Use a hydrometer to determine the alcohol content of a drink.
A hydrometer is an instrument that measures the density (thickness) of liquids relative to the density of water. This measure is known as specific gravity. Hydrometers are calibrated so that water at a set temperature (usually 20C) measure 1.000.
The combination of 5 gallons of water with 2.7kg of malt extract will result in specific gravity of 1.035 because when any solids are dissolved into the water, the density increases as the yeast ferments the dissolved sugars into alcohol and carbon dioxide, the density of the liquid drops because alcohol is less dense than water.
Most hydrometers are calibrated to read accurately at 20 C (it should say on the hydrometer or in the instructions).
If you were to measure the specific gravity of a liquid at 27 C, then you would get an inaccurate figure. This figure is because as liquids heat up, they become less dense,
For example, try heating some honey, the warmer it gets, the runnier it becomes, and the same things happen to beer. For every 5.6 C, your measurements will be off by .002.
So at 27C, your brew measures 1.038 you've got to add .0025 points to know the real specific gravity is 1.040, or wait until the temperature drops.
To calculate the alcohol content of your brew, you take a hydrometer reading before adding the yeast (Original Gravity OG) and then another before bottling/barreling (Final Gravity FG), taking into account any temperature differences.
You then multiply by 105 to get the alcohol per cent by weight.
Original Gravity: 1.050
Final Gravity: 1.010 = 0.040 x 105 = 4.2%
To convert the alcohol by weight to alcohol by volume (ABV) you have to multiply the weight by 1.25
ABV = 4.2 x 1.25 = 5.25%
ABV is the measure that is used by most commercial brewers although you do see OG and FG figures on some brews.
Potential Alcohol: Hydrometers usually have these measurements on it too which you may wish to use. Just record the starting figure, then subtract the finishing figure to get the % alcohol.