TryBuildCalcCompost Calculator (Carbon:Nitrogen Ratio Recipe)
Find your exact brown/green material mix instantly.
🕒 Last updated: September 22, 2026
Inputs
ℹ️Total volume of the finished, mixed pile or bin you're filling. A roughly 3 ft x 3 ft x 3 ft bin/pile (27 cu ft / 1 cu yd) is the commonly-cited minimum size to self-insulate and reach hot composting temperatures.
Target C:N Ratio
ℹ️The commonly-recommended range for active, hot composting is roughly 25:1-30:1 (Cornell Waste Management Institute cites ~30:1 as the ideal). A lower ratio composts faster but risks losing nitrogen as ammonia odor; a higher ratio decomposes more slowly.
Brown Material (Carbon)
ℹ️Carbon-rich, dry/woody material — the ingredient that balances a nitrogen-rich green.
Cornell Waste Management Institute's own On-Farm Composting Handbook table gives dry leaves a C:N ratio averaging 54:1 (range 40-80:1, this preset uses a representative 60:1), 0.9% nitrogen (dry weight), and a loose/dry bulk density of roughly 100-300 lb per cubic yard (this preset's ~7 lb/cu ft is the midpoint). Leaf C:N varies noticeably by tree species (Cornell's own leaf-species table ranges from 15:1 for alder to 113:1 for larch) — use Custom C:N Ratio for a known single-species leaf pile.
Green Material (Nitrogen)
ℹ️Nitrogen-rich, moist material — the ingredient that balances a carbon-rich brown.
Cornell's On-Farm Composting Handbook table gives fresh grass clippings a C:N ratio averaging 17:1 (range 9-25:1), 3.4% nitrogen (dry weight), and 82% moisture — consistent with Cornell's own separate "Getting the Right Mix" chapter, which cites roughly 80% moisture for grass clippings. Loose bulk density (~300-400 lb/cu yd, this preset's ~13 lb/cu ft is the midpoint) roughly doubles if the clippings settle/compact.
Cost (Optional)
Brown Material Needed
12.34
cu ft (0.457 cu yd) — 86.4 lb (39.2 kg)
Green Material Needed
14.66
cu ft (0.543 cu yd) — 190.6 lb (86.4 kg)
Target C:N ratio 30:1 achieved (as mixed: 30:1) for a 27 cu ft (1 cu yd) pile
Mixture moisture (as mixed): 67.3% by weight
This blend works out to about 67% moisture by weight — above the commonly-recommended 40-60% range (Cornell: a pile should feel like a wrung-out sponge). Above roughly 60%, water crowds out the air pockets a pile needs and it becomes prone to compacting and going anaerobic (slow, smelly decomposition) — regardless of how close the C:N ratio is to the target. Mix in more dry brown material, or turn the pile more often to help it dry out.
Brown Material (Carbon)
Material: Dry Leaves
C:N ratio: 60:1
Volume: 12.34 cu ft (0.349 m³)
Weight: 86.4 lb (39.2 kg)
Green Material (Nitrogen)
Material: Grass Clippings (fresh)
C:N ratio: 17:1
Volume: 14.66 cu ft (0.415 m³)
Weight: 190.6 lb (86.4 kg)
Estimated Finished Compost Yield
Roughly 8.1-13.5 cu ft (0.3-0.5 cu yd) once fully broken down and settled.
A rough estimate only — composting typically reduces starting volume by roughly 50-70% as organic matter breaks down, moisture evaporates, and the pile settles, but real shrinkage varies hugely by materials, moisture, turning frequency, and climate. Do not treat this as a precise figure.
Assumptions Used
This calculator blends the brown and green materials by MASS (weight), not by volume — a physically correct C:N ratio depends on the actual carbon and nitrogen MASS in the mix, and materials like dry leaves and wet food scraps differ enormously in weight per cubic foot. Dry Leaves is used at 60:1 C:N, 0.9% nitrogen (dry weight), 35% moisture, and 7 lb/cu ft bulk density; Grass Clippings (fresh) is used at 17:1 C:N, 3.4% nitrogen (dry weight), 82% moisture, and 13 lb/cu ft bulk density. Cornell Waste Management Institute's own On-Farm Composting Handbook table gives dry leaves a C:N ratio averaging 54:1 (range 40-80:1, this preset uses a representative 60:1), 0.9% nitrogen (dry weight), and a loose/dry bulk density of roughly 100-300 lb per cubic yard (this preset's ~7 lb/cu ft is the midpoint). Leaf C:N varies noticeably by tree species (Cornell's own leaf-species table ranges from 15:1 for alder to 113:1 for larch) — use Custom C:N Ratio for a known single-species leaf pile. Cornell's On-Farm Composting Handbook table gives fresh grass clippings a C:N ratio averaging 17:1 (range 9-25:1), 3.4% nitrogen (dry weight), and 82% moisture — consistent with Cornell's own separate "Getting the Right Mix" chapter, which cites roughly 80% moisture for grass clippings. Loose bulk density (~300-400 lb/cu yd, this preset's ~13 lb/cu ft is the midpoint) roughly doubles if the clippings settle/compact. This calculator estimates a recipe split and finished-yield range only — it does not manage moisture, turning/aeration, or pile temperature, all of which also matter for successful composting.
