Keystone Light (in any container) contains 4.20% ABV.
Introduced in 1989, Keystone Light is a refreshing light lager that is Always Smooth, Never Bitter™. Keystone Light is 4.2% alcohol by volume and has 100 calories per 12-ounce serving. Read consumer reviews on RateBeer.com.
Alcohol by volume (ABV) simply represents what portion of the total volume of liquid is alcohol. And to determine the ABV of a beer, a brewer typically uses what's called a hydrometer, which is an instrument that aids in measuring the density of liquid in relation to water (it essentially free-floats in a cylinder or liquid).
The hydrometer will be calibrated to read 1.000 in water (at 60°F), and the denser the liquid (example: add sugar to the liquid), the higher the hydrometer reading.
Learn how to calculate ABV on BeerAdvocate.com.
For information about calories, carbohydrates and the alcohol content of different beers, go to RealBeer.com.