How this calculation works
Convert each ingredient to dry matter and water before applying its dry-basis carbon and nitrogen percentages. Sum the element masses rather than averaging ingredient ratios, then compare with your entered range.
C:N = sum(dry mass × dry carbon fraction) / sum(dry mass × dry nitrogen fraction); dry mass = wet mass × (1 − moisture)A worked example
The example materials contain 9 kg dry matter, 6 kg water, 4 kg carbon and 0.14 kg nitrogen. Combined C:N is about 28.571:1 and moisture is 40%; adding 3 kg pure water would reach the entered 50% moisture.
Good to know
Entered mass balance only, not maturity, sanitation, fertilizer or crop advice. No decomposition or nutrient loss. Pure-water adjustment assumes unchanged dry matter and is not a drainage prescription.
Reference: Oregon State Extension: interpreting compost analyses (opens in a new tab) ↗
Read dry-basis analysis values correctly
A dry carbon percentage applies to dry ingredient mass, not the original wet load. First subtract the water fraction, then multiply the remaining dry matter by the carbon and nitrogen fractions.
Organic matter is not automatically carbon. Enter the relevant analysis values rather than treating an organic-matter percentage as the same measurement.
Combine element masses instead of averaging ratios
A small nitrogen-rich ingredient can change the mixture ratio substantially. Averaging two C:N labels ignores the amount of each ingredient and the nitrogen carried by each.
The calculation sums carbon kilograms and nitrogen kilograms independently and divides once. A zero total nitrogen input cannot produce a finite ratio and is rejected.
Water changes moisture without changing dry C:N
Adding pure water changes total wet mass and water mass but does not add carbon or nitrogen in this ideal balance. Removing pure water is the reverse mathematical scenario.
Actual drainage may carry dissolved nutrients, and real composting also loses matter through biological processes. The comparison range is entered by you and does not certify a finished or suitable product.
Use the result with the next measurement
The grain moisture shrink and dry matter loss calculator also separates water from dry matter. Its drying and sale-value assumptions differ from an initial compost ingredient balance.
Entered mass balance only, not maturity, sanitation, fertilizer or crop advice. No decomposition or nutrient loss. Pure-water adjustment assumes unchanged dry matter and is not a drainage prescription.
A few questions, answered
Can I average ingredient C:N ratios?
Not without correctly weighting the underlying element masses.
Are carbon and nitrogen percentages wet basis?
No, they apply to dry matter.
Does adding pure water change dry C:N?
No, not in this mass-balance model.