“Crude protein” on a feed or food label is not measured directly — it is nitrogen in disguise. The Kjeldahl method measures total nitrogen, and the label value is that nitrogen multiplied by a conversion factor. This Crude Protein Calculator runs the full Kjeldahl arithmetic from your titration (sample titre, blank, acid normality, sample mass) to nitrogen percent and crude protein percent — and also converts a directly measured nitrogen percentage when you already have one.
Worked Example: A Feed Sample by Titration
A 1.000 g feed sample titrates at 12.9 mL of 0.100 N acid against a 0.5 mL blank. The corrected volume is 12.4 mL, and each millilitre of 0.1 N acid neutralizes ammonia equivalent to 0.1 × 14.007 = 1.4007 mg of nitrogen — so the sample holds 12.4 × 1.4007 = 17.37 mg N, which is 1.737% nitrogen on the 1 g sample. Multiply by the general factor 6.25: crude protein = 10.86% — a plausible figure for a grain-based feed. The direct route agrees by construction: a sample independently measured at 3.2% nitrogen converts to 3.2 × 6.25 = 20.0% crude protein, the arithmetic printed on a thousand feed sacks. Both paths run through the same assumption — that protein is 16% nitrogen on average, so its amount is nitrogen × 100/16 = × 6.25 — which the word “crude” in the name is honestly flagging.
Why “Crude”: The Factor Table and Its Limits
The 6.25 factor assumes all nitrogen in the sample is protein nitrogen and that protein averages 16% N. Both are approximations. Non-protein nitrogen — urea in ruminant feed, free amino acids, nucleic acids, even adulterants like melamine (the 2008 scandal was precisely an exploit of this method) — inflates the result. And true nitrogen content varies by protein source, which is why official methods use food-specific Jones factors: milk and dairy 6.38 (casein is nitrogen-poorer per gram), wheat flour 5.7 (its proteins are nitrogen-richer), rice 5.95, soybean 5.71, egg and meat 6.25, gelatin 5.55. Using 6.25 for wheat flour overstates protein by roughly 9.6% relative (6.25/5.7) — a labeling-grade error hiding in a single constant. The calculator takes the factor as an input for exactly this reason: the method’s output is only as honest as the factor you choose, and the factor belongs to the sample type, not to the arithmetic. Moisture basis matters just as much — results are conventionally reported on a dry-matter or as-fed basis, and a protein percent without its basis is an unfinished sentence.
The Kjeldahl Method in Brief
Three stages produce the titre this page consumes. Digestion: the sample is boiled in concentrated sulfuric acid with a catalyst, converting organic nitrogen to ammonium sulfate. Distillation: alkali releases the ammonia, which is steam-distilled into a receiving acid (boric acid in modern setups). Titration: the captured ammonia is titrated with standardized acid — the volume difference against a reagent blank is the (V − Vb) in the formula N% = (V − Vb) × N × 14.007 / (sample mg) × 100. The Dumas combustion method, the main alternative, burns the sample and measures N₂ gas directly: faster, no acid waste, same × factor conversion at the end — and the same “crude” caveat, because both methods measure nitrogen and infer protein. Understanding that the whole industry rests on an inference is the difference between quoting a label and understanding one.
Common Mistakes
Forgetting the blank subtraction — reagent and apparatus nitrogen is real, and at low protein levels the blank is a sizable fraction of the titre. Applying 6.25 to dairy or wheat samples with regulatory Jones factors (6.38, 5.7). Mixing moisture bases between the nitrogen result and the reported protein. Treating crude protein as true protein in nutrition decisions for monogastrics, where non-protein nitrogen has near-zero value while ruminants can use some of it. And arithmetic slips in the normality: the formula needs equivalents — a 0.1 N acid and a 0.1 M diprotic acid are not the same titrant.
Reporting Basis: As-Fed Versus Dry Matter
One worked conversion prevents most label confusion. The example’s 10.86% protein is on an as-fed basis; if the feed carries 12% moisture, its dry-matter protein is 10.86 / (1 − 0.12) = 12.34%. Ration software works in dry matter, sacks print as-fed, and comparing the two bases directly manufactures a phantom 1.5-point discrepancy out of plain water. State the basis beside every protein figure — the calculator’s arithmetic is basis-neutral, but the interpretation never is.
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Frequently Asked Questions
How is crude protein calculated?
Crude protein % = nitrogen % × conversion factor. From a Kjeldahl titration: N% = (titre − blank) × normality × 14.007 / sample mass in mg × 100, then multiply by the factor — 6.25 generally, or a food-specific Jones factor.
Why is the factor 6.25?
Because average protein is about 16% nitrogen by mass: 100/16 = 6.25. It is a population average, not a constant of nature — milk protein uses 6.38 and wheat 5.7 in official methods.
What does the worked example give?
A 1 g sample titrating 12.9 mL against a 0.5 mL blank with 0.1 N acid holds 17.37 mg of nitrogen = 1.737% N, giving 10.86% crude protein at the 6.25 factor.
What is the blank titre for?
It measures the nitrogen contributed by reagents and apparatus alone, run without a sample. Subtracting it ensures the titre reflects only sample nitrogen — skipping it overstates protein, badly for dilute samples.
Is crude protein the same as true protein?
No. Crude protein counts all nitrogen as if it were protein, including non-protein nitrogen. True protein methods precipitate the protein first and measure only its nitrogen — results run lower.
Does the Dumas method give the same answer?
Nearly — Dumas measures the same total nitrogen by combustion and applies the same factors, so values track Kjeldahl closely, with small method biases handled by calibration against reference materials.