Social Security Questions? Ask a Social Security Expert
Social Security calculates your average indexed monthly earnings during the 35 years in which you earned the most. They apply a formula to these earnings and arrive at your basic benefit, or “primary insurance amount.” This is how much you would receive at your full retirement age.
If you have not yet done so you should check your earnings record to make sure that all are shown.
You can create an account with SSA online here
Then you can see all your information and make sure all your earnings are shown.