The standard working length of a reamer in endodontics is 16 mm because it corresponds to the average length needed to adequately prepare and clean the root canal system without damaging periapical tissues. This length allows for efficient removal of dentin while maintaining control within the canal. Using a reamer of 16 mm helps in reaching the apical third of most anterior and premolar teeth, ensuring thorough debridement. Longer or shorter reamers may compromise cleaning efficacy or cause procedural errors. Therefore, the 16 mm length balances accessibility and safety during root canal preparation.

Reference: Endodontics: Principles and Practice, Mahmoud Torabinejad, Edition 5.