Uncle Gimpy’s Intellectifier
An old reference sheet with simple formulae for length, area, volume as well as useful conversions. It’s called Uncle Gimpy’s Intellectifier, and is meant to be folded up and kept in a wallet or purse. This is certainly from a time before internet enabled personal devices (probably at least the 1980s), but these sorts of reference sheets are still just as useful today, if not less common.
It is mostly legible in its current state, but I intend to retype and upload a clean copy in the future.