The present invention generally relates to a writing instrument that includes a means for calculating a non-linear equation of any type.  For example, one use of the invention would be to calculate a person's body mass index (BMI), which may be determined based on the following equation: M/L2, where M is the body weight in kilograms, and L is the body height in meters.  It is recognized that other equations may be used for calculating the BMI.  For instance, the body weight may be expressed in pounds, and the body height by inches.  Nonetheless, the present invention may be used as a calculating device to calculate the resultant number from a predetermined non-linear equation. 
Numerous pens and other devices are available to determine the resultant figure from a simple linear equation, such as an ordinary mathematical multiplication table.  In order to calculate a non-linear function, however, a logarithmic type of scale must be introduces, as well as a means for accessing the appropriate numerical values on both the logarithmic scale and the remaining factor(s) of the equation. 
Each such implement would typically be capable of calculating a limited scope of non-linear functions.  For example, the BMI calculating implement discussed above would be a device which would typically be dedicated to performing only that calculation.  The limiting factor for the scope of calculation functions possible would generally be the space limitation on the barrel of the device, as that surface area would include the printed values for at least one of the resultant tables.  The incorporation of different colors for different calculations, however, as an example, could permit a single device to perform multiple independent non-linear calculations.  To ability to utilize a writing implement as a device to perform quick and accurate calculations would benefit the user by reducing their dependence upon number of separate products including, for example, various medical instruments, tables, books and charts, pieces of diagnostic equipment, stethoscopes, height/weight scales, writing pens, patient charts, prescription pads, and the like.  Although conventional electronic devices may be effective in calculating non-linear equations, the user must handle and operate these devices separately in addition to various other items.  Many health-care providers, technicians, mechanics and other users would like to reduce the number of items that they must handle during the performance of their duties. 
U.S. Pat.  No.  5,028,764 to Kuhlengel discloses a pen with rotatable cylindrical elements that are used to perform a simple calculation in the multiplication of two numbers.  U.S. Pat.  No.  6,104,603 to Wang discloses a writing pen with an integral electronic calculator including printed circuit board, keyboard and power supply.  In each of these patents, the invention provides its user no capability to perform a specific non-linear calculation, as the former will not permit a non-linear calculation and the latter relies upon the user to supply a formula, in addition to its dependence on electronic, rather than manual components.  Therefore, neither patent involves a device which may be manually employed to quickly determine the numerical outcome of a specific non-linear relationship equation, such as the Body Mass Index of a person, based jointly on multiple variables, in this case the two factors being their height and weight.