Compost C:N Ratio Calculator
Compost C:N ratio calculator: mix kitchen scraps, dry leaves, grass, manure and more and see your carbon-to-nitrogen ratio against the 25–30:1 target.
How to use
- Choose whether you measure in kg or in buckets (set your bucket size, filled loosely).
- For each ingredient, pick the material and enter the amount. You can use up to five.
- Read the blended C:N ratio and the advice under it.
- If the advice says "add browns" or "add greens", adjust the amounts until the ratio is between 25:1 and 30:1.
How it's calculated
Compost microbes need carbon (C) for energy and nitrogen (N) to build their bodies. The ratio of the two in your pile decides how fast and how cleanly it breaks down.
You cannot just average the ratios of the ingredients. Wet kitchen scraps are mostly water, while dry leaves are mostly dry matter, so a kilo of each contributes very different amounts of carbon and nitrogen. The calculator works out the real mass of C and N in each ingredient:
- Dry matter = weight × (1 − moisture %)
- N = dry matter × N % of dry matter
- C = N × the material's typical C:N ratio
- Blended C:N = total C ÷ total N
For buckets, the volume is first changed to kg using a typical loose bulk density for each material. Moisture of the blend is total water ÷ total weight.
The material values are typical approximations taken from common composting references, for example kitchen scraps around 15:1, grass clippings around 20:1, dry leaves around 60:1, rice straw around 60:1, sawdust and cardboard several hundred to one. Your own materials will differ.
Worked example
1 kg of dry leaves + 1 kg of kitchen scraps:
- Dry leaves: 1 × 0.85 dry = 0.85 kg dry matter → N = 0.85 × 0.8% = 6.8 g → C = 6.8 × 60 = 408 g
- Kitchen scraps: 1 × 0.20 dry = 0.20 kg dry matter → N = 0.20 × 2.0% = 4 g → C = 4 × 15 = 60 g
- Blend: 468 g C ÷ 10.8 g N ≈ 43:1
A simple average of 60 and 15 would say 37.5:1, which is wrong. At 43:1 the pile is too brown, and the calculator suggests adding about 2.4 kg more kitchen scraps to reach 30:1.
The default mix in the calculator (3 kg kitchen scraps, 1 kg dry leaves, 0.5 kg coffee grounds) comes out at about 29:1 with around 63% moisture — a good balance.
Tips for Malaysian home compost
- Greens (nitrogen): vegetable scraps, fruit peels, grass clippings, coffee grounds, okara, chicken or goat manure.
- Browns (carbon): dry leaves, rice straw, rice husk, shredded cardboard, coconut husk, untreated sawdust.
- In our humid weather, piles usually end up too wet rather than too dry. Keep a sack of dry leaves or shredded cardboard next to the bin and add a handful with every load of scraps.
- Aim for the feel of a wrung-out sponge: damp, not dripping.
- Chop big pieces. Coconut husk and sawdust break down very slowly, so use them in small amounts.
- Turn the pile every week or two to bring in air and keep it from smelling.
- Don't add meat, fish, oily food or dairy to an open home heap — they attract pests.
FAQ
Why is 25–30:1 the target? It is the range most composting guides recommend for an active pile. Much lower and nitrogen escapes as ammonia smell; much higher and the pile sits for months.
My compost smells bad. What should I add? A bad smell usually means too many greens or too much water. Add browns such as dry leaves or cardboard and turn the pile.
Can I use buckets instead of weighing? Yes. Choose buckets and set the bucket size. Light materials like dry leaves weigh very little per bucket, which the calculator accounts for.
Are the C:N values exact? No. They are typical values; real materials vary with plant type, age and how wet they are. Use the result as a guide and watch how your pile behaves.