Crude Protein Calculator
Convert Kjeldahl nitrogen content into crude protein percentage using standard or custom nitrogen-to-protein conversion factors.
🌾 What is Crude Protein?
Crude protein is an estimate of a food or feed sample's total protein content, calculated by multiplying its measured nitrogen percentage by a nitrogen-to-protein conversion factor. It is called "crude" because it counts all the nitrogen in a sample, not just the nitrogen that is actually bound up in true protein molecules, which makes it a fast, indirect estimate rather than a direct measurement of amino acids.
Crude protein is the standard figure used across animal feed labels, agricultural quality reports, dairy testing, brewing and malting analysis, and food nutrition labeling. A feed mill blending livestock rations relies on crude protein numbers to balance a diet's nutrition, a dairy lab reports crude protein to certify milk quality for pricing, a grain elevator tests wheat's crude protein to grade it for milling or export, and food manufacturers use it to satisfy nutrition labeling regulations.
A common misconception is that crude protein and true protein are the same thing. They are not. Because the underlying nitrogen test cannot tell the difference between protein-bound nitrogen and non-protein nitrogen (from free amino acids, nucleic acids, or additives like urea), crude protein is always an approximation, usually a slight overestimate of the true protein content. It is also easy to forget that the conversion factor itself varies by food type, using the wrong factor for a dairy or grain sample introduces a small but systematic error.
This calculator is useful because the underlying nitrogen assay (the Kjeldahl method or a modern combustion nitrogen analyzer) only reports percent nitrogen, not percent protein directly. Converting that number into a usable crude protein percentage, and further into a total mass of protein in a specific sample, requires picking the right conversion factor and doing the arithmetic correctly every time, which this tool automates.
📐 Formula
| Sample type | Conversion factor (F) |
|---|---|
| General (default, most foods and feeds) | 6.25 |
| Dairy / milk products | 6.38 |
| Wheat / grain products | 5.70 |
| Soy products (reference) | 5.71 |
| Rice (reference) | 5.95 |
| Gelatin / collagen (reference) | 5.55 |
📖 How to Use This Calculator
Steps
💡 Example Calculations
Example 1 — General Food Sample
Nitrogen = 2.56%, Factor = General (6.25), Sample = 100 g.
Example 2 — Milk Sample
Nitrogen = 0.53%, Factor = Dairy/milk (6.38), Sample = 100 g.
Example 3 — Wheat Grain Sample
Nitrogen = 1.75%, Factor = Wheat/grain (5.70), Sample = 500 g.
❓ Frequently Asked Questions
🔗 Related Calculators
What is the formula for crude protein from nitrogen content?
Crude protein percent equals percent nitrogen multiplied by a conversion factor, %CP = %N times F. The most common factor is 6.25, based on the assumption that protein averages 16% nitrogen by weight, so 100 divided by 16 equals 6.25.
Why is 6.25 the standard nitrogen to protein conversion factor?
Most proteins in food and feed contain close to 16% nitrogen by mass on average. Dividing 100 by 16 gives 6.25, so multiplying percent nitrogen by 6.25 estimates percent crude protein for a typical, unspecified food or feed sample.
Why does milk use a different conversion factor than 6.25?
Milk protein (mostly casein and whey) contains a slightly lower percentage of nitrogen than the 16% average assumed by the general factor. To correct for this, dairy and milk products use a factor of 6.38 instead of 6.25 when converting Kjeldahl nitrogen to crude protein.
Why does wheat use a factor of 5.70 instead of 6.25?
Wheat and other cereal grains contain nitrogen in forms and proportions that make the general 6.25 factor overestimate true protein. A factor of 5.70 is used for wheat and grain products so the crude protein figure lines up better with their actual amino acid content.
What is the Kjeldahl method?
The Kjeldahl method is a lab technique developed in 1883 that measures the total nitrogen content of a sample by digesting it in acid, converting organic nitrogen into ammonium, then distilling and titrating that ammonium to find percent nitrogen. That percent nitrogen is then multiplied by a conversion factor to estimate crude protein.
Is crude protein the same as true protein?
No. Crude protein is an estimate based on total nitrogen, which includes non-protein nitrogen sources such as free amino acids, nucleic acids, and additives like urea or melamine. True protein specifically measures nitrogen bound in actual protein structures, so crude protein is usually slightly higher than true protein content.
How do you calculate grams of crude protein in a sample?
Multiply the sample mass by the crude protein percentage, then divide by 100. For example, a 100 gram sample with 16% crude protein contains 100 times 16 divided by 100, which equals 16 grams of crude protein.
What nitrogen percentage is typical for common foods?
Nitrogen content varies widely by food type. Meat and most animal-based proteins run around 3.0 to 3.5% nitrogen, cereal grains like wheat run around 1.5 to 2.5% nitrogen, and milk runs around 0.5 to 0.6% nitrogen, each multiplied by its own appropriate conversion factor.
Can I use a custom conversion factor?
Yes. Select Custom factor from the dropdown and enter any nitrogen-to-protein conversion factor published for your specific food, feed, or ingredient category, common published values include 5.55 for gelatin, 5.71 for soy, and 5.95 for rice.
Why do animal feed labels always list crude protein instead of exact amino acids?
Measuring every individual amino acid in a feed sample is expensive and slow. The Kjeldahl nitrogen test and its conversion factor give a fast, inexpensive, and reasonably reliable estimate of overall protein content, which is why crude protein remains the standard figure on animal feed and food labels worldwide.