Crude Protein Calculator
Kjeldahl measures nitrogen and assumes it came from protein — which is why melamine once passed as milk.
The formula
crude protein = nitrogen percent * conversion factor
Nitrogen stands in for protein
The Kjeldahl method measures nitrogen, not protein. Protein averages about 16 % nitrogen, so multiplying by 100/16 — that is, 6.25 — converts one to the other. The word "crude" in crude protein is doing real work: it is an estimate resting on an average.
Why the factor varies
Different proteins have genuinely different nitrogen contents, because their amino acid compositions differ. Dairy protein is nitrogen-poor enough to warrant 6.38, wheat flour nitrogen-rich enough to warrant 5.70. Using 6.25 for everything overstates wheat protein by about nine per cent.
The method's blind spot
Any nitrogen in the sample counts, whether or not it is in protein. Nucleic acids, urea and free amino acids all inflate the figure. So does melamine, which is 66 % nitrogen — the reason the 2008 Chinese milk scandal worked at all was that a Kjeldahl test cannot tell melamine from protein. Modern methods for high-stakes testing measure amino acids directly.
Empirical is not molecular
Every method on these pages returns a ratio, because that is all a composition can determine. Glucose, formaldehyde and acetic acid all reduce to CH₂O, and no amount of elemental analysis will separate them. A molar mass from mass spectrometry or a freezing-point depression is what turns the ratio into a formula, and it has to come from a second, independent measurement.