Three steps: an average, a formula, an age
First SSA averages up to 35 years of your earnings, each year lifted to the wage level of the year you turn 62, and divides by the 420 months in those 35 years. Years you did not work count as zero, which is why the divisor is always 35 years rather than however long you happened to work. The result is rounded down to the dollar, and it is called the average indexed monthly earnings, or AIME.
Then the AIME goes through three brackets. For 2026 the first $1,286 of it is paid at 90 percent, the amount between $1,286 and $7,749 at 32 percent, and everything above $7,749 at 15 percent. The total is rounded down to the next lower ten cents and is the primary insurance amount (PIA) - the benefit you would receive if you started exactly at full retirement age. Those bend points are republished every autumn; the ones on this page are the 2026 figures.
The last step is the age. Start before full retirement age and the benefit is cut, permanently. Start after and it grows by delayed retirement credits, at 2/3 of one percent a month, which is 8 percent a year. The final payment is the adjusted PIA rounded down to the whole dollar.
What the claiming age costs, month by month
The early-retirement reduction is two rates, not one. The first 36 months before full retirement age cost 5/9 of one percent each; every month beyond that costs 5/12 of one percent. With a full retirement age of 67, sixty months early is 36 months at 5/9 of one percent plus 24 months at 5/12 of one percent, which is 30 percent - so a benefit started at 62 is 70 percent of the PIA, for the rest of the recipient's life.
The published percentages at each whole year, for anyone born in 1960 or later, are 70 at 62, then 75, 80, 86 2/3, 93 1/3 and 100 at 67 - and 124 at 70. Delayed credits stop once 69 is over: a benefit started at 70 and one started at 75 pay exactly the same, so waiting past 70 buys nothing but three fewer years of payments.
Full retirement age itself depends on the year you were born, and it is not a birth date you can look up in one line: someone born in 1959 has a full retirement age of 66 and 10 months, someone born in 1955 reaches it at 66 and 2 months, and anyone born in 1960 or later waits until 67. Anyone born on 1 January uses the previous year's row, which is SSA's own rule.
What this page does not model
Several things are left out on purpose, each one a place where a calculator can be confidently wrong rather than merely approximate. The first is the wage indexing of your actual record: this page takes your earnings at face value, so if your pay kept pace with average wages the estimate is close, and if it started low and rose fast the real figure is higher.
The second is the cost-of-living adjustments that arrive every December once a benefit starts. This is a 2026-formula figure in today's dollars, which is the right way to compare claiming ages and the wrong way to forecast a check in 2041. The third and fourth are the federal income tax on benefits, which can reach 85 percent of them depending on other income, and the Medicare Part B premium, which is deducted before the payment reaches you. Both are real and both are published elsewhere; neither changes the comparison this page is for.
Spousal and survivor benefits. They are percentages of somebody else's PIA - a spouse's benefit is at most half of the worker's PIA and no less than 32.5 percent of it starting at 62, and a survivor's benefit is the larger of the two records - but they also depend on the other person's claiming age and record, which this page never sees. The family maximum caps what a worker and dependents can be paid between them, and it is not worked out here either.
Two ways in, and what each one proves
Typing the average indexed monthly earnings from a statement makes the page exact: it applies the published bend points to a number SSA has already worked out for that record. Typing what you earn now instead produces an estimate, because the page does not know the other 34 years of the record or how each year was indexed before the average was drawn.
Both paths use the same bend points and the same age percentages, so the 62-to-70 table is worth the same in either case once the AIME is settled. The page says which of the two it is answering, because one is a quotation and the other is an estimate, and the difference matters for planning.