The objectives of this study were to evaluate a practical method of determining more accurately conversion factors for calculating the protein content of food from the total nitrogen content (N : P factors) and to propose a reliable procedure for routinely converting total nitrogen data to the protein content. First, mushrooms and vegetables were selected as model foods to explore a novel method of determining N : P factors for the reason that these foods contain high levels of nonprotein nitrogen. These foods were analyzed for total nitrogen content, amino acid nitrogen and nonprotein nitrogen and then the N : P factors were calculated. Based on the results, the novel method for calculating N : P factors was proposed; the N : P factor was defined as the proportion of the amino acid residue content to the total nitrogen content. The average N : P factors calculated by this novel method were 3. 99±0. 76 (mean±SD) for mushrooms and 4. 10±0. 60 for vegetables. Protein contents estimated using these factors were in good agreement with the contents determined by amino acid residues. Second, applying this novel calculation method, the N : P factors for some other foods were investigated. Cereal products and several pulses were determined the novel N : P factors, based on the total nitrogen content and amino acid residues. The averaged factors were 5. 26 for rice, 5. 47 for wheat, 5. 54 for other cereal products and 5. 51 for pulses. Then, the N : P factors for 295 foodstuffs in “the Revised Standard Tables of Food Composition in Japan, Amino Acid Composition of Foods”, were calculated. These results suggested that the N : P factor of 6. 25 commonly used for many foods was not a realistic factor for estimating the protein content of food by converting the total nitrogen. Third, the protein contents of 11 school lunch menus and 2 convenience store lunch boxes were calculated from the total nitrogen content using the novel N : P factors. The proportion of the protein contents multiplied by the novel N : P factors to the traditional protein contents was approximately 84%. The averaged novel N : P factor for 13 menus was 5. 16±0. 10. The protein content of diet meals containing various foods should be multiplied by this value.   It was concluded that this novel calculation method should be practical and suitable for the evaluation of the N : P factors.