Calculate crude protein percentage from nitrogen content using the Kjeldahl method. Supports the standard N × 6.25 factor plus food-specific conversion factors for wheat, dairy, gelatin, and more. Enter nitrogen % or Kjeldahl titration data to get results instantly.
✓ Verified: AOAC Method 990.03 & AAFCO Official Feed Terms — April 2026
Choose how your nitrogen measurement is expressed
Total nitrogen as % of dry matter or as-receivedEnter a valid nitrogen percentage (0–100).
Enter sample titrant volume (mL).
Enter blank titrant volume (mL).
Normality of HCl or H2SO4 titrantEnter acid normality.
Enter sample mass (g).
Enter mass of nitrogen in sample (g).
Enter total sample mass (g).
Select sample type for correct N-to-protein factor
Enter a valid conversion factor.
Enter to also calculate dry matter basis CP
Crude Protein
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Sources & Methodology
🛡️Formula based on AOAC Official Method 990.03 (Kjeldahl protein determination). Conversion factors verified against AAFCO Official Feed Terms 2026 and FAO/WHO food composition standards.
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AOAC International — Official Method 990.03
Standard Kjeldahl method for protein in animal feed (using N × 6.25). aoac.org
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AAFCO Official Feed Terms 2026
American Association of Feed Control Officials — official definitions and calculation methods for crude protein in animal feeds including guaranteed analysis requirements.
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FAO Food Composition Tables — Protein Conversion Factors
FAO Food and Nutrition Paper 77 (2003): specific nitrogen-to-protein conversion factors by food group. fao.org
Step-by-step Kjeldahl method:
1. Digest sample with H₂SO₄ + catalyst → converts organic N to (NH₄)₂SO₄
2. Add NaOH → releases NH₃ gas, distill into boric acid
3. Titrate with standard acid (HCl): mL acid × Normality × 14.007 = mg N
4. % N = (mg N / sample mass in g) / 10
5. Crude Protein (%) = % N × conversion factor (default 6.25)
CP% = %N × F | where F = 100 / %N in protein (default 16% → F=6.25)
Example: Feed sample with 3.2% N, standard factor 6.25:
CP = 3.2 × 6.25 = 20.0% crude protein
If moisture = 10%, DM basis CP = 20.0 / (1 − 0.10) = 22.2% CP DM
How to Calculate Crude Protein: Kjeldahl Method Guide
Crude protein (CP) is the most fundamental proximate analysis measurement in animal nutrition, food science, and agronomy. It represents the total nitrogen-containing compounds in a sample, converted to a protein equivalent using an established conversion factor. The Kjeldahl method — developed by Johan Kjeldahl in 1883 — remains the international reference standard for measuring total nitrogen in organic materials.
The term "crude" is used because the calculation cannot distinguish between true protein nitrogen and non-protein nitrogen (NPN) sources. Both are captured equally by the Kjeldahl digest. In most solid feed materials, NPN is a small fraction of total nitrogen. However, in some samples — particularly fresh grass silage, liquid milk, or any urea-supplemented feed — NPN can account for 15–40% of total nitrogen.
Nitrogen-to-Protein Conversion Factors by Food Type
Food / Feed Category
N Factor
Standard
Reason
Most animal feeds & mixed proteins
6.25
AOAC 990.03
~16% N assumed in protein
Wheat flour & cereal grains
5.70
ICC 105 / AOAC
Higher N% in gluten proteins
Cow milk & dairy products
6.38
IDF / CODEX
Lower N% in casein proteins
Soybean
6.32
AOCS
Specific to legume proteins
Gelatin & collagen
5.55
AOAC
High N% in hydroxyproline
Sunflower seed
5.46
FAO FNP 77
Specific to oilseed proteins
Peanut / groundnut
5.18
FAO FNP 77
Specific amino acid composition
Typical Crude Protein Values in Animal Feeds
Understanding expected CP ranges helps verify calculation accuracy. A result well outside the normal range for a given feed type should trigger review of the nitrogen measurement or the conversion factor used.
