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

Compost Mix Check

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Target {{ targetRangeLabel }} • Moisture {{ moistureRangeLabel }} • Dry basis mode

Measurement system:

Use lb for US scale readings or kg for metric batches; all material weights share this unit.

Target ratio minimum:

Typical active compost starts near 25:1; enter the low cutoff as a whole number.

: 1

Target ratio maximum:

Use about 30:1 for hot compost or 35:1 for a looser active-pile band.

: 1

Account for moisture:

Keep on for fresh as-handled weights; turn off only when every weight is already dry.

Materials:

Add one row per feedstock; use presets as starting points, then edit to match your batch.

No materials added

Add your first material to start calculating the blend.

Preset:

Pick the closest feedstock, then adjust the numbers if your batch is wetter, drier, or woodier.

Name:

Optional; leave blank to use the preset name or enter a bin-ready label like Fall leaves.

C:N ratio:

Enter parts carbon per 1 part nitrogen; accepted 5-600, e.g. 60 for leaves.

: 1

Moisture:

Enter 0-90% water by weight; disabled when moisture accounting is off.

Weight:

Use as-handled scale weight in the selected unit; minimum 0.1.

Target preset:

Moisture target minimum:

Enter 10-85%; 50% is a common lower edge for active compost.

Moisture target maximum:

Enter 15-90% and keep it above the minimum; 60% is a common upper edge.

Brown repair helper:

Choose a carbon-rich feedstock such as leaves, straw, sawdust, wood chips, or cardboard.

Green repair helper:

Choose a nitrogen-rich feedstock such as grass, scraps, coffee grounds, manure, or alfalfa.

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
{{ row.label }} {{ row.materialClass }} • {{ formatPercent(row.moisturePercent, 0) }} moisture • Dry basis	{{ row.ratioDisplay }}	{{ row.wetWeightDisplay }}	{{ row.dryWeightDisplay }}	{{ row.waterDisplay }}	{{ row.carbonDisplay }}	{{ row.nitrogenDisplay }}

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Add at least one material to chart dry mass distribution.

No nutrient chart yet

Add at least one material to chart carbon and nitrogen contributions.

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Add materials to generate the mass components chart.

Compost mix waiting for materials

Add brown and green ingredients to see the blended C:N ratio, moisture window, and repair guidance.

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A compost pile depends on a living balance: microbes need carbon for energy, nitrogen for growth, water for metabolism, and air space so the process stays aerobic. The carbon-to-nitrogen ratio, usually written as C:N, is one way to estimate whether a fresh mix has enough browns and greens to heat, break down steadily, and avoid the most common odor problems.

The ratio is based on mass, not scoops, buckets, or visible volume. Dry leaves, straw, cardboard, grass clippings, food scraps, coffee grounds, and manures all carry different amounts of dry matter and water. A heavy pail of wet scraps may add less dry carbon and nitrogen than it seems, while a small amount of sawdust can add a large carbon load because most of its mass is dry.

Common compost feedstock roles
Feedstock role	Typical materials	Practical effect
Carbon-rich browns	Dry leaves, straw, cardboard, sawdust, wood chips	Raise the C:N ratio, add structure, and slow excess wetness.
Nitrogen-rich greens	Grass clippings, food scraps, coffee grounds, manures, alfalfa	Lower the C:N ratio and help microbial growth, but often add water.
Bridge materials	Ingredients near the working range, such as some manures or mixed plant waste	Shift the pile less dramatically and can smooth an otherwise sharp correction.

Wet feedstock split into water and dry matter before carbon and nitrogen are blended into a C:N ratio. — C:N math follows dry matter. Moisture changes how much of a fresh load actually contributes carbon and nitrogen.

Active composting commonly aims near 25:1 to 35:1, with tighter hot-pile recipes often near the lower part of that range. Those numbers are guides, not guarantees. The useful carbon in wood chips and sawdust may become available slowly, chopped greens can collapse and exclude air, and a pile that is mathematically balanced can still stall if it is too dry, too small, too compact, or not turned.

