CompoCalc - C:N Calculator

Use CompoCalc to estimate the Carbon:Nitrogen (C:N) ratio of your ingredients before you build a batch. Aim near 30:1 for general compost, then adjust based on how your bin looks, smells, and heats up.

Every compost ingredient contains some Carbon (C) and some Nitrogen (N). The C:N ratio describes how much carbon there is compared to nitrogen.

High C:N (drier, woody, lots of browns) → slower to break down.
Low C:N (lots of greens/manures) → can heat up fast, go smelly, or get slimy.
A balanced starting point for most backyard compost is around 30:1.

CompoCalc helps you estimate this balance before you build or adjust the pile.

Carbon “Browns” (raise C:N, dry the mix):

Dry leaves, shredded paper/cardboard, straw, sawdust, wood shavings.
OLD/DRY plant stems, corn stalks, dried grass, wood chips.

Nitrogen “Greens” (lower C:N, add moisture and activity):

Fresh grass clippings, food scraps, coffee grounds, tea, manures.
Green weeds or trimmings, spent garden plants that are still moist.

When your mix is too dry and woody, add more greens. When it is slimy, smelly, or heavy, add more absorbent browns.

Select a material from the list or type your own.
Check / tweak the C:N value if you have better data for your material.
Enter the amount (by weight) and choose the unit.
Click + Add Material to add it to the mix.

Behind the scenes, CompoCalc converts each material into estimated total carbon and total nitrogen, then shows you:

The combined Estimated C:N ratio for the entire mix.
A gauge that indicates if the mix is very high, balanced, or very low.

Use this estimate as a guide — it is not a lab test, but it is extremely helpful for staying in a “good range” before you load a bin or pile.

CompoCalc focuses on proportion, not exact batch size. A few tips:

Think in parts or approximate weight, not perfection.
Stay consistent with units (all pounds, all kilograms, etc.).
Use rough bucket estimates if you don't have a scale (for example, “1 bucket = 1 part”).
Once the ratio looks good on-screen, you can scale the whole recipe up or down to match the size of your bin or pile.

If the C:N is too high (very carbon-heavy, > ~45:1):

Add more greens: food scraps, fresh grass, manures, coffee grounds.
Mist some water in as you go if the mix is very dry.

If the C:N is too low (very nitrogen-heavy, < ~20–25:1):

Add absorbent browns: dry leaves, shredded cardboard, straw.
Fluff and mix in more structure to prevent a soggy, airless mat.

After each tweak, add the new materials in CompoCalc and watch the ratio move toward your target range.

Smelly like ammonia or rotten eggs

Likely too many greens and not enough air.
Add generous dry browns, mix and fluff, avoid sealing it airtight.

Very dry and not breaking down

Often too many browns and not enough moisture.
Add moist greens and a light watering, then mix.

Hot pile that feels extreme

High nitrogen can drive temperatures well over 140°F (60°C).
Turn the pile, break up clumps, and balance with extra browns if needed.

For worm bins (vermicomposting), you generally want:

A moderate C:N ratio (not extremely high or low).
Materials that are pre-cooled and only gently warm or room temperature.
No strong ammonia smell and no visible chemical residues.

If you pre-compost a hot mix, let it cool below about 80°F / 27°C before introducing large numbers of worms. Start with a small test corner if you're unsure.

C:N means Carbon to Nitrogen. It compares how much carbon is available for every “part” of nitrogen.

A common target for hot compost is around 30:1. Vermicompost often does well a bit above or below that, but this range balances activity and stability.

High C:N (very brown) → slow, cool, often dry.
Low C:N (very green) → odors, wet or heavy material.

Tip: Use CompoCalc as a “preview” before mixing a large batch. Adjust amounts until the C:N lands where you want it, then build the real mix.

Carbon materials raise the C:N ratio and usually make the mix lighter and more fibrous.

Examples of good “browns”:

Dried leaves, straw, and hay.
Cardboard and paper (shredded works best).
Coco coir, peat, and other bedding materials.
Wood shavings and sawdust (use in moderation).

Use more browns when: the C:N is low, the bin smells strong, or the material feels wet and compacted.

Nitrogen materials lower the C:N ratio and fuel microbial growth.

Examples of good “greens”:

Fruit and vegetable scraps.
Fresh grass clippings.
Coffee grounds.
Aged manures and fresh plant trimmings.

Use more greens when: C:N is very high, the pile feels dry, or decomposition is very slow.

Use CompoCalc to experiment before you build a batch in real life:

C:N much higher than target → add nitrogen materials or reduce some carbon inputs.
C:N much lower than target → add more dry carbon materials such as shredded cardboard or bedding.
Very wet or dense mix → add fluffy, absorbent browns and mix gently for airflow.

Tip: Change one ingredient at a time, watch how the number moves, then copy those changes into your actual recipe.

Some quick patterns:

Strong ammonia smell → often too much nitrogen; add dry carbon and mix lightly.
Very slow breakdown → often high C:N or too dry; add some nitrogen and moisture.
Soggy or compacted zones → break up clumps, add fluffy browns, and mix lightly.

Trust your senses: the C:N number is a guide, but your nose, eyes, and hands will tell you when the mix is improving.