The SELKIRK DSP8OTR Stove Pipe Adapter is designed to connect an 8-inch diameter oval stove pipe to an 8-inch round chimney pipe. Therefore, the "8 inch" measurement refers to the diameter of the round end of the adapter. If you specifically need an 8-inch diameter at the oval end, you would need to confirm with the manufacturer or supplier if this adapter can accommodate your requirements or if they offer a different size or model that meets your needs.