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Seems that when I was brewing my batch I forgot to take a gravity reading. Is there any other way to calculate ABV% without this measure?

Thanks in advance

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marked as duplicate by mdma Apr 9 at 21:15

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You can have a lab analysis done on it, but that's not practical.

Without knowing the sugar content of the pre-fermented wort, you really have no other accurate way of determining alcohol content.

But you can take a couple guesses.

First, examine your notes from previous and upcoming brews to determine your typical efficiency. Apply this to the recipe you brewed (perhaps using some beer recipe software). Depending on the consistency of your brew system, this is a good starting point.

Next, consider your fermentation. Once the beer is done fermenting, take the final gravity. Look at the typical characteristics for the yeast in terms of attenuation. Now work backwards to guess at your original gravity.

Example: Assume you have a beer that finished at 1.015 and that the yeast has a typical apparent attenuation of 75%. You can estimate the OG as follows:

15 * (1 / (1 - 0.75)) = 60

This would indicate a 1.060 original gravity. The generic formula is:

((FG - 1) * (1 / (1 - (atten / 100))) + 1

where:
FG is final gravity (e.g. 1.015)
atten is the typical attenuation of your yeast strain expressed as a % 

I'd use these two techniques to come up with a good guess, depending on your confidence in either one.

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