Loons are similar on the water, but lack hooked bills. Anhinga has a long, pointed bill and a much longer tail. All adult cormorant species in the U.S. are separable by the shape and color of the gular areas. No other species has orange lores and gular region that does not form a point at the gape. Neotropical Cormorant can be similar but is slimmer and longer-tailed, and has a differently shaped gular area. Great Cormorant is also similar but has a yellowish, pointed gular area surrounded with white as an adult. Immatures are dark-chested and pale bellied, unlike Double-crested.
Length and wingspan from: Robbins, C.S., Bruun, B., Zim, H.S., (1966). Birds of North America. New York: Western Publishing Company, Inc.