Crude Protein Calculator (nitrogen × 6.25 or Jones factor, Kjeldahl titration)

Understanding the crude protein calculator — concept and principle

Protein content in food and feed is usually estimated from nitrogen. This calculator converts a nitrogen result, or a Kjeldahl titration, into crude protein using a general or food-specific conversion factor, and gives the result on a dry-matter basis too.

Protein is estimated from nitrogen, so the factor and the moisture basis both change the number.

How it is calculated

Crude protein = N (%) × factor.
General factor 6.25, because proteins average about 16% nitrogen (1 ÷ 0.16 = 6.25). Food-specific (Jones) factors include 6.38 for milk, 5.83 for whole wheat and 5.71 for soybean.
Kjeldahl: N (%) = (Vsample − Vblank) mL × normality × 1.4007 ÷ sample g.
Dry-matter basis = as-is result ÷ dry-matter fraction.

The principle

Nitrogen is easy to measure accurately, while protein is not. Because amino acids contain nitrogen in fairly consistent proportions, total nitrogen times a factor approximates protein. Different foods have different amino-acid compositions, so Jones proposed specific factors. The result is called "crude" because non-protein nitrogen, such as urea, nitrate or free amino acids, is counted too.

Worked example

Default inputs: nitrogen 2.10%, whole wheat (factor 5.83), dry matter 88%.

Crude protein = 2.10 × 5.83 ≈ 12.24% of the sample.
Dry-matter basis = 12.24 ÷ 0.88 ≈ 13.91%.
With the general factor: 2.10 × 6.25 ≈ 13.13%.
Conversion factor used = 5.83; nitrogen = 2.1%.

Each point is the calculator re-run with only “Nitrogen (% of sample)” changed; the gold point is the default example. Change the inputs above and this chart follows your numbers.

Nitrogen (% of sample)	Crude protein (% of sample)
0.525	3.06
1.05	6.12
1.57	9.18
2.1	12.24
2.62	15.3
3.15	18.36
4.2	24.49

How to read the result

Always state which factor and basis you used: the same wheat is 12.2% protein with 5.83 but 13.1% with 6.25, and higher again on a dry basis. Feed formulation usually uses dry-matter values; food labels may use regulated factors. The difference between factors matters for trade and labelling.

Common mistakes

Mixing as-is and dry-matter results.
Using 6.25 where a regulation requires a specific factor, or the reverse.
Forgetting to subtract the blank in a Kjeldahl titration.
Assuming all nitrogen comes from protein.

Keep in mind: Official protein analysis must follow the applicable standard method (for example AOAC) and the factors set by your regulator.

Frequently asked questions

Why multiply nitrogen by 6.25?

Proteins contain on average about 16% nitrogen, and 100 ÷ 16 = 6.25.

What factor is used for wheat?

Jones's factor for whole wheat is 5.83; some standards use 5.7 for wheat flour.

What is the difference between crude and true protein?

Crude protein includes non-protein nitrogen; true protein counts only nitrogen in protein.

Sources: Jones, D. B. (1941). Factors for converting percentages of nitrogen in foods and feeds into percentages of proteins. USDA Circular No. 183 · FAO (2003). Food energy — methods of analysis and conversion factors. FAO Food and Nutrition Paper 77

Checked 9 Nov 2026. FAO (2003) Food energy – methods of analysis and conversion factors, FAO Food and Nutrition Paper 77, Chapter 2 “Methods of food analysis” (16% N assumption, general factor 6.25, Jones nitrogen-to-protein factors); Kjeldahl nitrogen from titration (standard formula, N = 14.007 g/mol).

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