Compost 7 min read

Compost Calculator: C:N Ratio and Moisture

A pile that heats to 60 °C (140 °F) within days isn’t luck — it’s a well-balanced mix. This calculator works out the carbon-to-nitrogen ratio (C:N) and moisture of your pile from what you actually throw in — in buckets, wheelbarrows or kilograms — and tells you how much of what to add to get hot composting going.

Open this tool in the app →

Why the C:N ratio makes or breaks compost

The microbes that break down organic matter burn carbon for energy and use nitrogen to build protein — roughly 30 parts carbon to 1 part nitrogen. Too much nitrogen (grass alone, chicken manure) and the pile smells of ammonia as nitrogen escapes to the air. Too much carbon (just leaves, sawdust, cardboard) and the microbes starve — the pile sits cold for months. For hot composting, aim for 25–35:1.

For each ingredient we work out its dry mass (mass × (1 − moisture)) and from that the carbon and nitrogen it brings. Pile C:N = total carbon ÷ total nitrogen. It is not an average of the ingredients’ ratios — wet grass and dry leaves contribute very different amounts of dry matter, which is why “averaging” calculators get it wrong. Buckets and wheelbarrows are converted to kilograms using each material’s typical bulk density.

Ingredient	C:N	Water
Grass clippings	19:1	77%
Vegetable peels and scraps	17:1	85%
Coffee grounds	20:1	60%
Chicken manure	7:1	70%
Horse manure with bedding	30:1	70%
Dry leaves	67:1	35%
Straw	77:1	12%
Wood chips	~330:1	35%
Sawdust	~420:1	30%
Cardboard	~560:1	8%

Moisture: like a wrung-out sponge

The second condition is water: 50–60% of the pile’s weight. Below about 40% the microbes all but stop; above 65% water pushes out the air and the pile rots instead of composting. The calculator works out the moisture of the whole mix and tells you how many liters of water to add, or how much dry brown material to mix in.

The squeeze test

Squeeze a handful of material from the pile hard. A few drops at most — perfect. A steady trickle — too wet, add straw or cardboard. If it falls apart dry — water it.

How to build a hot compost pile

Gather enough material for a pile of at least about 1 × 1 × 1 m (3 × 3 × 3 ft) — a smaller pile loses heat faster than it makes it.
Enter the ingredients in the calculator and adjust the mix until C:N and moisture sit in the green zone.
Layer greens and browns or mix them right away, moistening each layer.
After 2–4 days the core should be 55–65 °C (130–150 °F). Turn the pile when it drops below about 45 °C (113 °F) — usually every 3–7 days.
After 6–8 weeks the pile stops heating — let the compost cure for 1–2 months.

Finished compost does the most good where the soil needs it — check with the soil analysis and top up what’s missing with the fertilizer calculator.

Frequently asked questions

At the start of hot composting about 25–35:1, ideally around 30:1. Finished compost is down to about 10–15:1, because part of the carbon leaves as CO₂.

It depends on moisture, but by weight grass needs roughly a third as much dry leaves to reach 30:1 (10 kg grass + 3.5 kg leaves ≈ 30:1). By volume that is about 1.5 buckets of leaves per bucket of grass, but to keep the mix from being too wet you need more like 2–3 buckets. The calculator checks both conditions for your quantities.

An ammonia smell means too much nitrogen (greens). A rotten smell means too wet with too little air. In both cases mixing in dry browns (leaves, straw, cardboard) and turning the pile helps.

Usually too much carbon (not enough greens), a mix that is too dry, or a pile that is too small — under about 0.75 m³ (1 cubic yard) heat escapes faster than it is produced.

Only if the pile really holds 55 °C (130 °F) or more for several days and that material goes into the center. In a cold compost pile it is better to leave them out.