How Many Days Are in a Year (and Why 366 Happens)

365 days — plus almost six hours

A common year is 365 days long. But one full orbit of the Earth around the Sun takes a little more — about 365 days, 5 hours and 49 minutes. Over four years that remainder adds up to nearly a whole day.

Why leap years exist

To stop those accumulated hours from shifting the calendar against the seasons, an extra day — February 29 — is added every four years. Such a year is 366 days long and is called a leap year.

The rule is slightly more precise than "every four years": years divisible by 100 are not leap years, except those divisible by 400. So 2000 was a leap year but 1900 wasn't. This keeps the calendar accurate for thousands of years.

Days in the months

A year has 12 months of different lengths: seven with 31 days (January, March, May, July, August, October, December), four with 30 (April, June, September, November) and February with 28 or 29. A simple way to remember is the "knuckle rule": the bumps on your fist are 31-day months, the dips between them are the shorter ones.

Days in weeks and over several years

A week has 7 days, so a common year is 52 weeks and 1 day, and a leap year 52 weeks and 2 days. Over 10 years that's 3,652 or 3,653 days; over 30 years, about 10,957.

The exact number of days between two dates depends on how many leap years fall between them. Rather than counting by hand, use the age in days calculator to see how many days you've lived, or the days until a date calculator for a countdown.

In short

• A common year is 365 days, a leap year 366.
• Leap years come every 4 years, except century years not divisible by 400.
• Month lengths vary, so the exact number of days is easier with a calculator.