John Calvin speaks about free will in the following six ways:
1. Adam had free will and freely chose to sin.
2. Adam lost free will because free will requires the mind to deliberate and to discern good from evil before choosing one of these ends. In this sense, free will no longer exists in humans because our natures have become corrupted.
3. Yet all people by nature have an impulse toward the good. This impulse, however, is an animal impulse which does not follow from reason deliberating between good and evil and choosing one or the other (a requirement for free will; Inst. 2.2.26).
4. The regenerate have free will restored to them in part now, and then in full in heaven.
5. As a consequence, fallen people lack the ability to do good (since that requires free will). Fallen people then only do moral evil (since the impulse towards good seems, for Calvin, to be vice since not rational).
6. When fallen people sin then, they do so by necessity but not by compulsion (Inst. 2.3.5). In this way, Calvin seeks to affirm that humans sin willingly without compulsion, even if they necessarily sin.
As a side note, he follows the logic of Bernard of Clairvaux and Augustine as he distinguishes how people willingly sin apart from compulsion yet do so by a necessity of nature (remember: free will requires reason to distinguish good from evil before the will chooses it; Adam lost us this creaturely capacity, and so our nature is totally corrupt in all its faculties. Now, we do good by animal impulse not reasonable choice).