How to store a legal chess position in log_2(8726713169886222032347729969256422370854716254) ~ 152.6 bits ~ 19.1 bytes using rankings:

https://github.com/tromp/ChessPositionRanking

But the major point of this project is to allow for random sampling of positions with a decent likelihood of getting a legal one, which allows for accurate estimation of the number of legal positions.