This famous sonnet by John Donne does not follow a regular metrical scheme, and indeed does not possess a very tight organisation as a poem. We can say that on the whole it is written as a loose iambic pentameter, with the word "loose" denoting the fact that quite a few lines have more than ten syllables, and that the metre of the lines is definitely not regular or constant. Consider the first two lines as an example of the poem's variation on form:
Batter my heart, three-person'd God ; for you
As yet but knock ; breathe, shine, and seek to mend...
Both lines have ten syllables, but it is clear that there is not a regular iambic meter, as the first line starts off with the stress on the first syllable, and the second line contains a number of spondees, serving to emphasise the words "knock; breathe, shine." Meter therefore is something that is fluid in this poem and used by the poet to emphasise certain aspects of the poem rather than something to be slavishly adhered to.