How many days are in each month of a common year?
A common year has 365 days divided among 12 calendar months. Unlike a leap year, it does not include the extra day added to February, so February has 28 days instead of 29.
The month lengths follow the Gregorian calendar used in most of the world. Seven months contain 31 days, four contain 30 days, and February is the shortest month with 28 days in a common year.
Knowing the number of days in each month helps with date calculations, payroll schedules, project deadlines, school terms, travel planning, and countdowns. It also makes it easier to estimate how many days remain in a year or how far apart two dates are.
The month lengths at a glance
January, March, May, July, August, October, and December each have 31 days. April, June, September, and November each have 30 days. February has 28 days when the year is common.
The distribution can seem irregular because the months do not alternate perfectly between 30 and 31 days. A quick reference is useful when adding dates by hand or checking whether a monthly period has ended.
| Month | Common year | Leap year | Difference |
|---|---|---|---|
| January | 31 days | 31 days | None |
| February | 28 days | 29 days | +1 day |
| March | 31 days | 31 days | None |
| April | 30 days | 30 days | None |
| May | 31 days | 31 days | None |
| June | 30 days | 30 days | None |
| July | 31 days | 31 days | None |
| August | 31 days | 31 days | None |
| September | 30 days | 30 days | None |
| October | 31 days | 31 days | None |
| November | 30 days | 30 days | None |
| December | 31 days | 31 days | None |
Why February has 28 days
February is shorter because of the way the Roman calendar was reorganized before the modern Gregorian system developed. Calendar designers needed to fit the year to the length of Earth’s orbit while keeping a practical pattern of whole days and months.
The solar year is slightly longer than 365 days, so the calendar occasionally adds an extra day. In a common year, February ends on the 28th. In a leap year, February reaches 29 days, bringing the annual total to 366.
How common years reach 365 days
Adding the standard month lengths gives seven months of 31 days, four months of 30 days, and one month of 28 days. The calculation is 217 days from the 31-day months, 120 days from the 30-day months, and 28 days from February, for a total of 365 days.
A common year is therefore one day shorter than a leap year. The difference affects annual date intervals, recurring events, interest calculations, and any schedule that crosses February in a leap year.
Common years and leap years compared
The only month whose length changes is February. Every other month has the same number of days in both types of year. This makes it easier to compare calendars: identifying whether February has 28 or 29 days is usually enough to determine the annual total.
Gregorian leap-year rules are precise. A year is generally a leap year when it is divisible by four, except century years must also be divisible by 400. Thus, 2000 was a leap year, while 1900 was not. Years such as 2024 and 2048 are leap years; 2023 and 2050 are common years.
Applying month lengths to date calculations
Counting days between dates requires attention to the starting date, ending date, and whether the count is inclusive. For example, the number of days from March 1 to April 1 is 31 when measuring the elapsed interval, while counting both calendar dates produces 32 listed dates.
Monthly deadlines can also be misleading when a recurring date does not exist in every month. A schedule set for the 31st may need a rule for April, June, September, November, and February. Businesses often move the date to the final day of the month or use a fixed number of elapsed days instead.
For weekly planning, month length also affects how dates align with weekdays. A month of 31 days shifts the next month’s starting weekday by three days, while a 30-day month shifts it by two. A full-week countdown can help when a date-based plan depends on complete Monday-to-Sunday periods rather than calendar months.
Reliable ways to check a calendar date
A printed calendar, date calculator, or month-length reference can prevent small errors that compound across a long schedule. When a calculation crosses December, remember that the year changes after December 31 and that the next January begins a new annual sequence.
For quick mental checks, the familiar knuckle method can identify 31-day months. Starting with January on a knuckle, move across the knuckles and dips; months on knuckles have 31 days. February remains the exception, and the method repeats after July by treating July and August as consecutive 31-day months.
Useful habits include:
- Check whether the year is common or leap before calculating dates around February.
- Memorize the four 30-day months: April, June, September, and November.
- Treat January, March, May, July, August, October, and December as 31-day months.
- Clarify whether a deadline uses elapsed days, calendar dates, or business days.
- Verify calculations that cross the end of a month or year.
Month abbreviations and calendar references
Standard month abbreviations are Jan, Feb, Mar, Apr, May, Jun, Jul, Aug, Sep, Oct, Nov, and Dec. They are useful in spreadsheets, schedules, forms, and compact date displays, although some systems use “Sept” instead of “Sep.”
When recording a date, a format such as 15 Feb 2025 avoids confusion between day-month and month-day systems. Including the year is especially important near February, since the month may contain either 28 or 29 days.
A common year always contains 365 days, but the number of weekdays and weekends varies depending on the year’s starting weekday. For exact results, use the complete date rather than relying only on the month’s usual length.
Use the monthly figures above as a quick reference, then apply the correct leap-year rule whenever February is involved. For precise elapsed-day counts, check the full calendar dates and account for the counting method being used.