What this calculator does
Crude protein is measured nitrogen multiplied by a conversion factor. At the general-purpose factor of 6.25, a nitrogen content of 2.5% gives 15.63% crude protein.
The word crude is doing real work in that name. The method measures nitrogen, not protein, and assumes all of it comes from protein averaging 16% nitrogen. Non-protein nitrogen from urea, nucleic acids or added melamine counts just the same, which is exactly how the 2008 Chinese milk adulteration scandal passed testing.
The formula
The nitrogen percentage is multiplied by the conversion factor. The factor is the reciprocal of the average nitrogen fraction in the protein of that food: 6.25 assumes 16% nitrogen, while dairy at 6.38 and cereals at 5.4 reflect their different amino acid compositions. Choosing the right factor matters more than most users expect.
| Term | Meaning |
|---|---|
| Crude protein | Nitrogen times a factor. An estimate, not a direct measurement. |
| Kjeldahl method | The classic nitrogen determination behind most crude protein figures. |
| Conversion factor | The reciprocal of the protein nitrogen fraction for that food type. |
| Non-protein nitrogen | Nitrogen from other sources, which the method cannot distinguish. |
The inputs explained
| Field | What to enter |
|---|---|
| Nitrogen content (%) | Nitrogen content as a percentage, typically from a Kjeldahl or Dumas determination. |
| Conversion factor | Conversion factor. Use the one matching the food type rather than the general 6.25 where a specific value exists. |
When to use it
Feed and food labelling
Protein content on animal feed and many food labels is a crude protein figure derived this way.
Comparing ingredient quality
Crude protein is the standard specification for feed ingredients, though it says nothing about amino acid balance.
Agronomy
Grain protein content, which affects price and end use, is measured as crude protein.
Worked examples
Every figure in the tables below is produced by this page’s own calculator at build time, so the numbers and the tool always agree. Select any row to load that scenario.
How much does the factor change the answer?
The same nitrogen measurement with different conversion factors.
| Conversion factor | Crude protein | Nitrogen used | Factor used |
|---|---|---|---|
| 5.4 (cereals) | 13.50 % | 2.50 % | 5.4 |
| 5.9 (fish and meat) | 14.75 % | 2.50 % | 5.9 |
| 6.25 (general) | 15.63 % | 2.50 % | 6.25 |
| 6.38 (dairy) | 15.95 % | 2.50 % | 6.38 |
Questions
Why is it called crude protein?
Because it is an estimate from nitrogen rather than a measurement of protein. Any nitrogen in the sample counts, whether or not it came from protein. The word crude is a warning about the method, not about the quality of the food.
Why 6.25?
Because protein averages about 16% nitrogen, and 100 divided by 16 is 6.25. It is a general-purpose average. Real proteins vary, which is why dairy uses 6.38 and cereals 5.4, reflecting their particular amino acid compositions.
Can crude protein be manipulated?
Yes, and it has been. Adding a nitrogen-rich non-protein compound raises the measured figure without adding any protein. The 2008 melamine adulteration of Chinese milk products exploited exactly this, and it caused serious harm before being detected.
Does crude protein indicate protein quality?
No. It gives a quantity, not a quality. Two feeds with identical crude protein can differ greatly in amino acid balance and digestibility, which is what actually determines nutritional value. Amino acid analysis is needed for that.
For nitrogen in a different context, see the combustion analysis calculator. For converting mass to moles, see the grams to moles calculator.