My co-workers found a bunch of our old backup tapes and I have been trying to find out the length of the tape that fits on these spools. We found out that sticking a magnet on the ceiling, and then the unpacked tape spool to the magnet, that the whole spool would unravel itself into a big pile of tape (fun practical joke).

Out of curioisity, I have been trying to find the length of the tape on the spool, but have not had much luck searching about. I’ve taken some rough measurements and am hoping someone could do the calculations for me because my first result was something in the neighborhood of 4,900 km. I’m fairly certain that my math skills have diminished since graduating a decade ago.

Entire spool diameter = **9cm**

Inner spool that the tape is wrapped around = **4.2cm**

Rough estimate of tape thickness(based of a cassette tape) = **10 micrometers (.001cm)**

Thanks ahead of time!