A Hydrometer Measures the density or specific gravity of a liquid. Used to give a reading to the gravity of your wort. 
Suspend the Hydrometer in a sample of your wort to get the reading in one of our 250mL Glass Cylinders or our 100mL Plastic Cylinder.
To calculate your % Alcohol By Volume 
(OG - FG) x 131.25 = ABV %
Where OG is your Original Gravity e.g. 1.050 as shown by your hydrometer before you pitch your yeast.
and FG "Final Gravity" e.g. 1.010 as shown by the hydrometer after it has fermented.
= ( 1.050 - 1.010 ) * 131.25 
= 5.25% ABV