There are several methods out there to do the calculations, but perhaps the most common one is to use the hydrometer again when fermentation is complete to get the Final Gravity reading (reached when the number doesn't change between separate readings 3 days apart). From there it's a bit of math:
(Original Gravity - Final Gravity) x 131.25 = ABV (%)
1.053 - 1.008 = 0.045 x 131.25 = 5.9% ABV
Hope this helps.
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Secondary: Ex-Pat 1800 English IPA
Bottled: Black IPA, Hoppiness is an IPA
Drinking: Port O' Palmer Robust Porter, Belgian Honey Wit, Nae Foolish Wee Heavy, Devil's Milk barley wine