Like T.J. I use iEngineer on my iPhone and iPod and iPad. When in my shop, however, I just look at the walls that are plastered with
large print charts for conversions, tap/drill sizes etc. On thing that I like about iEngineer is that it also easily shows things like major/minor diameters, drill sizes for different fits (tight-normal etc), strengths of screws in multiple materials and grades (proof load, advised torque, lubricated torques, tensile strengths, clamping force, allowable shear forces through threads or shoulders, head types and sizes) etc. etc. etc.
All this is readily available without having to refer to multiple pages and charts in books ( I have nothing against books, I love them ) etc.
Its one of the few phone apps that I find are easier to use than the charts, tables and books.
CHuck the grumpy old guy