Here are a few tips if you plan to calculate the C:N ratio of a mixture. First, you need more than just the C:N ratio of the starting materials. You cannot calculate the %N or %C from just the C:N ratio. If you have 100 lbs of material with a C:N ratio of 30, you do not know how much C or N is present. There could be 30% C and 1% N, or 60% C and 2% N, or 15% C and 0.5% N. Some tables will give the %N also and this is essential. You will likely have to calculate the %C but it is in some tables. There are three basic formulas to keep in mind. The C:N ratio = the %C / %N. The %N = the %C / C:N. The %C = C:N / %N.