The protein content in food is estimated by multiplying the nitrogen content by a nitrogen-to-protein conversion factor, traditionally 6.25. This factor, established over 100 years ago, assumes proteins contain 16% nitrogen. However, this approach is flawed as protein nitrogen content varies, and other nitrogenous compounds in food also contribute to nitrogen content. Specific conversion factors for different foodstuffs, such as those proposed by Jones, are scientifically flawed but widely used. Recent studies suggest that the average conversion factor is closer to 5.6 than 6.25, and specific factors for different foodstuffs provide more accurate estimates. The paper reviews the limitations of the 6.25 factor and proposes a set of specific conversion factors, including a default factor of 5.6, for more accurate protein estimation. These factors are particularly important when "protein" refers to amino acids. The paper also discusses the implications of using these factors for nutritional and regulatory purposes, highlighting the need for more accurate methods to assess protein quality and quantity in food. The use of 6.25 and Jones' factors has significant financial implications, especially in the dairy industry. The paper concludes that while the 6.25 factor has been widely used, it is no longer scientifically sound and that specific conversion factors should be adopted for more accurate protein estimation.The protein content in food is estimated by multiplying the nitrogen content by a nitrogen-to-protein conversion factor, traditionally 6.25. This factor, established over 100 years ago, assumes proteins contain 16% nitrogen. However, this approach is flawed as protein nitrogen content varies, and other nitrogenous compounds in food also contribute to nitrogen content. Specific conversion factors for different foodstuffs, such as those proposed by Jones, are scientifically flawed but widely used. Recent studies suggest that the average conversion factor is closer to 5.6 than 6.25, and specific factors for different foodstuffs provide more accurate estimates. The paper reviews the limitations of the 6.25 factor and proposes a set of specific conversion factors, including a default factor of 5.6, for more accurate protein estimation. These factors are particularly important when "protein" refers to amino acids. The paper also discusses the implications of using these factors for nutritional and regulatory purposes, highlighting the need for more accurate methods to assess protein quality and quantity in food. The use of 6.25 and Jones' factors has significant financial implications, especially in the dairy industry. The paper concludes that while the 6.25 factor has been widely used, it is no longer scientifically sound and that specific conversion factors should be adopted for more accurate protein estimation.