The C and N contents of feedstocks is generally known from lookup tables listing common types of feedstock. It is important to deduct the moisture content if the listed value is for dry material.
For foodstuffs with a nutrition analysis, the N content may be estimated from the protein content as , reversing the crude protein calculation. The C content may be estimated from crude ash content (often reported in animal feed) or from reported macronutrient levels as .
Given the C:N ratio and one of C and N contents, the other content may be calculated using the very definition of the ratio. When only the ratio is known, one must estimate the total C+N% or one of the contents to get both values.