A C:N estimate is most helpful before materials are mixed, when a correction is still easy. It can show the direction of an imbalance, compare possible repairs, and catch extreme recipes. It cannot identify herbicide residues, pathogens, salt, pH, particle-size problems, or whether a finished compost is mature enough for a planting bed.

How to Use This Tool:

Use one material row per feedstock so weight, moisture, and C:N ratio are handled before the pile totals are combined.

Choose Imperial (lb) or Metric (kg). Existing material weights convert when the measurement system changes, so check rounded values after switching units.
Set the Target ratio minimum and Target ratio maximum, or choose a Target preset in Advanced. The default active-pile band is 25:1 to 35:1, while the tighter hot-pile band is 25:1 to 30:1.
Leave Account for moisture on for fresh, as-handled material weights. Turn it off only when every entered weight has already been converted to dry weight.
Add materials, pick the closest preset, then edit C:N ratio, Moisture, and Weight when your batch is wetter, drier, fresher, aged, or mixed with bedding.
Choose the Brown repair helper and Green repair helper if you want the repair plan to size a specific correction material.
If an error appears, fix the field named in the message. Target maximum must exceed target minimum, moisture targets must stay from 10% to 90% with high above low, material ratios and weights must be positive, and material moisture must stay below 100% when moisture accounting is on.
Read Blend Check first, then use Repair Plan for low-edge, midpoint, and high-edge additions. Material Ledger shows the per-material dry weight, water, carbon, nitrogen, and share values that explain the result.

Interpreting Results:

Start with Final C:N ratio, Classification, Blend moisture, and Moisture status. A good ratio can still be too wet or too dry, and a good moisture percentage cannot rescue a mix that is far outside the selected C:N target band.

Compost C:N and moisture interpretation boundaries
Output cue	Boundary	What to verify next
Nitrogen heavy	Final C:N ratio is less than the target minimum.	Review the brown-helper rows and check whether the projected moisture becomes too dry.
Balanced	Final C:N ratio is greater than or equal to the minimum and less than or equal to the maximum.	Check Blend moisture, Largest dry matter share, and feedstock estimates before assuming the pile will heat.
Carbon heavy	Final C:N ratio is greater than the target maximum.	Review the green-helper rows and confirm the added wet material does not overshoot the moisture window.
Too dry	Blend moisture is less than the moisture target minimum.	Use the moisture correction and consider whether dry browns are dominating the dry mass.
Too wet	Blend moisture is greater than the moisture target maximum.	Compare dry brown additions with pile structure, drainage, and air space.

Treat small decimal differences as planning noise unless the material data came from testing. In most garden and farm settings, the useful decision is the direction of the imbalance, the scale of a realistic repair, and whether moisture or dry mass is the harder constraint.

Technical Details:

Compost C:N math is a dry-matter calculation. Each feedstock starts with a fresh weight and, when moisture is included, a water fraction. Removing that water gives dry matter. The material C:N ratio then splits the dry matter into carbon and nitrogen shares, and the pile-level ratio comes from the sum of those carbon shares divided by the sum of those nitrogen shares.

The arithmetic is sensitive to water because moisture changes the dry mass behind a scale reading. A 20 lb material at 70% moisture contributes 6 lb of dry matter, while a 20 lb material at 10% moisture contributes 18 lb. The same entered C:N ratio has three times as much influence in the drier material because more dry mass is available to split into carbon and nitrogen.

Formula Core:

The governing equations remove water, split each dry material by its C:N ratio, and combine all material contributions.

\begin{array}{lcl} D_{i} & = & W_{i} \times (1 - m_{i}) \\ C_{i} & = & D_{i} \times \frac{R_{i}}{R_{i} + 1} \\ N_{i} & = & D_{i} \times \frac{1}{R_{i} + 1} \\ R_{mix} & = & \frac{\sum_{i = 1}^{n} C_{i}}{\sum_{i = 1}^{n} N_{i}} \end{array}

Compost formula symbols
Symbol	Meaning	Unit or form
W	Entered material weight	lb or kg, converted internally to a common mass unit
m	Material moisture	Percent as a fraction, such as 70% 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 a 20 lb dry-leaves row at 10% moisture and 60:1 C:N, dry matter is 18 lb. The carbon share is 18 × 60 / 61, or about 17.70 lb, and the nitrogen share is 18 / 61, or about 0.30 lb. Those two values are added to the carbon and nitrogen totals from every other row before the final ratio is computed.

