Carbon-to-Nitrogen Ratio (C:N) Calculator
Written by Thierno Sadou Diallo, formula verified per our methodology • Last checked on 10/10/2026
The carbon-to-nitrogen (C:N) ratio is calculated with mass of carbon ÷ mass of nitrogen, in the same unit. A ratio between 25:1 and 30:1 is generally considered favorable for composting, a balance between energy supply (carbon) and nutrients (nitrogen) needed by decomposer microorganisms.
Explanation
The C:N ratio (carbon over nitrogen) of an organic material is a central indicator in decomposition ecology and composting, since it largely determines how fast microorganisms (bacteria, fungi) can break that material down. Carbon supplies the energy microbial activity needs, while nitrogen is essential for protein synthesis and the growth of decomposer microbial populations: a balance between the two is therefore necessary for efficient decomposition. A material rich in carbon relative to nitrogen (high C:N ratio, typically straw, dead leaves, or wood, ranging roughly from 50:1 to over 200:1 depending on the material) decomposes slowly, since microorganisms lack enough available nitrogen to fuel their population growth rate. Conversely, a material rich in nitrogen (low C:N ratio, typically fresh grass clippings or manure from plants with high photosynthetic activity, often below 15:1) decomposes quickly, but the nitrogen excess that microorganisms can't use may be lost as gaseous ammonia, causing unpleasant odors and wasting nutrients. For effective composting, a C:N ratio generally cited between 25:1 and 30:1 (some sources widening this range to 20:1-40:1) offers a good compromise, typically mixing carbon-rich ("brown," dry) materials with nitrogen-rich ("green," fresh) materials. This ratio naturally decreases over the course of decomposition, with finished compost generally showing a ratio closer to 10:1 to 15:1.
Example: 300 g of carbon for 10 g of nitrogen
Inputs
Mass of carbon: 300 g. Mass of nitrogen: 10 g.
Calculation
C:N ratio = 300 ÷ 10 = 30:1, at the upper end of the range generally favorable for composting.
Result
This 30:1 ratio falls within the range generally considered favorable for effective composting.
Frequently asked questions
Why does a too-high C:N ratio slow down decomposition?
Because decomposer microorganisms need nitrogen to synthesize the proteins necessary for their growth and multiplication: with insufficient nitrogen relative to available carbon, their population grows slowly, which mechanically slows down the entire decomposition of the organic matter, even if energy (carbon) isn't lacking.
Why can a too-low C:N ratio create bad odors?
Because when available nitrogen far exceeds what microorganisms can actually use for growth, the excess turns into gaseous ammonia (NH₃), which escapes into the air and produces the characteristic smell associated with an unbalanced compost, too rich in nitrogen-heavy "green" materials (fresh grass clippings, for example) without enough carbon-rich "brown" materials to balance it.
How do I adjust the C:N ratio of an unbalanced compost?
If the ratio is too low (too much nitrogen, often accompanied by unpleasant odors), adding carbon-rich materials like dry dead leaves, straw, or shredded cardboard rebalances the mix. If the ratio is too high (decomposition visibly very slow), adding nitrogen-rich materials like fresh grass clippings, fruit and vegetable peelings, or manure speeds up the process.