Most Efficient Way To Perform A Basic Table Lookup?

I want to create a simple program which can convert Units of Measure (e.g inches into centimeters, milliliters, meters, feet, etc). I want to create a function which has two parameters (unit to convert from and unit to convert to) and returns a conversion value (e.g. funciton passed parameters values inch and centimeter and returns value 2.54). In terms of strategy for the least processing overhead I am exploring a few options. I guess the slowest way would be some kind of nested If/Then or Case loop, so I wanted to use something similar to a 2D table lookup in excel (where the top row and the left column (key?) would have the unit types, and the conversion values would occupy the cells in the table body). Is this possible in VB? Could I load all of this into a 2D array or something? I read a bit about hashtables and dictionaries as well, Ideally Im looking for the least processor-hungry code since this function may be called upon frequently. Ill be using other functions to perform similar lookups on other 2D tables elsewhere too (i.e. matching a value on the top row and the left column and returning the coincident cell value in the table body).