About

Crude protein determination relies on total nitrogen measurement - typically via the Kjeldahl digestion method (AOAC 2001.11) - multiplied by a nitrogen-to-protein conversion factor F. The general factor 6.25 assumes protein contains 16% nitrogen, but this is an approximation. Real protein matrices deviate: dairy casein uses 6.38, wheat gluten uses 5.70, and gelatin uses 5.55. Applying the wrong factor inflates or deflates reported protein by up to 12%, which can cause regulatory non-compliance under Codex Alimentarius or FDA 21 CFR 101.9. Non-protein nitrogen (urea, nitrates, melamine) is not distinguished - the 2008 melamine contamination crisis demonstrated the vulnerability of this method.

This calculator computes crude protein from either a direct nitrogen percentage or from raw titration data (titrant volume, blank volume, acid normality, sample mass). It applies the correct Jones factor for your sample matrix. Note: results represent crude protein. True protein requires correction for non-protein nitrogen, which this tool does not perform. Pro tip: always run a reagent blank and at least two replicates per sample to keep your coefficient of variation below 2%.

Formulas

The Kjeldahl crude protein equation multiplies measured total nitrogen by the appropriate Jones conversion factor:

CP = N × F

Where CP is crude protein content in % (or g/100g), N is total nitrogen in %, and F is the nitrogen-to-protein conversion factor (default 6.25).

When computing nitrogen from titration data:

N% = (V_s − V_b) × N_acid × 14.007W × 100

Where V_s is titrant volume for the sample in mL, V_b is the blank titrant volume in mL, N_acid is the normality of the standard acid in mol/L, 14.007 is the atomic mass of nitrogen in g/mol, and W is sample weight in mg.

Reference Data

Food / Feed Matrix	Jones Factor (F)	Assumed N Content (%)	Standard / Reference
General (default)	6.25	16.00	Codex Alimentarius
Milk & Dairy	6.38	15.67	Jones 1931 / AOAC
Wheat & Flour	5.70	17.54	Jones 1931
Rice	5.95	16.81	FAO/WHO
Maize / Corn	6.25	16.00	AOAC
Soybean	5.71	17.51	Jones 1931
Barley, Oats, Rye	5.83	17.15	Jones 1931
Peanuts / Groundnuts	5.46	18.32	Jones 1931
Meat & Fish	6.25	16.00	AOAC 2001.11
Egg (whole)	6.25	16.00	FAO
Gelatin	5.55	18.02	Jones 1931
Casein (pure)	6.40	15.63	AOAC
Almonds & Tree Nuts	5.18	19.31	Jones 1931
Sesame Seed	5.30	18.87	FAO
Sunflower Seed	5.30	18.87	FAO
Cotton Seed	5.30	18.87	AOCS
Animal Feed (mixed)	6.25	16.00	ISO 5983
Mushrooms	4.38	22.83	Fujihara et al.
Vegetables (leafy)	4.39	22.78	Salo-Väänänen & Koivistoinen
Fruits	4.39	22.78	Salo-Väänänen & Koivistoinen

Frequently Asked Questions

Each protein source has a distinct amino acid composition, which changes the nitrogen-to-protein mass ratio. Wheat gluten contains more nitrogen-rich amino acids like glutamic acid, yielding a lower factor of 5.70. Dairy casein has a lower nitrogen fraction, requiring a factor of 6.38. Using the general 6.25 factor introduces systematic error of up to 12% depending on the matrix.

The Kjeldahl method measures total nitrogen, not just protein nitrogen. Sources of NPN include urea, free amino acids, nucleic acids, nitrates, and alkaloids. In milk, NPN accounts for roughly 5% of total nitrogen. In fresh vegetables, NPN can exceed 40%. For high-NPN samples, consider a TCA precipitation step to isolate true protein nitrogen before applying the conversion factor.

Standard practice uses 0.1N HCl or H₂SO₄ for samples with nitrogen content above 1%. For micro-Kjeldahl with small sample masses (under 100mg), use 0.01N acid to improve endpoint resolution. Always standardize your acid against a primary standard (potassium hydrogen phthalate or Na₂CO₃) before each batch.

Yes. The Dumas method (AOAC 992.15) also reports total nitrogen as a percentage. The same conversion factor applies: CP = N × F. Simply enter your nitrogen percentage in direct mode and select the correct matrix. Note that Dumas typically yields slightly higher nitrogen values than Kjeldahl because it also captures nitrate and nitrite nitrogen.

AOAC and ISO 5983 guidelines specify that the coefficient of variation between duplicates should not exceed 2% relative. If your replicates differ by more than this, check for incomplete digestion (green or brown digest color indicates residual organic matter), titration endpoint overshoot, or sample heterogeneity. For regulatory submissions, run at least three replicates.

For samples below 0.5% nitrogen (fruits, refined sugars, oils), increase sample mass to at least 2g and use 0.01N acid for titration. The blank correction becomes critical at low levels - a blank volume error of 0.1mL at 0.01N acid changes reported nitrogen by 0.007mg, which is significant for a 500mg sample.