The simple answer to how you measure the percent alcohol by volume (%ABV) of your cider is that you subtract your final specific gravity (FG) from your original specific gravity (OG) and multiple this number by 128.
If you’ve read some of my other articles, you probably realize there are rarely simple answers when discussing hard cider. That is because there are so many variables in the cider making process. Without directly measuring it with expensive equipment means it’s about impossible to get it exact. You can get close but there are just so many factors that can change. That is why this calculation for measuring the percent alcohol or %ABV is just an estimate. You can find “more accurate” calculations but, the calculated difference is small because the margin of error is large. In other words, you can use a calculation that appears to create a more accurate number but, the variables used in the calculation all have a significant amount of error.
For example, the refractometer or hydrometer you are using has a margin of error. You can get a more accurate device, but does that help? The largest factor of error usually comes from the yeast you use. All yeast are different. They each have different efficiencies in their ability to convert sugar into ethanol. Saccharomyces cerevisiae is relatively efficient at processing sugar into ethanol but each yeast strain will be slightly different. Non-Saccharomyces strains are less efficient. They tend to produce more alternative compounds like glycerol versus ethanol. Nutrients also play a part in the efficiency of yeast as yeast have preferences for sugar and nutrient sources as well as different genes or pathways for processing them. You might also be wondering why 128 as there are often references to use 131. This difference is an example of the nutrient make-up of the juice or must you are fermenting. The 131 is a better estimator for beer but, the compounds more commonly found in cider make the 128 a better estimator of the %ABV.
Ultimately, the difference associated with all the variables is small enough that it is usually negligible for reporting %ABV in most countries so the estimate is good enough. If you need better accuracy, send it to a lab (Ex: White Labs) that can provide a detailed analysis of your cider. They also usually provide a full range of the many compounds, acids, tannins, sugar, and polysaccharides, that are present.
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