You can use your hydrometer and just need the reading from when you started and the finished reading, a link is here to one of the calculations that can be used, some hydrometers have their own calculations included on the instructions too;
With the OG reading 1.040 and the FG reading 1.002 we have worked out the approximate alcohol content using two different formulas, one based on the hydrometer instructions and the chart they give, and one using a calculation, both came out with very similar readings at around 5%
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You can use your hydrometer and just need the reading from when you started and the finished reading, a link is here to one of the calculations that can be used, some hydrometers have their own calculations included on the instructions too;
http://homebrewonline.vanillaforums.com/discussion/81/how-to-use-your-hydrometer#latest