To be legally sold, Calvados must be at least 40% abv. This alcohol content is obtained by the gradual addition of distilled or demineralised water to the eau-de-vie. This is known as reduction.
However, some “cask strength” Calvados are unreduced. They have a “natural” alcohol content created by the evaporation of the eau-de-vie over time. Taken directly from the top of the barrel, they are usually bottled on demand and show a wonderful concentration of aromas. Some “cask strength” Calvados can contain up to 55% vol.