Disregarding individual values, effort values, and Nature, a level 100 Pokémon's stats in Attack, Defense, Speed, Special Attack, and Special Defense will be exactly 5 more than double its base stats in each, while the HP stat will be 110 plus double the base stat (except in the case of Shedinja, whose HP is always 1). Source.
So, here are your formulas (where x is the base stat):
HP = 2x + 110
All other stats = 2x + 5
So, for example, say a Pokemon has an attack stat of 125. You would disregard Natures, EVs, and treat this stat as if it had a 0 IV. The equation you would get is:
125 = 2x + 5, or x = 60. So the base attack stat of said Pokemon is 60.
This is how Pokemon sites find out Pokemon's base stats. They discover patterns and plug in numbers.