A hydrometer is an essential tool for measuring your wort’s specific gravity. It is a hollow glass tube that is perfectly weighted, usually with lead, containing a precision scale of numbers. The hydrometer is placed inside a tall, skinny, cylindrical container which holds the liquid you are attempting to measure. The hydrometer will float and reveal the liquid’s specific gravity at the liquid’s top line.
A standard hydrometer (well, mine anyway) will provide a range of numbers from 0.980 to 1.120. You can also purchase hydrometers with a smaller range if you are concerned with precision, or if your vision isn’t its greatest. By measuring your wort’s specific gravity with a hydrometer before and after fermentation, you’re able to calculate the alcohol content of the finished beer.
An alternative approach to measuring specific gravity without a hydrometer, would be to use a refractometer.