In the sonnet, the poet compares his beloved to the summer season, and argues that his beloved is better. The poet also states that his beloved will live on forever through the words of the poem. Scholars have found parallels within the poem to Ovid's Tristia and Amores, both of which have love themes. Sonnet 18 is written in the typical Shakespearean sonnet form, having 14 lines of iambic pentameter ending in a rhymed couplet. Detailed exegeses have revealed several double meanings within the poem, giving it a greater depth of interpretation.
The poem starts with a line of adoration to the beloved—"Shall I compare thee to a summer's day?" The speaker then goes on to say that the beloved being described is both "more lovely and more temperate" than a summer's day. The speaker lists some things that are negative about summer. It is too short—"summer's lease hath all too short a date"—and sometimes the sun shines too hot—"Sometime too hot the eye of heaven shines." However, the beloved being described has beauty that will last forever, unlike the fleeting beauty of a summer's day. By putting his love's beauty into the form of poetry, the poet is preserving it forever by the power of his written words. The hope is that the two lovers can live on, if not through children, then through the poems brought forth by their love which, unlike children, will not fade.
Scholars have outlined this poem's similarities to a portion of Ovid's Tristia. Near the end of this book of poems, Ovid writes (translated from the original Latin): "What a monument I have raised to thee in my books, O my wife, dearer to me than myself, thou seest. Though fate may take much from their author, thou at least shall be made illustrious by my powers. As long as I am read, thy fame shall be read along with me." Ovid's Amores follows a similar vein: "So likewise we will through the world be rung / And with my name shall thine always be sung." Shakespeare is known to have used Ovid in many of his other works as well. In sonnet 18 he seems to have borrowed the general idea of immortality of the writer and his lover through poetry.
"Complexion" in line six, can have two meanings: 1) The outward appearance of the face as compared with the sun ("the eye of heaven") in the previous line, or 2) the older sense of the word in relation to the five humours. In the time of Shakespeare, "complexion" carried both outward and inward meanings, as did the word "temperate" (externally, a weather condition; internally, a balance of humours). The second meaning of "complexion" would communicate that the beloved's inner, cheerful, and temperate disposition is sometimes blotted out like the sun on a cloudy day. The first meaning is more obvious, meaning of a negative change in his outward appearance.
The word, "untrimmed" in line eight, can be taken two ways: First, in the sense of loss of decoration and frills, and second, in the sense of untrimmed sails on a ship. In the first interpretation, the poem reads that beautiful things naturally lose their fanciness over time. In the second, it reads that nature is a ship with sails not adjusted to wind changes in order to correct course. This, in combination with the words "nature's changing course", creates an oxymoron: the unchanging change of nature, or the fact that the only thing that does not change is change. This line in the poem creates a shift from the mutability of the first eight lines, into the eternity of the last six. Both change and eternity are then acknowledged and challenged by the final line.
"Ow'st" in line ten can also carry two meanings equally common at the time: "ownest" and "owest". Many readers interpret it as "ownest", as do many Shakespearean glosses ("owe" in Shakespeare's day, was sometimes used as a synonym for "own"). However, "owest" delivers an interesting view on the text. It conveys the idea that beauty is something borrowed from nature—that it must be paid back as time progresses. In this interpretation, "fair" can be a pun on "fare", or the fare required by nature for life's journey. Other scholars have pointed out that this borrowing and lending theme within the poem is true of both nature and humanity. Summer, for example, is said to have a "lease" with "all too short a date." This monetary theme is common in many of Shakespeare's sonnets, as it was an everyday theme in his budding capitalistic society.