The formula is plain arithmetic. Social Security takes your `35` best earning years, adjusts them for wage growth, adds them up, and divides by `420`. That average is what your check is built on.
Say a year worth `$52,000` never made it onto the record. That pulls about `$124` a month out of the average. For a middle earner, roughly `32` cents of each of those dollars reaches the actual check. Call it `$40` a month. About `$480` a year, for as long as you live.
Over a `22`-year retirement that runs past `$10,000`. Every raise after that is a percentage of the smaller number. So the gap widens every January.
If you worked more than `35` years, the damage is smaller. Your next best year slides in and you lose only the difference. Worked `35` years or fewer? A zero slides in. That is the full hit.
Look at who lands in that second group. People who raised kids. People who worked overseas. Owners who ran one thin year and let it go. The people with the least room to absorb it.