Chimney caps come in varied sizes based on length and width. Sizes from 6-by-5 inches to 13-by-18 inches are most standard, although sizes can vary based on flue shape. Some chimney cap sizes are also measured by mesh height in inches.
Chimney caps have different heights and widths based on whether the flue shape is oval, rectangular, round or square. Flues can be made of traditional masonry or steel, which can also affect chimney cap size. The eFireplaceStore.com website offers a guide on how to measure a flue for chimney cap size, depending on flue type and material and how many flues a chimney has.