How age is calculated
Age is counted in completed years. You are 30 until the day you complete 31 full years, at which point you become 31 — the system used almost everywhere and the one this calculator implements.
Getting the months and days right is harder than it looks, because months have different lengths. The calculation works down from years to months to days, borrowing from the preceding month when the day-of-month would go negative. Because that preceding month may have 28, 29, 30, or 31 days, the same interval can produce different day counts depending on where it falls in the calendar.
| Years | Months | Weeks | Days | Hours | Minutes |
|---|---|---|---|---|---|
| 1 | 12 | 52 | 365 | 8,766 | 525,949 |
| 5 | 60 | 260 | 1,826 | 43,829 | 2,629,746 |
| 10 | 120 | 521 | 3,652 | 87,658 | 5,259,492 |
| 18 | 216 | 939 | 6,574 | 157,785 | 9,467,086 |
| 21 | 252 | 1,095 | 7,670 | 184,082 | 11,044,933 |
| 30 | 360 | 1,565 | 10,957 | 262,975 | 15,778,476 |
| 40 | 480 | 2,087 | 14,610 | 350,633 | 21,037,968 |
| 50 | 600 | 2,608 | 18,262 | 438,291 | 26,297,460 |
| 65 | 780 | 3,391 | 23,741 | 569,778 | 34,186,698 |
| 80 | 960 | 4,174 | 29,219 | 701,266 | 42,075,936 |
| 100 | 1,200 | 5,217 | 36,524 | 876,582 | 52,594,920 |
Leap years and February 29 birthdays
A leap year occurs every four years, except century years, except century years divisible by 400. So 2000 was a leap year, 1900 was not, and 2100 will not be. The rule keeps the calendar aligned with the solar year of about 365.2422 days.
People born on February 29 — roughly one in 1,461 — have a legal birthday that varies by jurisdiction in non-leap years. Some define it as February 28 and others as March 1, which occasionally matters for driving ages, alcohol purchase, and pension eligibility. This calculator counts completed years, so a leapling's age increments on February 28 in common years.
| Year | Leap year? | Why |
|---|---|---|
| 1900 | No | Divisible by 100 but not 400 |
| 1996 | Yes | Divisible by 4 |
| 2000 | Yes | Divisible by 400 |
| 2023 | No | Not divisible by 4 |
| 2024 | Yes | Divisible by 4 |
| 2026 | No | Not divisible by 4 |
| 2028 | Yes | Divisible by 4 |
| 2100 | No | Divisible by 100 but not 400 |
| 2400 | Yes | Divisible by 400 |
Different counting conventions
The Western convention of counting completed years is not universal. In the traditional East Asian age system, a person is one year old at birth and gains a year at the lunar new year rather than on their birthday, which can make someone up to two years older than their Western age. South Korea officially moved to the international system in 2023, though the traditional count persists socially.
Other conventions exist in specialized domains. Actuarial work sometimes uses the nearest birthday rather than the last one. Insurance underwriting occasionally uses age at nearest birthday, which can shift a premium band. And gestational age in pregnancy is counted from the last menstrual period, running about two weeks ahead of the embryo's actual age.
Age milestones in the United States
Many rules key off exact age, which is one reason to know it to the day. The thresholds below are federal or near-universal; states vary on driving, and retirement figures depend on birth year.
| Age | What changes |
|---|---|
| 16 | Driving licence in most states (with restrictions) |
| 18 | Vote, sign contracts, enlist, jury service; legal adulthood |
| 21 | Purchase alcohol and tobacco (federal) |
| 25 | Rent a car without a young-driver surcharge at most agencies |
| 26 | Age out of a parent's health insurance plan |
| 50 | Catch-up contributions to 401(k) and IRA accounts |
| 59½ | Withdraw from retirement accounts without the 10% penalty |
| 62 | Earliest Social Security retirement benefits, at a reduced rate |
| 65 | Medicare eligibility |
| 66–67 | Full Social Security retirement age, depending on birth year |
| 70 | Maximum Social Security benefit; no gain from waiting longer |
| 73 | Required minimum distributions begin (75 from 2033) |