Crude Protein Calculator
Estimate nitrogen content from Kjeldahl titration data, then convert that nitrogen percentage into crude protein using a food-specific conversion factor.
Calculation breakdown
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How to use this crude protein calculator
What this calculator does. This tool estimates crude protein as a percentage of sample mass. In Kjeldahl mode it first derives nitrogen content from the difference between the sample titration and blank titration, the titrant concentration, an acid standardization factor, and sample weight. It then multiplies nitrogen percentage by a selected protein conversion factor and the dilution factor. In Known nitrogen % mode it skips the titration calculation and starts from a laboratory-reported nitrogen percentage. The result is an analytical estimate, not a direct measurement of true protein or nutritional quality.
When to use it. It is useful for checking a food, feed, grain, dairy, or research-sample calculation after a Kjeldahl analysis; comparing how matrix-specific Jones factors affect reported protein; documenting a laboratory worksheet; or converting a nitrogen result produced by another validated method. The underlying laboratory concept is described in the FAO discussion of food protein analysis and nitrogen conversion factors.
How to calculate
- Choose Kjeldahl titration when you have titration data, or Known nitrogen % when nitrogen has already been measured.
- In Kjeldahl mode, enter Titrant blank test volume and Titrant volume in milliliters. The sample volume must be at least as large as the blank.
- Enter Titrant concentration in mol/L, Acid factor as a positive correction multiplier, and Sample weight in grams.
- Enter the Dilution factor. Use 1.000 when no aliquot or dilution correction is required.
- Select the Protein conversion factor that best matches the sample. Use the standard 6.25 only when a matrix-specific factor is not more appropriate.
- Read Crude protein percentage, the primary result, and review the supporting outputs and calculation breakdown. Use Download Excel to save the current typed inputs and outputs, or Reset to restore the initial example.
Input guide
Calculation mode is required and determines which formula path is used. Titrant blank test volume and Titrant volume accept nonnegative decimal values in mL, such as 0.20 and 1.00. Their difference is the corrected titrant volume; reversing them is a common error. Titrant concentration is required, must be positive, and is entered in mol/L, for example 0.100. Acid factor is a positive dimensionless standardization factor; 1.000 means no adjustment. Sample weight is required in grams and must be greater than zero; a smaller mass raises the calculated nitrogen percentage for otherwise identical titration data. In direct mode, Nitrogen content is required as a percentage from 0 to 100, for example 1.121%, and should not be entered as the fraction 0.01121. Dilution factor must be positive; increasing it increases crude protein proportionally. Protein conversion factor is selected from the supplied matrix list. Choosing a larger factor increases the reported crude protein without changing measured nitrogen.
Output guide
Crude protein percentage is the estimated protein-equivalent mass percentage and is driven by nitrogen content, dilution factor, and conversion factor. A zero result means zero corrected nitrogen or zero entered nitrogen; values above 100% usually indicate incompatible units, an incorrect dilution factor, or an unsuitable sample basis. Nitrogen content is the calculated or entered nitrogen mass percentage. Protein in sample converts the percentage into grams for the entered sample weight. Corrected titrant volume is sample titrant volume minus blank volume and is not used in direct mode. Selected conversion factor repeats the Jones factor used. The summary pills show the active method, nitrogen percentage, and factor. The breakdown table lists each stage, expression, result, and unit; these are explanatory identities, not separate recommendations.
Worked example
With a 1.00 mL sample titration, a 0.20 mL blank, 0.100 mol/L titrant, acid factor 1.000, and 1.000 g sample, the corrected volume is 0.800 mL. Nitrogen is 0.800 × 0.100 × 1.000 × 1.4007 ÷ 1.000 = 0.112056%; applying the default dilution factor of 1.000 leaves it unchanged. Multiplying by the standard factor 6.25 gives 0.70035% crude protein, and the represented protein mass is 0.0070035 g. The displayed values round to 0.11%, 0.70%, and 0.0070 g while the workbook retains the canonical numbers.
How the calculation works
The Kjeldahl calculation uses the nitrogen molar mass conversion constant 1.4007 when titrant volume is in milliliters, concentration is in mol/L, and sample mass is in grams. The blank correction removes reagent background. The calculator uses the conventional form:
Crude protein is then estimated as nitrogen percentage × protein conversion factor × dilution factor. The AOAC Official Methods of Analysis provides the formal method framework used by laboratories, while the FDA guidance on nutrition labeling helps distinguish laboratory composition data from consumer-facing label interpretation.
Interpretation and common mistakes
Crude protein assumes that measured nitrogen represents protein nitrogen after applying an average conversion factor. Non-protein nitrogen can bias the estimate upward, incomplete digestion can bias it downward, and one universal factor does not suit every food matrix. Keep all volume units in mL, concentration in mol/L, and mass in grams. Do not enter milligrams without converting to grams. Confirm whether your laboratory result is reported on a dry-matter or as-received basis before comparing samples. For animal feeds, the USDA reference on nitrogen-to-protein conversion provides additional context on factor selection and analytical limitations.