Introduction: Body mass index (BMI) provides little information on body composition. For example, two people with the same BMI might have different body compositions. In this sense, the development of a new BMI able to provide body composition information is of clinical and scientific interest. The aim of the study was to suggest a new modified BMI formula. Material and methods: A total of 108 subject, females 56 and males 52, 0-73 years old, in various physiopathological conditions were evaluated. Data were collected and processed by a program that through anthropometric measurements calculates classic BMI, volume, surface, V/S (that we can defined like a body-thickness "pseudospessore") and the new BMI-BFMNU. Results: The basic formula (BMI =Body Mass [kg]/Height [m2]) uses the height squared as the value of the body surface, although this is only an approximation of the real surface, whereas using the real surface instead, the new BMI reflects better the ratio between the body volume and its surface. The ratio called 'pseudospessore' is already used in literature from the BFMNU (Italian acronym refereed to Biologia e Fisiologia Modellistica della Nutrizione Umana) method and has been shown to be related to the amount of fat. Conclusions: Using the BMI-BFMNU, it is possible to obtain an indication of the body structure related to the amount of fat. The consequence is that the obtained numerical values do not coincide with the traditional BMI's values and will refer to different normal ranges. For instance, a person may be in the range of normal weight for both BMI measurements, but only the BMI-BFMNU detects whether a person has a higher or lower fat content considering the individual's category. This study opens up to new possible future developments on the application of the new BMI that will allow a more accurate assessment and classification of patients.