Calculate Required Amount of Alcohol Needed to Obtain a Specific Proof in a Finished Liquor/Liqueur

Calculate how much base alcohol at a given percentage is required to make a finished product with a specific alcohol content for any volume (eg. 316ml of 95% alcohol is needed to make a 750ml bottle of 40% vodka- the rest of the bottle can be filled with water, sugar, essence, etc., without altering the proof of the finished liquor).
%abv alcohol to use

%abv desired

ml total desired

 ml of base needed