As general rules of thumb: If more than half of a tooth's original crown portion (the part of the tooth normally visible above the gum line) has been lost, a post is needed to assist with anchoring the core to the tooth. If more than half of the tooth's crown still remains, a core by itself will probably suffice.

Why do I need a post for a crown?

What is a post crown? In root-filled teeth it may be necessary to insert a post into the tooth root before placing a crown. A post gives support and helps the crown to stay in place. The surface of the tooth may be removed down to the level of the gum.

Do dental crowns need a post?

Post and core is a dental restoration treatment that's sometimes performed after a root canal. When a significant portion of a tooth's structure is removed, a post and core can help keep a dental crown in place. Dental crowns are put on top of teeth, to protect them from further damage or infection.