NPK rating is used to label fertilizer based on the relative content of the chemicals nitrogen

Nitrogen

Nitrogen is a chemical element that has the symbol N, atomic number of 7 and atomic mass 14.00674 u. Elemental nitrogen is a colorless, odorless, tasteless, and mostly inert diatomic gas at standard conditions, constituting 78.08% by volume of Earth's atmosphere...

(N), phosphorus

Phosphorus

Phosphorus is the chemical element that has the symbol P and atomic number 15. A multivalent nonmetal of the nitrogen group, phosphorus as a mineral is almost always present in its maximally oxidized state, as inorganic phosphate rocks...

(P), and potassium

Potassium

Potassium is the chemical element with the symbol K and atomic number 19. Elemental potassium is a soft silvery-white alkali metal that oxidizes rapidly in air and is very reactive with water, generating sufficient heat to ignite the hydrogen emitted in the reaction.Potassium and sodium are...

(K) that are commonly used in fertilizers.

Unlike the N number, the numbers for P and K do not reflect the amount of elemental phosphorus and potassium in the fertilizer. Rather they represent the amount of oxide in the form of P₂O₅ and K₂O that would be present in the fertilizer if all the elemental phosphorus and potassium were oxidized into these forms.

Method for converting an N-P-K value to an actual composition

The factors for converting from P₂O₅ and K₂O values to their respective P and K elemental values are as follows:

• P₂O₅ consists of 56.4% oxygen and 43.6% elemental phosphorus. The percentage (mass fraction
  Mass fraction
  In aerospace engineering, the propellant mass fraction is a measure of a vehicle's performance, determined as the portion of the vehicle's mass which does not reach the destination...
  
  ) of elemental phosphorus is 43.6% so P = 0.436 x P₂O₅

• K₂O consists of 17% oxygen and 83% elemental potassium. The percentage (mass fraction) of elemental potassium is 83% so K = 0.83 x K₂O

• Nitrogen values represent actual nitrogen content so these numbers do not need to be converted.

Using these conversion factors we can determine that an 18−51−20 fertilizer contains by weight:

• 18% elemental (N)
• 22% elemental (P), and
• 16% elemental (K)

In the U.K., fertilizer labeling regulations allow for reporting the elemental mass fractions of phosphorus and potassium. The regulations stipulate that this should be done in parentheses after the standard N-P-K values.

NPK values for commercial fertilizers

NPK values for various synthetic fertilizers include:

• 82-00-00 Anhydrous Ammonia
• 21-00-00 Ammonium Sulfate
• 46-00-00 Urea
• 35-00-00 Ureaform (~85% slow release, sparingly soluble ureaformaldehyde)
• 40-00-00 Methylene Ureas (~70% slow release)
• 31-00-00 Isobutylidene Diurea (~90% slow release)
• 30-00-00 to 40-00-00 Sulfur-coated Urea (slow release)
• 33-00-00 Ammonium Nitrate
• 15-00-00 Calcium Nitrate
• 13-00-44 Potassium Nitrate
• 00-17-00 to 00-22-00 Superphosphate (Monocalcium phosphate monohydrate with gypsum)
• 00-44-00 to 00-52-00 Triple superphosphate (Monocalcium phosphate monohydrate)
• 10-34-00 to 11-37-00 Ammonium Polyphosphate
• 11-48-00 to 11-55-00 Monoammonium Phosphate
• 18-46-00 to 21-54-00 Diammonium Phosphate

NPK Values for mined fertilizer minerals

• 00-3-00 to 00-8-00 Raw Phosphate Rock (would be 00-34-00 if it was soluble)
• 00-00-60 Potassium Chloride
• 11-08-02 to 16-12-03 bird guano
  Guano
  Guano is the excrement of seabirds, cave dwelling bats, and seals. Guano manure is an effective fertilizer due to its high levels of phosphorus and nitrogen and also its lack of odor. It was an important source of nitrates for gunpowder...
  
  

NPK values for biosolids fertilizers and others

• 04-12-00 Bone meal
  Bone meal
  Bone meal is a mixture of crushed and coarsely ground bones that is used as an organic fertilizer for plants and formerly in animal feed. As a slow-release fertilizer, bone meal is primarily used as a source of phosphorus....
  
  
• 06-02-00 Milorganite
  Milorganite
  Milorganite is the trademark of a biosolids fertilizer produced by the Milwaukee Metropolitan Sewerage District. The name Milorganite is a contraction of the phrase Milwaukee Organic Nitrogen, and was the result of a 1925 naming contest held in National Fertilizer Magazine...
  
  
• 03-02-02 poultry manure
• 01-00-00 dairy manure
• 01-00-01 horse manure