Short answer: a common year has 365 days, and a leap year has 366.
When each applies
Most years have 365 days. Leap years, which occur roughly every four years, add February 29 for a total of 366. The most recent leap years were 2020 and 2024. The next one is 2028.
The full rule: a year is a leap year if divisible by 4, except century years, which must also be divisible by 400. That is why 2000 was a leap year but 1900 was not.
Days in each month
| Month | Days |
|---|---|
| January | 31 |
| February | 28 (29 in leap years) |
| March | 31 |
| April | 30 |
| May | 31 |
| June | 30 |
| July | 31 |
| August | 31 |
| September | 30 |
| October | 31 |
| November | 30 |
| December | 31 |
A classic memory aid is the knuckle trick. Make a fist and count the months across your knuckles, starting with January on the first knuckle. Every month that lands on a knuckle has 31 days; every month in a valley has 30, except February. It works because the 31-day months genuinely alternate that way, with July and August as the one place two knuckles sit side by side.
The century rule’s long game
The “divisible by 400” exception is easy to dismiss until you see its effect at scale. Without it, every century year would be a leap year, and the calendar would gain about three extra days per 400 years. With it, each 400-year cycle contains exactly 97 leap years instead of 100, tuning the average year to 365.2425 days.
In practice this means: 2000 had a February 29, 2100 will not, and neither will 2200 or 2300. If you are computing spans that cross 2100, say a long-range projection or a very long payment schedule, those missing leap days change the total. For everyday spans under a century, the only century year that ever matters is 2000, and it behaved normally.
Why the answer is not exactly 365.25
Earth’s orbit takes about 365.2422 days. The leap year system (adding a day every 4 years, skipping most century years) averages the calendar year to 365.2425 days, astonishingly close to reality. The leftover drift amounts to about one day every 3,000 years.
When the difference matters
- Age calculations: total days lived changes depending on how many leap days fall in your lifetime.
- Contracts and interest: a 366-day year shifts daily-rate math slightly.
- Anniversaries: events anchored to February 29 need a rule for non-leap years.
- Loan and savings interest: banks often compute daily interest as the annual rate divided by 365, even in leap years (the “actual/365” convention), while others use 366. On a large balance, that one-day difference is real money.
- Subscriptions and warranties: a one-year subscription bought on March 1, 2023 ends March 1, 2024, which spans 366 days because it includes February 29, 2024. Bought March 1, 2024, the same “one year” is 365 days.
- Payroll: salaried staff paid monthly earn the same per month either way, but daily-rate contractors working through a leap year log one extra billable day.
Our Age Calculator counts every leap day in your lifetime automatically.
A year is not always 365 or 366 days
Astronomers use several different “years.” The tropical year (365.2422 days) is what the calendar tracks. The sidereal year (365.2564 days) measures one full orbit relative to the stars. And calendar history has oddities: when Britain adopted the Gregorian calendar in 1752, the year had only 355 days because 11 days were skipped outright. For every modern calculation, though, the answer stays 365 or 366.