Calculate crude protein content from Kjeldahl nitrogen analysis with conversion factors for different food types and feed ingredients.
Crude protein determination is one of the most widely performed analyses in food science, animal nutrition, and agriculture. The Kjeldahl method, developed in 1883, measures the total nitrogen content of a sample through digestion, distillation, and titration. The nitrogen value is then multiplied by a conversion factor (typically 6.25) to estimate the crude protein content.
The general factor of 6.25 assumes that proteins contain 16% nitrogen on average (100/16 = 6.25). However, actual nitrogen-to-protein ratios vary by food type: dairy products use 6.38, wheat and flour use 5.70, soybeans use 5.71, and rice uses 5.95. Using the correct Jones factor for your specific sample type is critical for accurate protein reporting, especially for regulatory compliance and nutritional labeling.
This calculator processes your Kjeldahl titration data — accounting for sample weight, acid normality, titrant volume, and blank corrections — to compute the percent nitrogen and crude protein with the appropriate conversion factor. It includes presets for common food matrices, a reference table of conversion factors, and handles both macro-Kjeldahl and semi-micro methods.
This calculator saves time processing Kjeldahl lab data, automatically applies the correct conversion factor for your sample type, and reduces calculation errors that are common when working through the multi-step formula manually. This crude protein calculator helps you compare outcomes quickly and reduce avoidable mistakes when making day-to-day care decisions. Use the estimate as a planning baseline and confirm final decisions with a qualified professional when risk is high.
% Nitrogen = [(V_sample − V_blank) × N_acid × 14.007 × 100] / (W × 1000), where V = titrant volume (mL), N = acid normality, W = sample weight (g), 14.007 = atomic weight of nitrogen. Crude Protein (%) = % Nitrogen × Conversion Factor.
Result: % N = 2.09%, Crude Protein = 13.03%
For a 1.0 g sample: % N = (15.2 − 0.3) × 0.1 × 14.007 × 100 / (1.0 × 1000) = 2.09%. Crude protein = 2.09% × 6.25 = 13.03%.
The analysis begins with acid digestion, where the sample is heated with concentrated sulfuric acid and a catalyst (typically copper sulfate and potassium sulfate). This converts organic nitrogen to ammonium sulfate while destroying the organic matrix. The digestion typically takes 45–90 minutes at 380–400°C. After cooling, the digest is treated with excess sodium hydroxide, which liberates ammonia gas. This ammonia is steam-distilled into a receiving flask containing boric acid, forming ammonium borate. Finally, the trapped ammonium is titrated with a standard acid (HCl or H₂SO₄) to determine the amount of nitrogen.
The generic 6.25 factor assumes 16% nitrogen in protein, but actual proteins vary from 15% (collagen/gelatin, factor 5.55) to 19% (many seeds). The Codex Alimentarius specifies 6.38 for milk products, 5.70 for wheat, and 6.25 as the default. In 2003, FAO recommended using 5.71 for soybeans instead of the previously used 6.25, which significantly affects soy protein content claims on food labels.
While Kjeldahl remains the official reference method in many regulations (AOAC 928.08, ISO 8968), the Dumas/combustion method (AOAC 990.03) is increasingly adopted for routine analysis due to faster throughput and elimination of corrosive chemicals. NIR calibrations based on Kjeldahl reference data enable rapid at-line protein screening in food and feed manufacturing. Regulatory bodies like FDA and EFSA require specific conversion factors for nutritional labeling accuracy.
It's a three-step procedure: (1) acid digestion converts organic nitrogen to ammonium sulfate, (2) distillation liberates ammonia into a trapping solution, (3) titration quantifies the ammonia. This keeps planning practical and lowers the chance of preventable errors.
Because the Kjeldahl method measures ALL nitrogen, not just protein nitrogen. Non-protein nitrogen (NPN) from nucleic acids, urea, free amino acids, and alkaloids is also included.
Specific nitrogen-to-protein conversion factors developed by D.B. Jones for different food types, accounting for their actual amino acid compositions. The generic 6.25 is a default.
For dairy (use 6.38), wheat/flour (5.70), rice (5.95), barley (5.83), soybeans (5.71), gelatin (5.55), and nuts (5.18). Using 6.25 overestimates protein for these foods.
Kjeldahl is highly repeatable (±0.5-1% relative), but accuracy depends on complete digestion, proper blank correction, and using the right conversion factor. NPN can cause overestimation.
Dumas/combustion method (faster, no toxic chemicals), near-infrared (NIR) spectroscopy (rapid screening), and the Bradford/Lowry assays (for solutions). Each has trade-offs in speed, cost, and applicability.