Take a closer look at your hydrometer. Most have three scales showing specific gravity, degrees Balling, and potential alcohol. Typically, homebrewers use the specific gravity scale, which measures the density of wort relative to that of pure water. So, a wort with an original gravity of 1.048 is 4.8 percent heavier than the same volume of water. Since the scale is read out to three decimal places, specific gravity is one of the most precise measurements you’ll make as a homebrewer, but precision is not the same as accuracy.
Most brewers just assume that their hydrometer is close enough to correct, but it could be off by enough to affect the quality of the beer or make it harder to reproduce recipes. Skipping calibration is not worth it, especially because you can easily calibrate this useful tool.
Picking Known Points
Just like you’d calibrate a thermometer against known temperatures such as freezing and boiling water, you’ll use a pair of known data points to assess your hydrometer’s accuracy. The first point is easy: distilled water should read as 1.000 at the hydrometer’s reference temperature (usually 60°F/15°C or 68°/20°C).