Crude Protein Calculator

What Is Crude Protein?

Crude protein is an estimate of protein content based on measured nitrogen rather than a direct assay of intact protein molecules. Because amino acids contain nitrogen, laboratories convert total nitrogen into a protein percentage using a conversion factor (often called a Jones factor). The result is called "crude" because some non-protein nitrogen can be included, so the value is an approximation of true protein.

Food science, animal nutrition, and feed analysis rely on crude protein for labeling, formulation, and quality control. When you already know nitrogen content, or when you have Kjeldahl titration data, this calculator returns crude protein percentage in real time. For related molecular work, see our Protein Molecular Weight Calculator and Protein Concentration Calculator.

Crude Protein Formula

Once nitrogen content is known, crude protein is:

$$\mathrm{CP}(\%) = N(\%) \times \text{Jones factor} \times F_2$$

where $N(\%)$ is nitrogen percent, the Jones factor converts nitrogen to protein, and $F_2$ is the dilution factor of the digested volume.

Nitrogen from the Kjeldahl Method

The Kjeldahl method digests a sample in strong acid, converts organic nitrogen to ammonium, and titrates the liberated ammonia. Nitrogen percent is:

$$N(\%) = \frac{(V_b - V_1) \times c \times F_1 \times 1.4007}{m}$$

where:

• $V_b$ = blank titrant volume (mL)
• $V_1$ = sample titrant volume (mL)
• $c$ = titrant concentration (mol/L)
• $F_1$ = acid factor (1 for HCl)
• $1.4007$ = nitrogen molar-mass scaling constant for mL and g
• $m$ = sample mass (g)

You can skip titration inputs and enter $N(\%)$ directly if you already measured nitrogen by another method. For molar relationships in solution chemistry, try the Molarity Calculator or Molar Mass Calculator.

Jones Conversion Factors

Proteins average about 16% nitrogen, so $100/16 = 6.25$ is the common default factor. Specific foods differ: milk often uses 6.38, soy about 5.71, cereals about 5.4, and red seaweed about 4.59. Choose the factor that matches your matrix for a better estimate.

How to Use This Calculator

1. Select Kjeldahl titration mode or known nitrogen % mode.
2. Enter titration volumes, concentration, acid factor, and sample mass, or enter $N(\%)$ directly.
3. Set the dilution factor $F_2$ and the Jones factor for your sample type.
4. Read crude protein % instantly; copy the details panel if you need a lab notebook record.

Worked Example

Suppose a blank uses $12.5\,\mathrm{mL}$ of $0.1\,\mathrm{mol/L}$ titrant, the sample uses $8.2\,\mathrm{mL}$, $F_1 = 1$, sample mass is $1.0\,\mathrm{g}$, $F_2 = 1$, and the Jones factor is $6.25$:

$$N(\%) = \frac{(12.5 - 8.2) \times 0.1 \times 1 \times 1.4007}{1.0} = 0.6023\%$$

$$\mathrm{CP}(\%) = 0.6023 \times 6.25 \times 1 = 3.76\%$$