Compost C:N Ratio Calculator: Balance Greens and Browns
Calculate the carbon-to-nitrogen ratio of your compost mix, then see how much brown carbon material
or green nitrogen material to add. Use the simple backyard estimate for quick planning, or switch to
moisture-adjusted mode when comparing wet grass, food scraps, and manure with dry leaves, straw, or cardboard.
Reviewed June 2026. Calculations run locally in your browser.
Materials & correction target
Simple mode: mass is treated as entered weight; moisture is not applied to the C:N math.
Material
Mass
C %
N %
Moisture %
Results and next action
Total C:N Ratio:-
Total Carbon:-
Total Nitrogen:-
Dry Matter Used:-
Target:30:1
Mode:Simple
BalancedReady to compost
Symptoms:Warm, earthy, and active pile.
Next actions:Keep moisture like a wrung-out sponge and turn as needed.
Correction solver
Calculate to estimate how much material to add for the target ratio.
Greens, browns, and scientific C:N are related but not identical
Backyard advice often says to use about 2:1 or 3:1 browns-to-greens by volume. That is a useful starting point
because dry leaves, straw, paper, and wood add bulky carbon, while grass clippings, food scraps, coffee grounds,
and many manures add more nitrogen and moisture. The scientific C:N target is different: it is carbon mass divided
by nitrogen mass, usually planned near 25:1 to 30:1 for an active starting mix.
Use rough volume ratios when you are layering a small bin. Use this calculator when the pile smells, stays cool,
contains unusually wet materials, or needs a more specific "how much should I add?" answer.
Compost C:N ratio explained
Compost microbes need carbon for energy and nitrogen for proteins and cell growth. A low ratio means excess nitrogen:
the pile may become wet, slimy, anaerobic, or ammonia-smelling. A high ratio means excess carbon: the pile may stay
cool and break down slowly. Most active starting mixes work best near 25:1 to 30:1, but texture, air, particle size,
and moisture still matter.
In simple mode, the calculator treats your entered material weights as the basis for a quick weighted average. In
advanced mode, it first removes the moisture percentage from each as-used weight so wet grass, food scraps, and manure
can be compared more fairly with dry leaves, straw, cardboard, and sawdust.
Formula
Simple mode computes Ci = mi x ci and
Ni = mi x ni. Advanced mode uses dry matter first:
di = mi x (1 - moisturei/100), then
Ci = di x ci and Ni = di x ni.
The final ratio is total carbon divided by total nitrogen.
The correction solver uses the same carbon and nitrogen contribution for the selected material and solves
(C + xCadd) / (N + xNadd) = target ratio for the added mass x.
Material C:N lookup and presets
Preset C and N percentages are editable planning assumptions. The visible C:N value shows the source value or typical range used to set the preset.
Browns
Autumn leaves30-80:1
Dry leavesabout 54:1
Straw40-100:1
Hay15-32:1
Greens
Grass clippings9-25:1
Vegetable scraps15-20:1
Fruit scrapsabout 40:1
Coffee groundsabout 20:1
Tea leavesabout 20:1
Fresh weeds, no seedabout 19:1
Manures
Cow manure11-30:1
Horse manure22-50:1
Chicken manure3-10:1
Manure with beddingabout 30:1
Paper/Wood
Shredded newspaper398-852:1
Corrugated cardboardabout 563:1
Sawdust200-750:1
Wood chipsabout 560:1
Custom
Lab-tested materialenter your own C, N, moisture
Unknown scrapsstart with a similar preset
Worked examples
Kitchen scraps plus leaves
Inputs: 8 lb food scraps at 40% C and 2.5% N, plus 4 lb dry leaves at 48% C and 0.9% N.
Carbon = 5.12 lb, nitrogen = 0.236 lb, so C:N = 21.7:1. To reach 30:1, add about 9.3 lb dry leaves.
Grass clippings plus dry leaves
Inputs: 10 lb grass clippings at 45% C and 2.65% N, plus 20 lb dry leaves at 48% C and 0.9% N.
Carbon = 14.10 lb, nitrogen = 0.445 lb, so C:N = 31.7:1. To reach 30:1, add about 2.2 lb grass clippings.
Manure plus straw bedding
Inputs: 25 lb cow manure at 39.9% C and 2.1% N, plus 10 lb straw at 42% C and 0.525% N.
Carbon = 14.18 lb, nitrogen = 0.578 lb, so C:N = 24.5:1. To reach 30:1, add about 12.0 lb straw.
Sources and assumptions
Presets are representative values, not lab guarantees. Compost materials vary by species, age, storage, bedding,
rain exposure, and decomposition stage. Adjust C, N, and moisture when you have local or lab data.
Cornell Composting: Compost Chemistry for the 30:1 planning target, high-carbon/high-nitrogen examples, odors, slow decomposition, and finished compost range.
Cornell Composting: C/N Ratio for the mixture formula using mass, carbon percent, nitrogen percent, and moisture percent.
A common starting target is about 25:1 to 30:1. The calculator defaults to 30:1 because it is a practical target for active composting.
Is compost C:N ratio measured by weight or volume?
Scientific C:N is measured by weight: carbon mass divided by nitrogen mass. Volume rules such as 2:1 or 3:1 browns-to-greens are rough backyard shortcuts.
What happens if C:N is too high?
The pile is carbon-rich and nitrogen-limited. It may stay cool, dry, and slow. Add greens such as grass clippings, food scraps, coffee grounds, or manure.
What happens if C:N is too low?
The pile is nitrogen-rich. It may turn wet or slimy and may smell like ammonia. Add dry browns such as leaves, straw, shredded cardboard, or wood chips.
How much brown material should I add to food scraps?
Enter the food scraps, choose dry leaves, straw, cardboard, or another brown as the correction material, and calculate. Wet food scraps often need more dry brown material than a small scoop by volume suggests.
Are coffee grounds green or brown?
Coffee grounds are usually treated as a green because their C:N value is near nitrogen-rich materials, even though they look brown.
Are eggshells green or brown?
Eggshells are mostly mineral calcium carbonate. They can be composted in small crushed amounts, but they do not meaningfully fix a carbon or nitrogen imbalance.
Should I include moisture?
Use advanced moisture-adjusted mode when you weigh wet materials such as grass, food scraps, or manure. Simple mode is fine for quick backyard estimates with mixed, approximate amounts.
What C:N ratio is finished compost?
Finished compost is often around 10:1 to 15:1 because carbon is lost as carbon dioxide while the pile decomposes.
Why does my pile smell?
Common causes are too much nitrogen, too much water, compaction, or not enough oxygen. Add dry browns, mix in bulky material, and turn or loosen the pile.
Are results private?
Yes, everything runs locally in your browser.
How it works
This calculator computes weighted carbon and nitrogen totals, reports the resulting ratio, diagnoses likely symptoms, and solves for a selected correction material. All computation runs client-side for privacy and speed.
Troubleshooting quick guide
Wet, slimy, or ammonia smell
Likely too nitrogen-rich, too wet, or compacted. Add dry leaves, straw, shredded cardboard, sawdust, or wood chips, then mix in air.
Cool and very slow
Likely too carbon-rich, too dry, too small, or lacking fresh nitrogen. Add grass clippings, food scraps, coffee grounds, or manure and check moisture.
Looks balanced but still inactive
Check pile size, moisture, oxygen, and particle size. C:N balance helps, but microbes also need air and water.
Disclaimer
This calculator provides estimates based on typical C and N percentages. Compost inputs vary; adjust with observation and local guidance.