Diagram simplified for clarity (not to scale) — pile shown schematically, layer heights reflect the real brown:green volume split.
What Is a Compost Calculator?
A compost calculator finds how much brown (carbon-rich) material and green (nitrogen-rich) material your compost pile or bin needs, to fill a target total volume at a healthy target carbon:nitrogen (C:N) ratio — commonly cited as roughly 25:1-30:1. Unlike a simple coverage or fill calculator, this is a genuine RECIPE/BLEND problem: it mixes two materials by MASS (weight), not just volume, since a real C:N ratio is a ratio of actual carbon mass to nitrogen mass, and materials like dry leaves and wet food scraps differ enormously in how much they weigh per cubic foot.
It reports the volume AND weight of each material needed, the as-mixed C:N ratio actually achieved, and a rough estimated finished-compost yield (since composting typically reduces starting volume by roughly 50-70% as organic matter breaks down). This is a different calculation from this site's Raised Bed Soil Calculator, which splits FINISHED materials (topsoil, compost, amendments) across a bed by simple volume percentage — this calculator instead blends RAW organic materials by C:N chemistry to build an active compost pile in the first place.
What this calculator does not do:
- It computes an initial recipe split and a rough finished-yield range only — it does not manage moisture, turning/aeration, or pile temperature over time, all of which also matter for successful composting
- It is not a raised bed soil-mix or mulch/topsoil coverage calculator — for those, this site's dedicated Raised Bed Soil Calculator, Mulch Calculator, and Topsoil Calculator are the right tools
Compost Calculator Formula: How Is It Determined?
A target pile volume, a target C:N ratio, and each material's own C:N ratio/%nitrogen/bulk density/moisture combine into a real mass-weighted blend — never a rough volume-only split.
Step 1 — Pile/Bin Volume
By Total Volume: Volume = entered value
By Bin Dimensions: Volume = Length x Width x Height
Either enter the target total volume directly, or enter a rectangular bin's length, width, and height.
Step 2 — Nitrogen Mass per Unit Volume
Nitrogen per cu ft = Bulk Density x (%Nitrogen / 100) x (1 - Moisture% / 100)
%Nitrogen is measured on a DRY-WEIGHT basis, so it only applies to the material's dry portion — bulk density (the material's as-piled weight per cu ft) times (1 - moisture%) gives the dry weight per cu ft, which is then multiplied by %nitrogen to get actual nitrogen mass per cu ft. This is the building block the whole blend depends on — see the Reference Tables section below for why skipping this step (mixing by volume alone) gives a physically wrong C:N ratio.
Step 3 — Volume Split to Hit the Target Ratio
Brown Volume = Total Volume x [Green N/cu ft x (Target - Green Ratio)] / Denominator
Denominator = Brown N/cu ft x (Brown Ratio - Target) + Green N/cu ft x (Target - Green Ratio)
Green Volume = Total Volume - Brown Volume
Derived from the identity that a blend's overall C:N ratio is the NITROGEN-MASS-weighted average of each material's own ratio (R = total Carbon mass / total Nitrogen mass, and each material's own Carbon mass = Nitrogen mass x that material's own C:N ratio). This engine's formula was independently re-derived and verified against Cornell Waste Management Institute's own published worked example (a real 10 kg grass + leaves mix targeting 30:1) — see the Reference Tables section for the comparison.
Step 4 — Weight
Material Weight = Material Volume x Material Bulk Density
Each material's resulting volume converts directly to weight using its own bulk density, so you can weigh out ingredients instead of (or in addition to) measuring volume.
