What is the ideal C:N ratio for compost?

The widely accepted target is 25:1 to 30:1 — meaning 25 to 30 parts carbon for every 1 part nitrogen by dry weight. In this range soil microbes have enough energy (carbon) to digest the nitrogen efficiently, producing heat, CO2 and water rather than ammonia. Below 15:1 the pile goes anaerobic and smells of ammonia; above 40:1 decomposition slows dramatically because microbes run out of nitrogen to build protein.

What are "browns" and "greens" in composting?

Browns are high-carbon, low-moisture materials: straw, dried leaves, cardboard, wood chips, newspaper. Greens are high-nitrogen, higher-moisture materials: fresh grass clippings, vegetable peelings, coffee grounds, and manure. The terms refer to nutrient content, not colour — fresh green grass is a 'green' but so is chicken manure, which is neither green nor fresh-looking.

Why does moisture content matter for the C:N calculation?

C:N ratios in reference tables are given on a dry-weight basis, but you add materials by fresh weight at the compost heap. A bucket of fresh grass (80% moisture) contains only 20% dry matter, whereas the same bucket of dry straw (10% moisture) is 90% dry matter. The calculator converts each fresh weight to its dry-weight equivalent before blending the ratios, so the result reflects what the microbes actually see.

My pile smells of ammonia — what went wrong?

An ammonia smell means the C:N ratio is too low — usually below 15:1. There is more nitrogen than the microbes can use, so the excess escapes as ammonia gas. The fix is to add high-carbon browns: shredded cardboard, dry straw, or crumbled autumn leaves. Add browns in layers, turn the pile, and the smell should clear within a few days.

How long does composting take at the right C:N ratio?

At an optimal 25–30:1 ratio, a hot-composted pile (turned every 3–5 days) can produce finished compost in 4–8 weeks in warm weather. Cold or passive composting at the same ratio takes 3–6 months. Getting the ratio right is the single biggest factor after keeping the pile moist (50–60% moisture, like a wrung-out sponge) and ensuring adequate aeration.

Can I compost wood chips or sawdust?

Yes, but with caution. Sawdust has a C:N ratio of around 325:1 and wood chips around 400:1. Adding them without a proportionate increase in nitrogen sources will stall decomposition for months. If you want to use large quantities of wood-based material, balance each kilogram of sawdust with several kilograms of fresh manure or grass clippings, or pre-compost the wood separately with a nitrogen activator like nettles.

Compost Ratio Calculator

Every compost pile is a living microbial ecosystem, and the carbon-to-nitrogen (C:N) ratio is the single most important number that determines whether that ecosystem runs hot and fast, limps along slowly, or simply smells bad. This calculator lets you enter any combination of ingredients, see your blended C:N ratio in real time, and get a precise correction recommendation — all in your browser, with no data uploaded anywhere.

How it works

Composting microbes need carbon as an energy source and nitrogen to build protein. The ratio of those two elements in the feed material sets the speed and character of decomposition. The standard target is 25:1 to 30:1 by dry weight.

The blended C:N ratio across a mix of ingredients is:

Blended C:N = Total carbon mass ÷ Total nitrogen mass

where total carbon and total nitrogen are each summed across all ingredients. For a single ingredient:

Dry weight = Fresh weight × (1 − moisture fraction)

Nitrogen = Dry weight ÷ (C:N ratio + 1)

Carbon = Dry weight − Nitrogen

The ÷ (C:N + 1) step comes from the definition: if C:N = r, then C = r·N and C + N = dry weight, so N = dry/(r + 1). This is a simplification that treats the organic fraction as purely carbon and nitrogen; in practice the remaining mineral ash fraction is small enough that it does not meaningfully change the ratio.

The calculator uses published dry-weight C:N values from the Cornell Composting Science programme and the NRAES-54 On-Farm Composting Handbook, with typical moisture percentages from the same sources.

Worked example

Suppose you have a typical autumn garden clearance:

Material	Fresh weight	C:N (dry)	Moisture
Fresh grass clippings	5 kg	17:1	80%
Autumn leaves	3 kg	50:1	40%
Vegetable scraps	2 kg	15:1	80%

Step 1 — Convert to dry weight:

Grass: 5 × 0.20 = 1.00 kg dry
Leaves: 3 × 0.60 = 1.80 kg dry
Veg scraps: 2 × 0.20 = 0.40 kg dry

Step 2 — Split each dry weight into C and N:

Grass: N = 1.00/18 = 0.056 kg; C = 0.944 kg
Leaves: N = 1.80/51 = 0.035 kg; C = 1.765 kg
Veg scraps: N = 0.40/16 = 0.025 kg; C = 0.375 kg

Step 3 — Sum and divide:

Total C = 0.944 + 1.765 + 0.375 = 3.08 kg
Total N = 0.056 + 0.035 + 0.025 = 0.116 kg
Blended C:N = 3.08 ÷ 0.116 = ~27:1 — right in the optimal zone.

That mix composts well as-is. Swap the autumn leaves for dry wood chips (C:N ≈ 400:1) and the ratio jumps to around 160:1 — the tool would then recommend adding roughly 18 kg of fresh grass clippings to correct it.

Formula note

All C:N ratios in the material database are dry-weight values. Fresh weights are used for input because that is how most gardeners and farmers measure materials at the heap. The moisture correction is applied internally and shown in the “Show working” panel so you can verify every step. No interpolation or approximation is made beyond the standard organic-fraction model described above.