iUnit is a unit conversion program for the Macintosh and Windows. You can convert between 1545 different units quickly and easily. iUnit also displays the values of 91 physical constants. In addition, there is the Basis Weight calculator, which converts between basis weight and thickness, taking into account the density of the material, the density can also be calculated from the other two. Just enter in any two of the three values, making sure to leave the other field blank. Select the correct measurement units, and click on the Calculate button.