Step 5 — Estimated Finished Yield
Low Estimate = Total Volume x 30% (i.e. 70% shrinkage)
High Estimate = Total Volume x 50% (i.e. 50% shrinkage)
Composting typically reduces starting volume by roughly 50-70% as organic matter breaks down, moisture evaporates, and the pile settles — shown as a RANGE with an explicit caveat, since real shrinkage varies hugely by materials, moisture, turning frequency, and climate.
Worked Example
This example walks through your current inputs above, using the same steps as the Formula section.
Input Values Used
| Input | Value | Why it is used |
|---|---|---|
| Total Volume | 27 cu ft (1 cu yd) | Sets the total to split between materials |
| Target C:N Ratio | 30:1 | Sets the blend goal |
| Brown Material | Dry Leaves (60:1, 0.9%N, 7 lb/cu ft, 35% moisture) | Carbon side of the blend |
| Green Material | Grass Clippings (fresh) (17:1, 3.4%N, 13 lb/cu ft, 82% moisture) | Nitrogen side of the blend |
Step 1-3 — Volume Split
| Calculation | Result |
|---|---|
| Brown Volume (mass-weighted split, solved for target 30:1) | 12.34 cu ft |
| Green Volume = 27 - 12.34 | 14.66 cu ft |
| As-Mixed C:N Ratio (independent check) | 30:1 |
| As-Mixed Moisture (weight-weighted blend) | 67.3% |
Step 4 — Weight
| Calculation | Result |
|---|---|
| Brown Weight = 12.34 cu ft x 7 lb/cu ft | 86.4 lb (39.2 kg) |
| Green Weight = 14.66 cu ft x 13 lb/cu ft | 190.6 lb (86.4 kg) |
Step 5 — Estimated Finished Yield
| Calculation | Result |
|---|---|
| Low Estimate = 27 cu ft x 30% | 8.1 cu ft |
| High Estimate = 27 cu ft x 50% | 13.5 cu ft |
Figures above are rounded for display; the real recipe split is always calculated from full, unrounded values, so it can occasionally look off by a hair if you redo the division yourself using the rounded figures shown.
Therefore, a 27 cu ft pile targeting 30:1 needs approximately 12.34 cu ft (86.4 lb) of Dry Leaves and 14.66 cu ft (190.6 lb) of Grass Clippings (fresh).
Essential Checklist+−
Complete these critical checks before approving the work or proceeding to the next construction stage.
✓Pile Size & Material Selection+-
- Target pile/bin volume confirmed from an actual measurement (bin dimensions or a realistic estimate of available materials), not assumed
- Selected brown/green material presets' C:N ratio, %nitrogen, and bulk density genuinely match the actual materials being used, not just the closest generic preset
- Meat, dairy, oily foods, diseased plant material, pernicious weeds, and pet waste excluded from the pile
✓Mixing & Layering+-
- Overall pile moisture checked by hand (aiming for the texture of a wrung-out sponge) after mixing, not assumed from the calculated volumes alone
✓Active Management+-
- A turning/aeration schedule planned for the active composting phase (e.g. every 1-2 weeks for a hot pile)
✓Finished Compost & Use+-
- Finished compost confirmed mature (dark, crumbly, earthy-smelling, no longer heating up, original materials no longer recognizable) before use, not used while still actively decomposing
Full QC Checklist+−
Verification checklist for a compost pile/bin C:N recipe — covering pile size and material selection, mixing and layering, active management, and finished compost use. Use the Essential Checklist for critical checks before finalizing, expand to Full QC Checklist for complete verification.
