I have a decayed tooth. how much tooth structure is needed for a crown only and how much warrants a post and core?

Depends: No numbers can be offered here. See a great dentist and get an opinion. If not satisfied, get another opinion.

Broken down tooth: There are no absolute values do determine if a crown or filling is needed. If the decay is very deep, then root canal therapy is needed in order to place the post. If you are unsure of the diagnosis, get a second opinion.

Depends: Depending on the extent of decay that needs to be removed, and the remaining tooth structure, root canal treatment, post and core, and a crown may be indicated. Every clinical situation is different and your dentist will determine what is needed in your case.

Professional opinion: If your tooth has had root canal because of decay, without seeing it my guess is you would need a post and core. If the decay didn't effect the nerve, than you tooth might require a pin buildup before a crown. If the decay goes below the gum line, you might need crown lengthening so as to not violate biological width. Your best option is to be carefully examined by a dentist and heed their advise!

Only 1.5 mm to 2 mm: of intact tooth structure above the crown. The purpose of a post is to retain a core that is needed because of extensive loss of coronal tooth structure after root canal. If concerned ask your dentist for more information.