Alchemy, in Full Metal Alchemist, is based on the concept of 'one needs to provide materials of equal value compared to the thing one want to create' (equivalent exchange).
But, how does it exactly work. Can this amount of required materials be calculated by the alchemist? If so, how? Are there some sort of lookup-tables?
Or does one need to guess and provides something of more value to be on the safe side? If so, can one become a more skilled alchemist by experience of successful guesses?
Also, different alchemists might have different specialized skills. How does that work? For the guessing part, I can imagine that one have a preference for certain kinds of alchemy and thus gain experience with a specialized skill.
Edit: Madara Uchiha's answer suggests that there are always materials required. I was also wondering about the situation where this is not the case and the alchemist was successful none the less.
In the end Edward trades his alchemy skill in exchange for returning Alphonse's body to him.