Sludge metal generally combines the slow tempos, heavy rhythms and dark, pessimistic atmosphere of doom metal with the aggression, shouted vocals and occasional fast tempos of hardcore punk. As The New York Times put it, "The shorthand term for the kind of rock descending from early Black Sabbath and late Black Flag is sludge, because it's so slow and dense." Many sludge bands compose slow-paced songs that contain brief hardcore passages (for example, Eyehategod's "Depress"). However, some bands emphasize fast tempos throughout their music. The string instruments (electric guitar and bass guitar) are heavily distorted and are often played with large amounts of feedback to produce an abrasive, sludgy sound. Additionally, guitar solos are often absent. Drumming is often performed in typical doom metal fashion, but drummers may employ hardcore d-beat or double-kick drumming during faster passages. Vocals are usually shouted or screamed, and lyrics are generally pessimistic in nature. Suffering, drug abuse, politics and anger towards society are common lyrical themes.
Many sludge metal bands from the Southern United States incorporate Southern rock and blues influence, although not all sludge bands share this style. So-called "atmospheric" sludge bands adopt a more experimental approach and compose music with an ambient atmosphere, reduced aggression and philosophical lyrics. Due to the similarities between sludge and stoner metal, there is often a crossover between the two genres, but sludge metal generally avoids stoner metal's positive atmosphere and its usage of psychedelia. Sludge metal also bears some musical and lyrical resemblance to crust punk, due to the usage of political lyrics and thick, 'dirty' guitar sounds.