Last updated: July 3, 2026
Crude Protein Calculator
Creators
Dharmendra SinghReviewers
Creators
Dharmendra SinghReviewers
Quick Answer
The crude protein calculator estimates protein percentage from nitrogen content using crude protein (%) = nitrogen (%) × conversion factor. The default factor is 6.25, based on the assumption that protein is 16% nitrogen; common alternatives include 6.38 for dairy and 5.70 for wheat.
Crude protein percent is calculated by multiplying nitrogen percent by a conversion factor. For example, two percent nitrogen times the standard factor six point two five equals twelve point five percent crude protein.
Key Takeaways
- Crude protein percent equals nitrogen percent multiplied by a conversion factor.
- The standard factor 6.25 assumes protein contains 16% nitrogen.
- Use matrix-specific factors when required, such as 6.38 for dairy and 5.70 for wheat.
- Crude protein includes non-protein nitrogen, so it is an estimate rather than a direct true-protein measurement.
- Always report the conversion factor and moisture basis with crude protein results.
Creators
Dharmendra SinghReviewers
Creators
Dharmendra SinghReviewers
Formula
crude protein (%) = nitrogen (%) × conversion factor
Where:
- N=Nitrogen content measured in the sample(%)
- F=Nitrogen-to-protein conversion factor(dimensionless)
- Crude protein=Estimated protein content by mass(%)
Worked Examples
Standard food or feed sample
A sample containing 2.0% nitrogen is converted with the standard 6.25 factor.
- 1Start with measured nitrogen: N = 2.0%.
- 2Use the standard conversion factor: F = 6.25.
- 3Multiply nitrogen by the factor: 2.0 × 6.25 = 12.5.
- 4The crude protein estimate is 12.5% by mass.
Higher-nitrogen sample with standard factor
A sample containing 3.5% nitrogen uses the same general factor of 6.25.
- 1Record the nitrogen content: N = 3.5%.
- 2Select the general protein factor: F = 6.25.
- 3Calculate crude protein: 3.5 × 6.25 = 21.875.
- 4The crude protein estimate is 21.875%.
Dairy sample with 6.38 factor
Milk and dairy proteins are commonly converted with factor 6.38 rather than 6.25.
- 1Start with measured nitrogen: N = 0.56%.
- 2Use the dairy factor: F = 6.38.
- 3Multiply: 0.56 × 6.38 = 3.5728.
- 4Rounded to three decimals, crude protein is 3.573%.
Introduction
Crude protein is an estimate of total protein based on how much nitrogen a sample contains. Classical Kjeldahl analysis measures nitrogen, then multiplies it by a nitrogen-to-protein conversion factor because proteins contain nitrogen in their amino-acid backbones. This calculator applies crude protein (%) = nitrogen (%) × factor and is useful for food labels, feed analysis, grain quality checks, and dairy testing. If you also need elemental mass checks, compare with the percent composition calculator or convert sample amounts with the mole calculator. Authoritative background is available from AOAC International and the FAO food energy report.
What is crude protein?
Crude protein is not a direct count of intact protein molecules. It is a nitrogen-based estimate: the lab measures total nitrogen and assumes a typical fraction of nitrogen in proteins. The word crude signals that the method can include non-protein nitrogen such as urea, nitrates, or free amino acids if they are present in the sample.
It is reported as percent by mass of the sample.
It depends strongly on the conversion factor chosen.
It is widely used in feed, grain, dairy, and nutrition analysis.
It should be interpreted with sample context and method notes.
Crude protein formula and conversion factors
The standard formula is crude protein (%) = nitrogen (%) × F. The general factor 6.25 comes from assuming protein is 16% nitrogen, because 100 ÷ 16 = 6.25. Real proteins vary in nitrogen content, so specialized factors are common: about 6.38 for milk and dairy, 5.70 for wheat, and other food-specific values in food-composition tables.
Use the factor required by your laboratory method, regulation, or product specification when reporting official results.
How to calculate crude protein step by step
First obtain nitrogen percent from a validated analytical method such as Kjeldahl digestion or a calibrated combustion method. Then select the conversion factor that matches the material. Enter both numbers, multiply, and round to the precision justified by the nitrogen measurement.
