Chimney Flue Size Guide

An undersized flue causes smoke spillback and carbon monoxide risk. An oversized flue draws cold air and reduces draft efficiency. The NFPA 211 standard requires the flue cross-sectional area to be at least 1/10 of the fireplace opening area (for flues under 15 ft) or 1/12 (over 15 ft). Enter your fireplace dimensions to find the minimum required flue size.