GC Number GC-ID
This tool uses a custom base-31 conversion to convert the GC Number to or from the GC-ID. The characters used for the base-31 conversion are 0123456789ABCDEFGHJKMNPQRTVWXYZ (the letters ILOSU are omitted). When converting from a base-31 GC-ID back to its original numerical ID, you have to subtract 411120 due to the change in cache numbering after cache GCFFFF where the base was changed from 16 to 31.
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Adapted from GeocachingToolBox.com