How do you calculate the severance payment?

It depends on your employer. First off, be sure that your employer even offers severance as one of their benefits. If they do, they may have a policy of a lump sum designated when you first sign on, or it may be a formula that increases with the number of years you are with the company. This article talks more specifically about severance. The most important part is knowing your employer's policy PRIOR to signing with the company.