Your benefit is based on your highest 35 years of earnings, adjusted for wage inflation. Those earnings are averaged into a monthly figure (AIME), and a progressive formula with two 'bend points' converts it into your Primary Insurance Amount (PIA) — the benefit you'd receive at full retirement age. The formula replaces 90% of the first slice of average earnings, 32% of the next, and only 15% above that, so it deliberately favors lower earners.
This calculator approximates that process from a single average-earnings figure, which is why it's an estimate rather than an exact number. Your real benefit reflects your specific 35-year record. The Social Security Administration publishes your actual projected benefit in your online statement at ssa.gov — always use that for real planning.