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Alcohol proofing calculator
Alcohol is sold by volume and blended by weight, and ethanol and water shrink when they meet, so the obvious sum is wrong. This converts the strength to weight first, which is the only way the answer comes out right.
The sum, in words
Alcohol strength is written by volume, as percent ABV, but a blend is made by weight. So first turn both strengths into percent by weight. Then the water to add is the weight of alcohol you have, multiplied by the ratio of the two weight percentages, less the alcohol you started with.
Perfumer's alcohol at 96 percent ABV is 93.86 percent ethanol by weight. The 80 percent ABV you want is 73.51 percent by weight. For 1,000 g of the 96 percent: 1,000 multiplied by 93.86 divided by 73.51 is 1,276.79 g, and taking away the 1,000 g you started with leaves 276.79 g of water to add. That is about 1,486 ml of finished alcohol at 20 °C.
Why the obvious sum is wrong
Treat the two percentages as if they were by weight and you would add 200 g of water to that 1,000 g, not 276.79 g, and the alcohol would come out too strong. The gap is not rounding. It comes from two facts that the volume figure hides.
The first is that ethanol is lighter than water, so a percentage by volume and a percentage by weight of the same liquid are different numbers. The second is that ethanol and water shrink when they are mixed: a liter of each does not make two liters. A sum done in milliliters is wrong for both reasons at once, and a sum done by weight from a measured table is wrong for neither.
Where the conversion comes from
From a table of measured densities of ethanol in water at 20 °C, at steps of percent by weight, with the volume percentage worked out from each row and the figure between two rows read in a straight line. It is the same table the Orris Bench application uses when it has to turn a strength into a weight, so the two give the same answer.
Denatured alcohol is close, not exact
The table is ethanol and water. Perfumer's alcohol is usually denatured, which means a few percent of something else has been added to it, often another alcohol. That moves the density slightly, so the answer here is close rather than exact. Weigh what you actually add, and check the strength of the result before it goes into a batch.
The water matters too
Use distilled or deionized water. Minerals in tap water can cloud a perfume and react with some materials over time, and a faint haze that appears weeks after bottling is a batch nobody can sell.
What this calculator does not do
It does not correct for temperature beyond the 20 °C the table was measured at, or for what the denaturant does to the density. And it only takes alcohol down: water cannot make alcohol stronger, so a target above the strength you have is an answer this will not give.