Crude Protein Calculator — Kjeldahl / Dumas Nitrogen Conversion

Formula

How this is calculated

Crude protein is determined indirectly: the sample's total nitrogen is measured (by Kjeldahl digestion or Dumas combustion), and a species-specific factor converts nitrogen mass to protein mass. The principle is that most food proteins contain approximately 16% nitrogen by mass, so dividing 100 by 16 gives the default factor of 6.25 (100 ÷ 16 = 6.25). Actual nitrogen content varies by amino-acid composition, so AOAC Official Method 990.03 and the Jones system define food-specific factors that are more accurate: 5.70 for wheat proteins (which contain more non-protein nitrogen), 6.38 for casein-rich milk proteins, 5.71 for soy, and 5.95 for rice.

To use the calculator, enter the nitrogen percentage measured in your dried sample. If you also enter the sample mass, the calculator shows nitrogen and crude protein in grams alongside the percentage. The non-protein fraction displayed is simply the sample mass minus protein — it bundles moisture, fat, ash and true non-protein nitrogen into one remainder.

The result is "crude" because the factor method cannot distinguish true protein nitrogen from non-protein nitrogen sources such as urea, ammonium salts, amino acids, DNA or purines. In whole foods this overestimate is modest, but in adulterated products (the 2008 melamine scandal is the notorious example) it can be large. More precise methods such as amino-acid hydrolysis quantify true protein independently of the nitrogen route.