Convert from zeptometer to digit

Unit Definition (zeptometer)
The zeptometre (or zeptometer), symbol: zm, is an SI unit of metric length measurement. 1 zeptometre is 1e-21 metres, which is a very small distance

Unit Definition (digit)
The digit is a historic unit of distance equal to the width of a person's finger. Used in all the ancient civilizations of the Middle East and Mediterranean, the digit was equal to 0.75 inch or 19 millimeters with only the smallest variations. Typically, there were 4 digits in a palm, 16 in a foot, and 24 (sometimes 28) in a cubit. The word digit is from the Latin word for a finger or toe, digitus.