Measure nitrogen percent on the same moisture basis as the desired report.
Choose 6.25 for a broad general estimate unless a specific factor is required.
Multiply nitrogen percent by the factor.
Report the result as crude protein percent and state the factor used.
For solution concentrations used during analysis, the molarity calculator can help prepare reagents.
Common nitrogen-to-protein factors
Different foods have different average amino-acid compositions, so the best factor can change by matrix. The table gives practical reference values often seen in food and feed work. Always follow the governing standard for compliance testing.
| Material | Typical factor | Why it is used |
|---|---|---|
| General foods and feeds | 6.25 | Assumes 16% nitrogen in protein |
| Milk and dairy | 6.38 | Traditional dairy protein factor |
| Wheat and wheat flour | 5.70 | Cereal proteins have a different nitrogen fraction |
| Gelatin | 5.55 | Collagen-derived proteins differ from average food proteins |
| Soy products | 5.71 | Used in some food-composition references |
Kjeldahl method notes
The Kjeldahl method digests organic nitrogen to ammonium, distills or measures it, and expresses the result as nitrogen. It is robust and historically important, but it does not distinguish protein nitrogen from other nitrogen compounds. Combustion methods such as Dumas analysis can also produce nitrogen values that are converted to crude protein. See LibreTexts nitrogen analysis for educational chemistry context and Codex Alimentarius for food-standard context.
When comparing two crude protein results, confirm that both use the same analytical method, factor, dry-matter basis, and rounding rule.
Limitations and interpretation
Crude protein is a useful routine index, but it is not the same as digestible protein, amino-acid balance, or true protein by sequence. Materials with non-protein nitrogen can read artificially high. Moisture also matters: a result on an as-fed basis will be lower than the same sample on a dry-matter basis if water is present.
State whether the result is as-fed, dry matter, or another basis.
Use amino-acid analysis for nutritional quality, not only crude protein.
Beware of non-protein nitrogen additions or contaminants.
Keep the factor visible in reports so results can be compared.
Quick Reference Card
Crude Protein — Quick Reference
Quick reference • Crude Protein Calculator
crude protein (%) = nitrogen (%) × conversion factorValid range: 0% to about 100%; typical foods and feeds are usually below 40% on an as-fed basis
Common Values
⚠ Watch Out
- •Do not mix dry-matter and as-fed nitrogen values.
- •Do not use 6.25 when a regulation or commodity standard requires a different factor.
- •Crude protein can be biased high by non-protein nitrogen.
- •Round only after multiplication, not before selecting the factor.
Pro Tips
- →State the factor in reports, for example protein (N × 6.25).
- →Use 6.38 for many dairy calculations and 5.70 for wheat when specified.
- →Compare samples only on the same moisture basis.
- →Use amino-acid analysis when protein quality, not just quantity, is the goal.
FAQs
How do I calculate crude protein from nitrogen?
Multiply the measured nitrogen percent by the selected conversion factor. For example, 2.0% nitrogen with factor 6.25 gives 12.5% crude protein.
Why is 6.25 the standard conversion factor?
The 6.25 factor assumes proteins contain 16% nitrogen on average. Since 100 divided by 16 equals 6.25, nitrogen percent is multiplied by 6.25 to estimate protein percent.
When should I use 6.38 instead of 6.25?
Use 6.38 for many milk and dairy products when that is the required dairy convention. It reflects the average nitrogen content of dairy proteins better than the general 6.25 factor.
Is crude protein the same as true protein?
No. Crude protein is based on total nitrogen and can include non-protein nitrogen. True protein or amino-acid analysis is needed when the exact protein molecules or nutritional quality matter.
Can crude protein be higher than 100%?
For ordinary foods and feeds it should not be near 100%. A value above 100% usually indicates invalid inputs, a wrong factor, dry-basis confusion, or a reporting error.
Does moisture affect crude protein percentage?
Yes. As-fed samples include water mass, so percent protein is lower than on a dry-matter basis. Compare results only when they use the same moisture basis.