Hydrometer

Put your liquid in the hydrometer tube, spinning the hydrometer and measuring the original gravity (OG) when it settles. Try to make sure there is as little carbonation (bubbles) as possible in this process. Once you think it's done fermenting, take another measurement with the hydrometer for final gravity (FG).

(OG - FG) * 105 = {% Alcohol by Weight}
{% Alcohol by Weight} * 1.25 = [% Alcohol by Volume]