Repair Math:

Repair rows solve for the dry mass of the selected helper material needed to reach a target C:N ratio. The result is then converted back to wet weight using that helper material's moisture percentage.

D_{add} = \frac{R_{target} \times N_{mix} - C_{mix}}{C_{helper} - R_{target} \times N_{helper}}

The denominator must be able to move the mix toward the target. A carbon-rich helper can raise a low ratio; a nitrogen-rich helper can lower a high ratio. If the helper is invalid or cannot move the ratio in the needed direction, the repair row is blocked instead of returning a misleading amount.

Compost calculation and validation boundaries
Boundary	Rule	Effect on results
Target ratio band	Minimum and maximum must be greater than 0, and maximum must exceed minimum.	Defines the Nitrogen heavy, Balanced, and Carbon heavy labels.
Moisture target window	When moisture is enabled, low and high targets must stay from 10% to 90%, with high above low.	Defines Too dry, In moisture window, and Too wet.
Material inputs	C:N ratio and weight must be positive; material moisture must be from 0% to 99%.	Prevents zero nitrogen, zero mass, and all-water rows from producing a false ratio.
Material class	35:1 or higher is Carbon-rich; 20:1 or lower is Nitrogen-rich; values between are Bridge material.	Labels rows for scanning but does not change the arithmetic.
Displayed precision	Final ratios display to one decimal, percentages usually to one decimal, and weights use fewer decimals as values grow.	Small display differences should not be treated as lab precision.

Blend moisture is the water mass divided by wet mass. When a pile is too dry, the moisture correction estimates water needed to move toward the middle of the target window. When it is too wet, the correction estimates water to remove or dry bulking matter needed to offset the excess water.

Worked Examples:

Starter mix that is balanced but dry

With 20 lb dry leaves at 60:1 and 10% moisture, 12 lb fresh grass clippings at 17:1 and 70% moisture, and 12 lb kitchen scraps at 15:1 and 70% moisture, Final C:N ratio is about 34.0:1. The Classification is Balanced in a 25:1 to 35:1 target band.

Blend moisture is about 42.7%, so Moisture status reads Too dry against a 50% to 60% window. The ratio does not need a brown or green repair, but the moisture correction points toward adding water.

Wet green-heavy pile

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. Classification becomes Nitrogen heavy, and Blend moisture is about 61.1%, just above the default moisture window.

Repair Plan sizes dry-leaves additions for the target edges and midpoint. Reaching the low edge near 25:1 takes about 15.9 lb wet leaves, but the projected moisture falls near 49.5%, so the moisture result must be checked before accepting that repair.

Brown-heavy mix with an unrealistic green repair

A dry batch made from 15 lb sawdust, 18 lb straw, 12 lb shredded cardboard, and 6 lb fresh grass can land above 113:1, with Moisture status far below the 50% to 60% window. The green-helper repair may show a very large fresh-grass addition because wet grass carries so much water.

When Repair Plan suggests a correction larger than the pile itself, use Material Ledger to find the dominant dry matter row. Removing or reducing the woody material can be more practical than trying to fix the whole mix with one wet green addition.

FAQ:

Is C:N based on weight or volume?

The calculator uses weight. Each material's entered weight is converted to dry matter, then split into carbon and nitrogen from its C:N ratio.

Should Account for moisture stay on?

Yes for fresh leaves, grass, scraps, manure, and other as-handled feedstocks. Turn it off only when all weights already represent dry matter.

Why can a balanced C:N ratio still need work?

The Classification only checks the selected C:N band. Check Blend moisture, Moisture status, pile structure, and the largest dry matter contributor before assuming the recipe is ready.

Why did a repair row show a huge addition?

The selected helper may be very wet, very mild, or far from the target direction. Try a different 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?

The most 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.