📅 Days in Month Calculator

What is the Days in Month Calculator?

Most months have a fixed length that never changes: seven months have 31 days (January, March, May, July, August, October and December) and four months have 30 days (April, June, September and November). February is the only month whose length depends on the year, holding 28 days in a common year and 29 days in a leap year. That single exception is the reason a calculator is useful at all, since everything else can be memorised with the old knuckle trick or the rhyme that begins thirty days has September.

The leap year rule exists because a year is not exactly 365 days long. The Earth takes about 365.2422 days to orbit the Sun, so a 365-day calendar drifts by almost a quarter of a day each year. To correct this, the Gregorian calendar adds a leap day to February every four years, but with two refinements: a year divisible by 100 is not a leap year unless it is also divisible by 400. So 2000 was a leap year, 1900 was not, and 2100 will not be either. The full rule is: a year is a leap year if it is divisible by 4 and either not divisible by 100, or divisible by 400.

Under the hood, this tool uses a reliable date trick rather than a lookup table. Asking for day zero of the following month, written as new Date(year, month, 0) in JavaScript, rolls back to the last day of the month you actually want, and reading that day number gives the length. Because the browser computes February using the real Gregorian leap rule, the answer is always correct for the year you enter, with no special cases to maintain.

When to use it

• Settling how many days February has in a given year, especially around leap years.
• Working out billing or subscription cycles that depend on the exact number of days in a month.
• Calculating a daily rate for rent, salary, or interest by dividing a monthly figure by the real day count.
• Planning project schedules or content calendars where you need each month length at a glance.

How to use the Days in Month Calculator

1. Choose the month you want from the dropdown list.
2. Type the year (for example 2026) into the year field.
3. Read off the number of days in that month and whether the year is a leap year.
4. Scan the 12-month table to compare every month in the chosen year at once.

Formula & method

Worked examples

Number of days in each month

Is the year a leap year? February day count for sample years

Common mistakes to avoid

• Assuming February always has 28 days. February gains a 29th day in every leap year, which is roughly one year in four. Treating it as always 28 days throws off any daily rate or schedule that falls in a leap February.
• Thinking every fourth year is a leap year. The divisible-by-4 rule has a century exception. Years like 1700, 1800, 1900 and 2100 are divisible by 4 but are not leap years because they are not divisible by 400. The year 2000 is the rare century that is.
• Off-by-one month numbering. In JavaScript the Date month argument is zero-based for normal use, but the day-zero trick new Date(year, month, 0) deliberately uses a one-based month. Mixing the two conventions gives the wrong month length.
• Confusing days in a month with days in a billing period. A billing cycle that runs from the 15th to the 14th spans parts of two months, so its length is not the same as any single calendar month. Use a date difference calculator for spans that cross a month boundary.

Glossary

Leap year

A year with 366 days, where February has 29 days instead of 28, added to keep the calendar aligned with the Earth orbit.

Common year

An ordinary 365-day year in which February has 28 days.

Gregorian calendar

The calendar in worldwide civil use today, introduced in 1582, which defines the modern leap year rule.

Century year

A year ending in 00, such as 1900 or 2000, which is a leap year only when it is divisible by 400.

Month index

A number identifying a month. This tool uses 1 for January through 12 for December.

Frequently asked questions