NPK rating is a rating system describing the amount of nitrogen, phosphorus, and potassium in a fertilizer. NPK ratings consist of three numbers separated by dashes (e.g., 10-10-10 or 16-4-8) describing the chemical content of fertilizers. The first number represents the percentage of nitrogen in the product; the second number, P2O5; the third, K2O. Fertilizers do not actually contain P2O5 or K2O, but the system is a conventional shorthand for the amount of the phosphorus (P) or potassium (K) in a fertilizer. A bag of fertilizer labeled 16-4-8 contains of nitrogen (16% of the 50 pounds), an amount of phosphorus equivalent to that in 2 pounds of P2O5 (4% of 50 pounds), and 4 pounds of K2O (8% of 50 pounds). Most fertilizers are labeled according to this N-P-K convention, although Australian convention, following an N-P-K-S system, adds a fourth number for sulfur, and uses elemental values for all values including P and K.