1 standard drink equals 12oz of 5% ABV beer, 5oz of 12% wine, or 1.5oz of 40% liquor. This is equivalent to ~14 grams of pure alcohol.
Guava uses the Forrest method of the Widmark formula, which takes into account the total grams of pure alcohol consumed, as well as BMI (body mass index, which equals Weight/Height^2). Then, 0.015% is subtracted per hour since beginning drinking.
The calculation method used here is more accurate in some scenarios than most online BAC calculators, because the Forrest method utilizes height in addition to weight. This is important because BAC is based on body water content, which is higher in lean mass than fat mass.
However, BAC can still differ significantly from estimates (at least +/- 20%) for a handful of reasons including different body types, age, and food consumption. In particular, a large carb-heavy meal immediately before drinking can reduce peak BAC by up to ~50%, but notably will not reduce the time it takes the body to metabolize the alcohol.
Even a low level like .02% can reduce your ability to safely drive or operate machinery. The legal limit for driving a car varies by country, but for most of the United States it is .08%. Keep in mind that BAC estimates can be off by more than 20%, so it's smart to give yourself a very large buffer when making decisions.