Base conversion is an approach commonly used for URL shorteners. Base conversion helps to convert the same number between its different number representation systems.

The conversion table is [0-9a-zA-Z]:

private const string BASE62 = "0123456789abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ";

The value 999 is converted into g7 as:

999 = 16 * 62^1 + 7 * 62^0 = "g7"

where g and 7 are the 16th and 7th elements of the BASE62.