The calculations are complicated. The amount of any benefit is determined by a formula based on the average of your yearly reported earnings since you began working.
But to complicate matters further, Social Security computes your average earnings differently depending on your age. If you reached age 62 or became disabled on or before December 31, 1978, Social Security averages the actual dollar value of your total past earnings -- and bases the amount of your monthly benefits on that amount.
If you turned 62 or become disabled on or after January 1, 1979, Social Security divides your earnings into two categories: Earnings from before 1951 are credited with their actual dollar amount, up to a maximum of $3,000 per year; and from 1951 on, yearly limits are placed on earnings credits, no matter how much you actually earned in those years.