✓Pile Size & Material Selection+-
- Target pile/bin volume confirmed from an actual measurement (bin dimensions or a realistic estimate of available materials), not assumed
- Pile/bin size checked against the roughly 27 cu ft (3 ft x 3 ft x 3 ft) commonly-cited minimum for a self-insulating, hot-composting pile
- Selected brown/green material presets' C:N ratio, %nitrogen, and bulk density genuinely match the actual materials being used, not just the closest generic preset
- Target C:N ratio and selected brown/green materials produce a physically sensible split (not an extreme 95%+/5% split of one material)
- Meat, dairy, oily foods, diseased plant material, pernicious weeds, and pet waste excluded from the pile
✓Mixing & Layering+-
- Bulky brown materials (cardboard, branches, whole leaves) shredded, chopped, or broken up rather than added in large pieces
- Brown and green materials mixed together through the pile (not left as strict, unmixed layers) once building is complete
- Overall pile moisture checked by hand (aiming for the texture of a wrung-out sponge) after mixing, not assumed from the calculated volumes alone
- A coarser brown material layer placed at the base of the bin/pile for drainage and airflow before adding the mixed remainder
- Fresh food scraps or other odor-prone green material covered with a layer of browns after adding, rather than left exposed on the pile surface
✓Active Management+-
- A turning/aeration schedule planned for the active composting phase (e.g. every 1-2 weeks for a hot pile)
- Pile core temperature monitored with a compost thermometer if aiming for hot/thermophilic composting (targeting roughly 130-160°F / 55-70°C)
- Moisture rechecked periodically through the composting process, not just at initial mixing
- Any odor (ammonia smell, rotten-egg smell) or pest issues addressed promptly by adjusting the brown:green balance or turning frequency
- Additional water added during extended dry weather to keep the pile from drying out and stalling
✓Finished Compost & Use+-
- Finished compost confirmed mature (dark, crumbly, earthy-smelling, no longer heating up, original materials no longer recognizable) before use, not used while still actively decomposing
- This calculator's finished-yield volume range treated as a rough planning estimate only, not a precise guaranteed output
- Finished compost screened/sifted before use in seed-starting mix or fine top-dressing, if any large unfinished chunks remain
- A curing/resting period allowed after active decomposition slows, before heavy use around sensitive young plants
- This calculator's estimated finished-yield range compared against the actual quantity of compost needed for the intended beds/use, before committing to a starting pile size
Compost Material Reference Tables
C:N ratio, %nitrogen (dry weight), moisture, and bulk density for each preset — cross-checked primarily against Cornell Waste Management Institute's own On-Farm Composting Handbook (Appendix A, Table A.1) and its "Getting the Right Mix" chapter. General reference figures, not fixed universal constants, since real material properties vary by species, freshness, and how tightly packed a material is.
Brown (Carbon) Materials
| Material | C:N Ratio | %Nitrogen (dry) | Moisture | Bulk Density |
|---|---|---|---|---|
| Dry Leaves | 60:1 (range 40-80:1) | 0.9% | 35% | ~7 lb/cu ft |
| Straw | 80:1 (range 48-150:1) | 0.7% | 12% | ~7 lb/cu ft |
| Wood Chips / Sawdust | 450:1 (wide real range, ~200-800+:1) | 0.2% | 35% | ~17 lb/cu ft |
| Cardboard / Newspaper (shredded) | 350:1 (see source note below) | 0.1% | 8% | ~9 lb/cu ft |
Green (Nitrogen) Materials
| Material | C:N Ratio | %Nitrogen (dry) | Moisture | Bulk Density |
|---|---|---|---|---|
| Grass Clippings (fresh) | 17:1 (range 9-25:1) | 3.4% | 82% | ~13 lb/cu ft |
| Vegetable & Fruit Scraps | 19:1 | 2.7% | 87% | ~45 lb/cu ft (estimated) |
| Coffee Grounds (used) | 20:1 | 2.3% | 60% (estimated) | ~40 lb/cu ft (estimated) |
| Aged Manure (Cow / Horse) | 20:1 | 2.2% | 60% (estimated) | ~50 lb/cu ft |
Why Mixing by Mass (Not Volume) Matters
A C:N ratio is a ratio of CARBON MASS to NITROGEN MASS, not volume. This calculator converts each material's volume to weight using its own bulk density, applies each material's %nitrogen to its DRY weight (after removing its own moisture content), and only then computes the blend's real C:N ratio — reproducing the same underlying formula Cornell Waste Management Institute's own "Getting the Right Mix" chapter publishes (Richard & Trautmann, Equations 5-6). Independently re-deriving this engine's own volume-split formula against Cornell's own published numeric example (10 kg of grass clippings at 2.4%N/45%C/77% moisture mixed with leaves at 0.75%N/50%C/35% moisture, targeting a 30:1 ratio) reproduces Cornell's own published answer of 3.5 kg of leaves needed almost exactly (this engine's formula computes ~3.47 kg for the same inputs).
Source disagreement disclosed: several popular consumer composting guides cite shredded cardboard at roughly 350:1 and shredded newspaper at roughly 175:1 — this calculator uses 350:1 for its combined Cardboard/Newspaper preset. Cornell's own EMPIRICAL lab-table data shows measured values running meaningfully higher (corrugated cardboard 563:1, newsprint 398-852:1), so treat 350:1 as a conservative, commonly-quoted figure rather than a precise constant — use Custom C:N Ratio if you have a more specific figure for your own material.
