Compost C:N Ratio & Moisture Calculator
Enter the wet weight, moisture, carbon, and nitrogen values for two compost ingredients. The calculator solves the balancing ingredient by dry-matter C:N ratio, then checks whether the resulting batch is dry enough or too wet for the moisture target.
Balancing material required (lb)3.47
- Dry-matter ratio
- 30.0:1
- Moisture before correction
- 66.17%
- Water to add (lb)
- 0
- Excess water above target (lb)
- 3.34
- Final batch weight (lb)
- 13.47
- Recipe status
- C:N target is reachable. The two-ingredient mix is wetter than the moisture target; adding water cannot correct it.
Dry-matter mass balance
| Measure | Starting material | Balancing material | Total before water |
|---|---|---|---|
| Wet weight | 10 | 3.47 | 13.47 |
| Dry matter | 2.3 | 2.26 | 4.56 |
| Water | 7.7 | 1.22 | 8.92 |
| Carbon | 1.04 | 1.13 | 2.16 |
| Nitrogen | 0.06 | 0.02 | 0.07 |
Weights are reported in lb; water mass uses the same unit as the ingredient weights.
Carbon and nitrogen are balanced on dry matter, while moisture is balanced on wet weight.
How to use this calculator
- Choose pounds or kilograms and enter the wet weight of the material you already have.
- Enter moisture, carbon, and nitrogen values for that starting material.
- Enter the same properties for the material you could add as the balancing ingredient.
- Set the target C:N ratio and moisture, then read the required ingredient weight and water correction.
How the compost recipe is calculated
This calculator follows the dry-matter mass balance used in Cornell composting examples. Moisture is handled first as a fraction of wet weight, so a 10 lb pile of grass clippings at 77% moisture contains 2.3 lb of dry matter and 7.7 lb of water. Carbon and nitrogen are then calculated from the dry matter, not from the wet weight.
For the starting material, carbon is Bc = Qb x (1 - Mb) x Cb and nitrogen is Bn = Qb x (1 - Mb) x Nb. For each unit of balancing material, carbon is Ac = (1 - Ma) x Ca and nitrogen is An = (1 - Ma) x Na. To hit a target ratio R, the required wet weight of balancing material is:
x = (R x Bn - Bc) / (Ac - R x An)
After that C:N solve, the calculator combines the dry matter and water from both ingredients. Moisture before correction is existing water divided by combined wet weight. For a target moisture fraction G, the water addition is y = G x D / (1 - G) - H, where D is total dry matter and H is existing water. If y is negative, the mix is already wetter than the target; the calculator reports that amount as excess water instead of pretending water can fix it.
What moves the answer most
Moisture has a large effect because it changes both the dry matter available for carbon and nitrogen and the water already in the batch. A wet green ingredient can require much less weight than it appears by volume because most of its weight is water. The carbon and nitrogen percentages matter just as much: two materials with similar C:N ratios cannot pull the batch very far toward a different target, no matter how much is added.
What the calculator leaves out
The result balances total carbon, total nitrogen, and water mass. It does not model how available that carbon is to microbes, how particle size affects decomposition, or whether the recipe has enough coarse structure for airflow. A mathematically balanced pile can still need turning, a drier amendment, or a bulky ingredient to keep it aerobic.
Worked example
Suppose you have 10 lb of fresh grass clippings at 77% moisture, 45% carbon on dry matter, and 2.4% nitrogen on dry matter. You plan to add dry leaves at 35% moisture, 50% carbon, and 0.75% nitrogen, with a target of 30:1 C:N and 55% moisture.
The grass contributes 1.035 lb of carbon and 0.0552 lb of nitrogen. Each wet pound of leaves contributes 0.325 lb of carbon and 0.004875 lb of nitrogen. The C:N equation gives 3.47 lb of leaves. The combined mix is 66.17% moisture, so it is already wetter than the 55% target by about 3.34 lb of water. The final batch weight stays 13.47 lb unless you add drier material or let water evaporate.
Common questions
What is a good starting C:N ratio for compost?
About 30:1 is a common starting target for a hot compost mix. It is not a magic value, because particle size, aeration, and carbon availability also affect how the pile behaves.
Why measure browns and greens by weight instead of bucket volume?
The chemistry depends on dry matter, carbon, nitrogen, and water mass. A bucket of leaves and a bucket of grass can have very different weight, moisture, and bulk density, so volume alone does not describe the recipe.
How do I find carbon, nitrogen, and moisture values for my materials?
Use laboratory values when the recipe matters, especially for farm or institutional composting. Published tables can be useful for planning, but plant species, age, storage, and weather can move real values away from the average.
Why can a recipe hit the C:N target but miss the moisture target?
The C:N equation only decides how much of the second ingredient is needed for carbon and nitrogen. Once that amount is fixed, the water already present may leave the mix dry, near target, or too wet.
Can I use this for grass clippings and autumn leaves?
Yes, if you enter wet-basis moisture and dry-matter carbon and nitrogen values for both materials. The default values mirror Cornell-style grass clipping and dry leaf examples, but your actual materials may differ.
What should I do when the calculated mixture is too wet?
Adding water cannot correct a wet recipe. Let the material drain or evaporate, turn the pile for aeration, or add another dry, carbon-rich ingredient and recalculate with the new blend.