Crude Protein Calculator
Estimate crude protein content from nitrogen percentage using the Kjeldahl method. Select the appropriate conversion factor for your sample type.
Percentage of nitrogen in the sample (from Kjeldahl analysis)
Conversion factor (protein = N × factor). Default 6.25 = 100/16 (16% N in protein).
Enter custom factor only if "Custom factor" selected above
What is crude protein?
Crude protein is an estimate of total protein content in a food or feed sample, calculated as Nitrogen % × Conversion Factor. It's called "crude" because it measures total nitrogen (including non-protein nitrogen) rather than true protein. The Kjeldahl or Dumas method measures nitrogen, which is then converted to protein.
Why is the conversion factor 6.25?
Most proteins contain about 16% nitrogen by weight (100 ÷ 16 = 6.25). This means 1% nitrogen ≈ 6.25% protein. However, different foods have different conversion factors because their proteins have different nitrogen contents: dairy uses 6.38 (15.65% N), wheat uses 5.70 (17.5% N).
What is the Kjeldahl method?
The Kjeldahl method digests the sample in sulfuric acid, converting organic nitrogen to ammonium sulfate. Then ammonia is distilled into boric acid and titrated. The nitrogen content is calculated from the titrant volume. It's the standard method for crude protein determination in foods and feeds.
What's the difference between crude protein and true protein?
Crude protein counts all nitrogen (including non-protein nitrogen like nitrates, amino acids, nucleic acids). True protein measures only nitrogen in actual proteins (typically by precipitating proteins first, then measuring nitrogen in the precipitate). Crude protein is always equal to or higher than true protein.