Crude Protein Calculator
Calculate crude protein percentage from Kjeldahl back-titration data. Enter the blank and sample NaOH titre values, NaOH normality, and sample weight.
Calculate Crude Protein Percentage
Crude Protein Formula
Where:
- \(B\) — blank NaOH titre value (mL)
- \(S\) — sample NaOH titre value (mL)
- \(N\) — normality of NaOH (N)
- \(W\) — sample weight (g)
- \(1.4\) — factor that converts the titre difference into nitrogen percentage for these units
- \(6.25\) — factor that converts nitrogen percentage into crude protein percentage
The titre difference represents the acid consumed by ammonia from the sample in this back-titration setup. The formula first estimates nitrogen as a percentage of sample weight, then multiplies it by 6.25 to estimate crude protein.
Crude Protein – Calculation Example
Example: A 0.5 g feed sample has a sample NaOH titre of 5.2 mL. The blank titre is 12.5 mL, and the NaOH normality is 0.1 N.
\[\text{Crude protein (\%)}=\frac{(12.5-5.2)\times0.1\times1.4\times6.25}{0.5}=12.775\%\approx12.78\%\]
Result: 12.78% crude protein by sample weight.
How to use the calculator
- Enter Blank titre value (ml): the volume of NaOH used to titrate the blank, which is processed without the sample.
- Enter Sample titre value (ml): the volume of NaOH used to titrate the sample under the same procedure. For this calculation, it must not exceed the blank titre.
- Enter Normality of NaOH (N): the concentration of the NaOH titrant in equivalents per litre. Use the standardized normality if it differs from the concentration you intended to prepare.
- Enter Sample weight (g): the mass of material analyzed, in grams. This value must be greater than zero.
- Read Percentage of Crude Protein. The calculator rounds the result to two decimal places.
What crude protein percentage means
Crude protein is an estimate based on the nitrogen measured in a sample, rather than a direct measurement of its protein molecules. In the Kjeldahl method, digestion converts much of the sample’s nitrogen to ammonium. Ammonia is then released during distillation and collected for titration. The measured nitrogen is multiplied by a conversion factor to report an estimated protein percentage.
This calculator uses a factor of 6.25, corresponding to an assumed protein nitrogen content of 16%: \(100/16=6.25\). That convention makes results useful for routine comparison of feed and food samples. The number is a mass percentage: for example, 12.78% represents an estimated 12.78 g of crude protein per 100 g of sample on the same weight basis used for the test.
“Crude” matters because Kjeldahl nitrogen may include nitrogen from non-protein compounds. Multiplying all measured nitrogen by 6.25 can therefore give a value higher than the amount of true protein. Some foods and reporting standards also use a protein conversion factor other than 6.25; this calculator does not allow that factor to be changed. For a different aspect of feed composition, use the Crude Fibre Calculator.
Assumptions and practical checks
The formula assumes a back-titration procedure in which the blank requires more NaOH than the sample. Their difference must correspond to acid neutralized by ammonia from the analyzed portion. Do not use these inputs unchanged for a direct titration, a procedure with a different blank correction, or a method that requires an aliquot or dilution factor: those methods may need a different calculation.
Check that both titre values are nonnegative, that NaOH normality is positive, and that the sample weight is correct. The calculator displays an error when sample weight is zero or negative or when the sample titre exceeds the blank, but its input check does not verify every other physically meaningful condition. A blank and sample titre that are equal produce 0% in the formula; if that is unexpected, review the titration and recorded values.
Report whether the sample weight was measured as received or after drying. A crude protein percentage on an as-received basis is not directly comparable with one on a dry-matter basis. Consistent sample preparation, complete digestion and distillation, a suitable blank, and standardized NaOH all affect the reliability of the result. If preparing NaOH by dilution before standardization, the C1V1 = C2V2 Calculator can help plan the dilution; use the verified normality for the protein calculation.
Frequently Asked Questions
How can I check a crude protein result by hand?
Calculate the nitrogen mass first: the titre difference in mL multiplied by NaOH normality and by 0.014 gives grams of nitrogen. Divide by the sample mass in grams, multiply by 100 to get nitrogen percentage, then multiply by 6.25. This check applies only when the titre difference represents the acid consumed by the sample’s ammonia; other titration procedures may require different corrections.
What is a good crude protein percentage for animal feed?
There is no single good percentage; it depends on the animal, its life stage, and the feed’s role in the diet. Hay may contain roughly 6–20% crude protein, while soybean meal commonly contains around 44–49%, depending on the product and moisture. Compare results on the same moisture basis and use the animal’s nutritional requirements—not crude protein alone—to assess a feed.
How do I convert crude protein from an as-fed basis to a dry-matter basis?
Divide the as-fed crude protein percentage by the dry-matter fraction. For example, a feed with 12% crude protein and 20% moisture has a dry-matter fraction of 0.80, so its dry-matter crude protein is \(12/0.80=15\%\). Use a moisture measurement from a representative portion of the same feed. This conversion changes the reporting basis, not the amount of protein present.
Can I use NaOH molarity instead of normality?
Yes, for NaOH in this acid–base titration, its normality equals its molarity: 0.1 mol/L NaOH is 0.1 N because each mole supplies one equivalent of hydroxide. That equality does not apply to every reagent or reaction. If the NaOH has been standardized, enter its measured concentration rather than the concentration calculated from its preparation.
Why might a crude protein result exceed 100%?
A result above 100% is a warning to investigate, not a plausible mass fraction of actual protein. Check the sample mass, titre entries, titrant concentration, dilution or aliquot factors, and whether the method matches the calculation. Exceptionally high non-protein nitrogen can also inflate a nitrogen-based protein estimate. Review the laboratory procedure before reporting or interpreting such a result.
How does Kjeldahl crude protein differ from Dumas crude protein?
Both methods estimate protein from measured nitrogen, but they measure nitrogen differently. Kjeldahl uses digestion, distillation, and titration; Dumas uses high-temperature combustion and gas analysis. Depending on the sample and method, they may recover different nitrogen-containing compounds and produce different results. When comparing reports, check the test method, conversion factor, and moisture basis.
How should I judge whether two crude protein test results agree?
Compare results reported on the same moisture basis and with the same nitrogen-to-protein factor. Small differences can arise from sample heterogeneity, weighing, digestion, distillation, titration endpoints, and titrant standardization. Duplicate analyses and your laboratory method’s repeatability limit provide a better test of agreement than the calculator’s two decimal places. If a difference affects a feed or food decision, ask the testing laboratory to review it.