Article — Crude Protein Calculator
Crude Protein Calculator: %CP from Nitrogen × Kjeldahl Factor 6.25
Crude protein (%CP) equals nitrogen percentage times the Kjeldahl conversion factor: %CP = %N × 6.25 for general feed and meat. The 6.25 comes from the empirical observation that average protein contains about 16% nitrogen by mass, established by Gerardus Mulder in 1839. Dairy products use a slightly different factor (6.38) because milk proteins are nitrogen-poor by 2%; wheat flour uses 5.70, soybean uses 5.71, and almonds use 5.30. The method was invented by Johan Kjeldahl in 1883 for the Carlsberg brewery and is still the global AOAC standard 140 years later. A typical commercial soybean meal at 47% CP corresponds to 7.52% N. Wheat at 12% CP has 1.92% N. Cow's milk is about 3.4% protein, requiring just 0.53% nitrogen.
This calculator handles all common product types. Pick the sample type (general, dairy, grain, wheat, soybean, nuts, gelatin), enter the measured %N from your Kjeldahl or Dumas analysis, and the calculator returns %CP plus optional masses of nitrogen and crude protein for any sample weight.
What is crude protein
Crude protein is the total nitrogen content of a sample multiplied by a conversion factor, typically 6.25. It is called crude because it lumps together true protein (amino acids, peptides) with non-protein nitrogen (NPN) — urea, free amines, nucleic acids, nitrates, and ammonia. CP has been the global standard for protein content in food, feed, and forage since the late 19th century, predating any modern method to measure true protein.
The reason CP persists in regulations and labelling is practical: it is fast, cheap (~$1 per sample for Kjeldahl), and standardised. For most feedstuffs, NPN is small relative to true protein, so CP is a good proxy. For ruminants, NPN is even nutritionally useful — rumen bacteria convert it into microbial protein.
The 2008 melamine scandal in Chinese infant formula exploited the crude protein test. Melamine has six nitrogen atoms per molecule (67% N by mass), so adding tiny amounts to watered-down milk raised the measured %N to make it look protein-rich. The fraud killed six infants and sickened 300,000. Manufacturers now use HPLC or specific amino-acid tests for infant formula in addition to crude protein, because Kjeldahl alone cannot tell the source of the nitrogen.
Crude protein formula and the 6.25 factor
The crude protein formula is %CP = %N × 6.25, where 6.25 = 100 / 16. The denominator 16 is the average nitrogen content of protein, expressed as a percentage of total protein mass. Mulder established this in 1839 by analysing diverse purified proteins from animals, plants, and microbes. The value is an average; individual proteins range from 13% N (some plant proteins) to 19% N (gelatin and milk caseins).
Using 6.25 universally introduces small biases that matter for high-precision pricing. Dairy products use 6.38 because milk proteins are 15.67% N, not 16%. Wheat flour uses 5.70 because wheat gluten is 17.54% N. The choice of factor can shift the reported protein percentage by 2 to 4%.
%CP = %N × f mainf = 100 / %N_in_protein derivationGeneral: 6.25 Feed, meatDairy: 6.38 Milk, cheeseWheat: 5.70 FlourSoybean: 5.71 MealKjeldahl method for crude protein
The Kjeldahl method for crude protein has three steps. Step 1 — digestion: the sample is boiled in concentrated sulfuric acid with a metal catalyst (usually copper sulfate or selenium) at about 400 °C for 1 to 3 hours. All organic nitrogen is converted to ammonium ion: organic-N → (NH₄)₂SO₄. Step 2 — distillation: the cooled solution is made strongly alkaline with NaOH, which releases ammonia as gas: (NH₄)₂SO₄ + 2 NaOH → 2 NH₃ + Na₂SO₄ + 2 H₂O. The NH₃ is steam-distilled into a trap containing boric acid (H₃BO₃), forming ammonium borate. Step 3 — titration: the trap solution is titrated with standard HCl using a methyl-red/methylene-blue indicator. Moles of HCl equal moles of N in the original sample.
From there: mass N = moles HCl × 14.007, %N = (mass N / sample mass) × 100, %CP = %N × 6.25. The whole sequence takes 2 to 4 hours per sample on a manual setup; automated Kjeldahl systems run 24 samples per day.
Crude protein vs true protein
Crude protein vs true protein differs by the inclusion of non-protein nitrogen. CP counts every nitrogen atom in the sample, whether from amino acids or urea. True protein (TP) counts only the nitrogen in amino acids and peptides. For most fresh meats, dairy, and grains, NPN is below 5% of total N, so CP and TP agree closely. For fermented feeds (silage), urea-supplemented feeds, and protein hydrolysates, NPN can be 10 to 30% of total N — the two figures diverge sharply.
Crude protein conversion factors by product
The crude protein conversion factor varies by product because nitrogen content of protein varies. Codex Alimentarius and the FAO publish a table of recommended factors: 6.25 for general feed and meat, 6.38 for dairy, 5.95 for cereals (general), 5.70 for wheat flour, 5.71 for soybean and other oilseeds, 5.30 for nuts and seeds, 5.55 for gelatin, and 4.23 for spirulina. The further from 6.25 your product is, the more important the choice of factor becomes.
Crude protein in animal feed and pricing
Crude protein in animal feed drives both nutrition and pricing. Cattle on a milking diet need 16 to 18% CP; growing pigs need 18 to 22%; poultry layers need 15 to 17%; broiler chicks need 21 to 23%. Higher CP commands a premium: soybean meal at 48% CP sells for more than 44% CP soybean meal. Wheat at 12% CP is feed grade; 13.5% CP is bread grade; 15% CP is durum or high-protein hard wheat. Each percentage point of CP on a commodity grain typically shifts the price by 2 to 5%.
Dumas and NIR as alternatives to Kjeldahl
Dumas combustion (AOAC 990.03) is the modern alternative to Kjeldahl. The sample burns at 950 to 1100 °C in pure oxygen. All nitrogen forms NOₓ, which are reduced over copper to N₂ gas, then measured by thermal conductivity after CO₂ and H₂O are removed. The procedure takes 5 minutes per sample, uses no mercury or chromium, and agrees with Kjeldahl to within ±1%. Near-infrared spectroscopy (NIR, AOAC 2012.04) is even faster — under a minute — but requires species-specific calibration.
Dumas is gradually replacing Kjeldahl in commercial labs because it eliminates the corrosive concentrated sulfuric acid step and the mercury or selenium catalyst. The capital cost is higher ($30,000 to $60,000 for a Dumas analyser versus $5,000 to $10,000 for Kjeldahl), but operating cost per sample is lower and there is no hazardous waste stream.
Common crude protein mistakes
The most common crude protein mistake is using factor 6.25 for everything when a product-specific factor is more appropriate. For dairy that introduces a 2% under-estimate; for wheat flour a 9% over-estimate. The mistake matters more for trade pricing than for nutrition. Second mistake: assuming high CP means high-quality protein. CP ignores amino-acid balance and digestibility — a feed with 18% CP but poor lysine balance is worse than 16% CP with complete balance. Third mistake: ignoring NPN in fermented feeds, where 10 to 30% of total N can be urea or ammonia.
Adding melamine, urea, or other cheap nitrogen sources artificially raises measured CP without adding real protein. The 2008 melamine scandal and the 2007 pet-food crisis both exploited the Kjeldahl test's inability to distinguish protein nitrogen from other nitrogen sources. Modern food safety regulations require additional amino-acid analyses (LC-MS, HPLC) for infant formula and high-value protein products.