When intervals are inverted their original quality of either consonance or dissonance may change. This is beacuse the register of the two notes, and spacing between them have been altered. Examples of Interval inversions is a perfect 4th to a perfect 5th or unison becomes an octave etc.
Below is a simple chart of interval inversions.
Intervals paired in the center column (highlighted in gray) are expressed using standard interval number-names. The two intervals in each line are inversions of each other. Corresponding half-step numbers (Cipher numbers) appear at the far left and right. Those numbers, if added together, always equal twelve degrees, i.e. one octave. Note, an octave interval, when inverted, becomes Unison — no distance between tones, i.e. two identically pitched tones. This interval is sometimes called Perfect-Prime (abbr.. PP) or P-1. Prime means first or one.
It would be a good idea to memorize these! | 
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