Primary sources: Cornell Waste Management Institute, On-Farm Composting Handbook, Appendix A, Table A.1 (cwmi.css.cornell.edu); Cornell Waste Management Institute, "Getting the Right Mix" (Composting in the Classroom, Chapter 3, Richard & Trautmann); Oregon State University Extension and UC ANR/UC Davis composting guidance for the recommended target-ratio range. Kitchen-scraps, coffee-grounds, and aged-manure bulk density/moisture figures are this calculator's lowest-confidence figures, flagged above as estimates where a direct extension-sourced measurement wasn't found.
When should you use this compost calculator?
- Building a new compost pile or bin and wanting to hit a healthy C:N ratio from the start, instead of guessing at a brown:green ratio.
- Comparing how much of a specific brown or green material you actually need for a given pile size, since bulk density varies enormously between materials.
- Troubleshooting a smelly or slow-composting pile by checking whether your rough brown:green mix is meaningfully off the recommended C:N range.
- Planning roughly how much finished compost a given amount of starting material will eventually yield.
- Using a specific known material (a particular mulch, manure, or food-processing byproduct) via Custom C:N Ratio rather than a generic preset.
- Budgeting material cost when browns or greens need to be purchased (e.g. bagged straw, bagged manure) rather than sourced for free from yard waste.
Quick Composting Tips
- Shred or chop bulky browns (cardboard, branches, whole leaves) before adding them — particle size affects decomposition speed independently of a correct C:N ratio.
- Check moisture by hand after mixing — aim for the texture of a wrung-out sponge (a drop or two of water, not a stream, when squeezed).
- Turn the pile regularly (every 1-2 weeks for active hot composting) to keep oxygen flowing to the decomposing microorganisms.
- Layer a coarser brown material at the very base of the bin for drainage and airflow before adding the rest mixed together.
- If a pile smells like ammonia, add more browns and turn it; if it smells like rotten eggs, it's likely too wet or compacted — add dry browns and turn it.
- A pile smaller than roughly 3 ft x 3 ft x 3 ft rarely gets hot enough to compost quickly — combine several small sources of material to reach a self-insulating size if possible.
Common Mistakes
- Mixing brown and green materials by VOLUME alone without accounting for how different their bulk density is — a bucket of dry leaves and a bucket of wet food scraps contain very different masses of actual carbon and nitrogen.
- Assuming all "browns" or all "greens" share one C:N ratio — this calculator's own presets range from 60:1 (dry leaves) to 450:1 (wood chips/sawdust) among browns alone, a more than 7x spread.
- Adding meat, dairy, oily foods, diseased plants, or pernicious weeds to a home compost pile — these attract pests or can survive typical home composting temperatures.
- Building a pile too small to self-insulate (below roughly 27 cu ft / 3 ft x 3 ft x 3 ft), then expecting hot-pile speed and pathogen/weed-seed kill from a pile that never gets hot.
- Treating the finished-yield estimate as a precise, guaranteed number rather than a rough planning range — real shrinkage varies hugely by conditions.
- Using immature, still-decomposing compost directly around young plants — unfinished compost can temporarily tie up soil nitrogen as it continues breaking down.
Limitations
- Computes an initial recipe split (volume and weight of brown and green material) and a rough finished-yield range only — it does not manage moisture, turning/aeration, or pile temperature over the composting process.
- Material preset figures (C:N ratio, %nitrogen, bulk density, moisture) are commonly-cited representative reference figures cross-checked across multiple sources, not a fixed universal standard — actual material properties vary by species, freshness, and how tightly the material is packed. Use Custom C:N Ratio with your own material's known figures for a more precise result.
- A handful of preset figures (kitchen-scraps, coffee-grounds, and aged-manure bulk density/moisture) are this calculator's own disclosed lowest-confidence estimates, since a direct extension-sourced measurement for these specific materials/states wasn't found — see the Reference Tables section above.
- The estimated finished-compost yield is a rough order-of-magnitude range (50-70% shrinkage), not a precise prediction — real decomposition and volume loss varies hugely by materials, moisture, turning frequency, pile size, and climate.
- Only blends TWO materials (one brown, one green) at a time — a real pile built from several different materials over time will need this calculator run per material pair, or a simplified single representative brown/green figure for the mix as a whole.
- Does not evaluate pathogen or weed-seed kill, which depends on the pile actually reaching and sustaining adequate hot-composting temperatures — not something a starting recipe alone can guarantee.
Disclaimer: This calculator provides approximate results for planning and estimation purposes only. Actual requirements may vary based on site conditions, materials, workmanship, and local building regulations. Always consult a qualified engineer, architect, or construction professional before making final decisions.