A session beer is defined as a beer with an alcohol by volume or ABV of less then 5%. The purpose of a session beer is to permit the drinker to enjoy multiple beers at a sitting without becoming overly intoxicated.
Alcohol by volume (ABV) represents what portion of the total volume of a liquid is alcohol.
To calculate the ABV of a beer, you will want to subtract the Final Gravity from your Original Gravity and multiply by 131.
The following is an example of how to calculate your alcohol by volume, assuming that your original gravity was 1.055 and your final gravity reading was 1.012.
Here is an example: 1.055 – 1.012 = 0.043 x 131 = 5.633%
In this example your alcohol by volume worked out to 5.633%.