changes how much dry material each fresh kilogram or pound contributes. Wet grass and dry leaves should not be compared by fresh weight alone.

C:N cannot describe oxygen supply, pile size, temperature, pH, particle structure, contamination, or how readily microbes can use a material's carbon. Woody carbon may break down much more slowly than the same numerical ratio suggests. Use the estimate to plan a blend, then manage the actual pile by observing temperature, odor, moisture, and air space.

How to Use This Tool:

Choose the composting target, then describe the fresh materials as they will actually enter the pile.

Select Active pile, Hot compost, or a custom C:N and moisture window. The built-in active band is 25:1 to 35:1; hot compost uses 25:1 to 30:1.
Keep Account for moisture on when entering fresh weights. Set it off only when every weight already represents dry matter or when you intentionally want a dry-basis comparison.
Add one row for each feedstock and enter its C:N estimate, moisture percentage, fresh weight, and unit. Replace preset values with measured or supplier-specific data when available.
Open Advanced to choose the brown and green materials used by the repair calculation. The selected helper must lie on the needed side of the target ratio.
Compare the Mix snapshot with the moisture badge, then inspect Repair plan before adding material. Recalculate after weighing the actual addition.

Interpreting Results:

The Final C:N estimate is balanced when it equals either edge or falls between the selected low and high values. Below the low edge is Nitrogen heavy; above the high edge is Carbon heavy.

Read the ratio and moisture status together. A blend can fall inside the C:N band while still being too wet or too dry. When moisture accounting is off, the result is labeled Dry basis and no fresh-blend moisture judgment is made.

Repair plan calculates separate additions for the low edge, midpoint, and high edge. A helper may be unable to reach a target when its own C:N ratio is on the wrong side of that target; in that case the plan reports no usable addition instead of returning a negative mass.

Technical Details:

The model first converts pounds to kilograms and removes water from each fresh material. It then derives carbon and nitrogen planning fractions from the entered C:N ratio. These are ratio-normalized proxies, not laboratory measurements of elemental carbon and nitrogen.

Formula Core

For material i, dry mass is fresh mass multiplied by the non-water fraction. Its C:N ratio r_i is split into proxy shares that sum to one.

\begin{matrix} d_{i} & = & w_{i} (1 - \frac{m_{i}}{100}) \\ C_{i} & = & d_{i} \frac{r_{i}}{r_{i} + 1} \\ N_{i} & = & d_{i} \frac{1}{r_{i} + 1} \end{matrix}

The blend ratio is the sum of carbon proxies divided by the sum of nitrogen proxies. Blend moisture is total water mass divided by total fresh mass.

\begin{matrix} R_{mix} & = & \frac{\sum C_{i}}{\sum N_{i}} \\ M & = & \frac{\sum (w_{i} - d_{i})}{\sum w_{i}} \times 100 \end{matrix}

For example, 10 kg of dry 60:1 material mixed with 10 kg of dry 15:1 material gives carbon and nitrogen proxies of about 19.211 kg and 0.789 kg. The result is about 24.35:1, not the simple average of 37.5:1, because equal dry masses contain different proxy nitrogen shares.

Repair Mechanism

The helper calculation solves directly for the dry addition needed to reach a chosen target ratio. It then divides by the helper's dry fraction to return the fresh mass to add.

x_{dry} = \frac{R_{target} N_{mix} - C_{mix}}{c_{helper} - R_{target} n_{helper}}

A positive finite solution is reported. Zero or negative solutions, or a helper ratio that makes the denominator ineffective, are marked as unable to reach the target. The projected moisture includes the water carried by the helper.

Rule Core

Compost classification and moisture boundary rules
Measure	Condition	Result
C:N balance	Ratio < selected low edge	Nitrogen heavy
C:N balance	Low edge ≤ ratio ≤ high edge	Balanced
C:N balance	Ratio > selected high edge	Carbon heavy
Moisture	Moisture < selected low edge	Too dry
Moisture	Low edge ≤ moisture ≤ high edge	In window
Moisture	Moisture > selected high edge	Too wet
Material role	C:N ≥ 35:1	Carbon-rich helper candidate
Material role	C:N ≤ 20:1	Nitrogen-rich helper candidate

Limitations:

Preset C:N and moisture values are approximate and feedstocks vary with species, age, storage, weather, and sampling. The ratio-only proxy infers component shares from C:N without requiring measured percent carbon or percent nitrogen, so it is less specific than a laboratory-based mixture calculation.

The active 25:1 to 35:1 and hot 25:1 to 30:1 bands are planning presets, not universal operating standards.
A balanced estimate cannot confirm adequate oxygen, safe temperature, pathogen reduction, maturity, or contaminant control.
Carbon availability differs among leaves, paper, straw, sawdust, and wood chips even when their listed ratios look similar.
Repair amounts assume the selected helper matches its preset ratio and moisture. Weigh and recalculate the actual addition.

References:

C/N Ratio, Cornell Composting.
Compost Chemistry, Cornell Composting.
Composting, U.S. Environmental Protection Agency.