20 CFR 404.312: How is my old-age benefit amount calculated?

(a) If your old-age benefits begin in the month you attain full retirement age (as defined in § 404.409), your monthly benefit is equal to the primary insurance amount (as explained in subpart C of this part).

(b) If your old-age benefits begin after the month you attain full retirement age, your monthly benefit is your primary insurance amount plus an increase for retiring after full retirement age. See § 404.313 for a description of these increases.

(c) If your old-age benefits begin before the month you attain full retirement age, your monthly benefit amount is the primary insurance amount minus a reduction for each month you are entitled before you attain full retirement age. These reductions are described in §§ 404.410 through 404.413.