GIFSQ Tools · Module 1 — Food Composition & Proximate Analysis · Tool 4 of 500
Protein is nitrogen times a factor. Enter the nitrogen % from your analysis, pick the Jones factor for the food, and the protein follows. Got a titration instead? Use Tool 5, Kjeldahl Nitrogen, first.
Try sample values — fills the form with the worked example below.
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Crude protein (%)
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Nitrogen (%)
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Protein %, dry basis
Equation: Crude protein % = Nitrogen % × Jones factor
From nitrogen weight: Nitrogen % = nitrogen (mg) ÷ (sample (g) × 10).
Dry basis: Protein % (dry) = Protein % ÷ (dry matter % ÷ 100).
Method: Jones factors applied to Kjeldahl nitrogen (AOAC 990.03 / 984.13) or Dumas nitrogen (AOAC 992.15). The factor is the reciprocal of the food's average nitrogen content: wheat protein is about 17.5 % nitrogen, so 100 ÷ 17.5 = 5.70.
Assumptions: all nitrogen in the sample is protein nitrogen; the chosen factor matches the food's amino acid profile.
Limitations: crude protein overstates true protein whenever non-protein nitrogen is present — urea, nitrates, free amino acids and ammonia all count as protein here (milk with added urea, vegetables with nitrate); the factor is an average for the food group, not exact for one sample; true protein needs amino acid analysis. Never mix factors: the same nitrogen gives 9.86 % protein with wheat's 5.70 and 10.81 % with the general 6.25.
Rounding: results to 2 decimal places; intermediate values are not rounded.
This is a calculation aid, not a laboratory report — confirm against your accredited method before citing a value.
From nitrogen weight: Nitrogen % = nitrogen (mg) ÷ (sample (g) × 10).
Dry basis: Protein % (dry) = Protein % ÷ (dry matter % ÷ 100).
Method: Jones factors applied to Kjeldahl nitrogen (AOAC 990.03 / 984.13) or Dumas nitrogen (AOAC 992.15). The factor is the reciprocal of the food's average nitrogen content: wheat protein is about 17.5 % nitrogen, so 100 ÷ 17.5 = 5.70.
Assumptions: all nitrogen in the sample is protein nitrogen; the chosen factor matches the food's amino acid profile.
Limitations: crude protein overstates true protein whenever non-protein nitrogen is present — urea, nitrates, free amino acids and ammonia all count as protein here (milk with added urea, vegetables with nitrate); the factor is an average for the food group, not exact for one sample; true protein needs amino acid analysis. Never mix factors: the same nitrogen gives 9.86 % protein with wheat's 5.70 and 10.81 % with the general 6.25.
Rounding: results to 2 decimal places; intermediate values are not rounded.
This is a calculation aid, not a laboratory report — confirm against your accredited method before citing a value.
Worked example: wheat flour, nitrogen 1.730 %, Jones factor 5.70. Crude protein = 1.730 × 5.70 = 9.86 %. With dry matter 87.00 %, protein on a dry basis = 9.86 ÷ 0.87 = 11.33 %. The same nitrogen as a weight: 17.299 mg in a 1.000 g sample = 17.299 ÷ 10 = 1.73 % nitrogen.
Enter values above to see results.
Typical crude protein: wheat flour 10–12 %, milk powder 26–34 %, soy flour 35–40 %, beef 20–26 %. Always state the factor with the number — 9.86 % (wheat, 5.70) and 10.81 % (general, 6.25) are the same nitrogen, different answers. Tool 4 of 500 — GIFSQ
