Almost every severance policy in the US is a variation of one formula. You take a number of weeks of pay for each year you worked there, and you multiply it by your weekly gross pay. Everything else is detail: whether a part year counts, whether bonus and commission are in the weekly figure, and whether there is a cap on the total.
The formula
Severance = weekly gross pay x weeks per year of service x years of service
Worked example
Someone earning $1,500 a week who has been there 6.5 years, on a policy of one week per year of service, is looking at 6 weeks of pay, which is $9,000 counting completed years only. If the policy prorates the half year as well, it becomes 6.5 weeks, or $9,750. That single detail is worth $750 here, which is why the calculator shows both.
One week per year of service is the baseline most policies start from. Two weeks per year shows up at larger employers and for longer tenures, and senior packages are often written outside the formula entirely as a set number of months. Use the custom option above if your agreement says something else, including half weeks or a different unit.