Feed Ingredient
Typical CP Range (%)
%N equivalent
Corn/maize grain
8–10%
1.3–1.6% N
Wheat grain
10–14%
1.8–2.5% N
Alfalfa hay (good quality)
18–22%
2.9–3.5% N
Grass hay
8–14%
1.3–2.2% N
Soybean meal (44%)
44–48%
7.0–7.7% N
Fishmeal (60%)
60–72%
9.6–11.5% N
Blood meal
80–85%
12.8–13.6% N
Urea (fertilizer grade)
281% CP equiv.
45% N — all NPN
💡 Dry Matter Basis: Feed crude protein is often expressed on a dry matter (DM) basis to allow fair comparison between feeds with different moisture contents. DM CP = As-Received CP / (1 − moisture fraction). For example, fresh grass at 78% moisture and 4% CP as-received = 4 / (1 − 0.78) = 18.2% CP DM — much higher than it appears fresh.
Why Urea Has 281% Crude Protein Equivalent
Urea (CO(NH₂)₂) contains 46.7% nitrogen. Multiplied by 6.25 gives 291% crude protein equivalent (some sources use 46.7% directly, giving 292%). This is why urea is such a cost-effective nitrogen supplement in ruminant diets — but it is pure NPN, with zero true protein. Rumen microbes can convert NPN to bacterial protein, which is then digested in the small intestine. In monogastrics (pigs, poultry), urea cannot be utilized and is toxic at high doses.
Frequently Asked Questions
Crude protein (CP) is total nitrogen-containing compounds calculated as: CP% = %N × 6.25. It is called 'crude' because it includes non-protein nitrogen (NPN) from urea, amino acids, and nucleic acids alongside true protein. It is the standard proximate analysis value reported in feed tags and food composition databases.
The Kjeldahl method measures total nitrogen through three steps: 1) Acid digestion — sulfuric acid with catalyst converts all organic N to ammonium sulfate. 2) Distillation — NaOH releases ammonia, collected in boric acid. 3) Titration — the ammonia is titrated with standard HCl to calculate nitrogen. Results are multiplied by 6.25 to get crude protein.
Crude Protein (%) = Nitrogen (%) × Conversion Factor. Standard factor = 6.25. Example: 2.5% nitrogen × 6.25 = 15.6% crude protein. Food-specific factors: wheat 5.70, dairy 6.38, soybean 6.32. The factor 6.25 assumes protein is 16% nitrogen by mass (100/16 = 6.25).
Because most proteins contain approximately 16% nitrogen by mass: 100/16 = 6.25. This is an average across amino acid compositions. In reality it ranges from ~15% (cereal proteins) to ~17.7% (some legumes). Using food-specific factors gives more accurate results than the universal 6.25.
NPN includes all nitrogen not in true protein: urea, ammonia, amino acids, nucleic acids, nitrates, alkaloids. Kjeldahl cannot distinguish NPN from true protein. In ruminant feeds, NPN like urea can be used by rumen microbes. In monogastrics, most NPN is not utilized and urea is toxic at high levels.
Common CP ranges: Corn 8-10%; Wheat 10-14%; Alfalfa hay 18-22%; Grass hay 8-14%; Soybean meal 44-48%; Fishmeal 60-72%; Blood meal 80-85%. High-CP protein concentrates are used to balance lower-CP forages in ruminant and poultry diets.
Use 5.70 for wheat and cereal grains. Wheat gluten proteins have a higher nitrogen percentage than assumed by 6.25, so using 6.25 overestimates wheat protein by about 9%. ICC Standard No. 105 and AOAC Method 990.03 both specify 5.70 for wheat and wheat flour products.
Crude protein = all nitrogen × 6.25. True protein is measured by TCA precipitation and includes only actual protein. The difference = NPN fraction. For dry grain feeds, CP and true protein are nearly equal. For fresh forages or milk, NPN can account for 5-20% of total nitrogen.
From titration: 1) Net mL = sample mL − blank mL. 2) g N = net mL × normality × 0.014007. 3) % N = (g N / g sample) × 100. 4) CP% = %N × 6.25. Run in triplicate and report the mean.
DCP = CP × digestibility coefficient. For fishmeal, digestibility is 85-95%. For poor roughages, 50-60%. DCP is more nutritionally meaningful than CP for ration formulation because it reflects protein actually absorbed by the animal after digestion.