The meter of “The Rime of the Ancient Mariner” is somewhat loose, though in general it approximates a form known as ballad meter. Ballad meter typically consists of four-line stanzas that feature alternating lines of tetrameter and trimeter. The first and third lines usually contain four stressed syllables, and the second and fourth lines contain three stressed syllables. As the term ballad indicates, this type of meter has a long history in songwriting, and particularly for songs that recount longer narratives and histories. In this regard, ballad meter is appropriate for “The Rime of the Ancient Mariner,” in which the titular Mariner recounts a lengthy tale about a troubling sea voyage he experienced earlier in his life. Yet unlike a traditional ballad, Coleridge didn’t intend for his poem to be sung. We can infer as much from his use of variable stanza lengths and their shifting patterns of tetrameter and trimeter lines. Though most of the stanzas are typical four-line ballad stanzas, Coleridge frequently includes longer stanzas that range between five and nine lines. The use of stanzas with odd numbers of lines would interrupt the expected rhythmic flow were a vocalist to attempt to sing the “Rime.”

Despite the irregularity of the stanza lengths, Coleridge maintains a somewhat regular use of iambic rhythm throughout. Iambic rhythm is based on the two-syllable iamb and its characteristic duh-DUM stress pattern. The four-syllable lines in the poem are thus written in iambic tetrameter, and the three-syllable lines are in iambic trimeter. The poem’s underlying iambic rhythm may not be initially obvious, given how the opening stanza features several variations on that rhythm (lines 1–4):

     It is / an an- / cient Mar- / i-ner,
     And he stopp- / eth one / of three.
     “By thy long / grey beard / and glitt- / er-ing eye,
     Now where- / fore stopp’- / st thou me?”

Only the first line demonstrates perfect iambic rhythm. Each of the other three lines features at least one anapest, which is a three-syllable foot with two unstressed syllables followed by a stressed syllable. The second and fourth lines each have one anapest, and the third line has two. Whereas the opening line attempts to establish iambic rhythm, the next three lines interrupt that rhythm in a way that reflects the Mariner’s interruption of the Wedding-Guest. Though initially destabilized by the Mariner’s sudden appearance, the Wedding-Guest finds his rhythm again in the second stanza (lines 5–8):

     “The Bride- / groom's doors / are op- / ened wide,
     And I / am next / of kin;
     The guests / are met, / the feast / is set:
     May'st hear / the mer- / ry din.”

The strict iambic rhythm of this stanza reestablishes metrical regularity, which remains firmly in place for the duration of the poem.