What is the actual amount of phosphorous in a fertilizer with a npk ratio of 15-15-20?

NPK ratio system is a conventional shorthand for the ratio of the nitrogen N), phosphorus (P) and potassium (K) in a fertilizer. It actually shows amount of P ₂O₅.

If we use 100 kg of P₂O₅:

m (P₂O₅) = 0,15 · 100 kg.

m (P₂O₅) = 15 kg.

m (P₂O₅) : M (P₂O₅) = m (P) : 2M (P).

15 kg : 142 = m (P) : 62.

m (P) = 6,55 kg.

ω (P) = 6,55 kg : 100 kg · 100% = 6,55%.