Days in Month Calculator

The days-in-month calculator tells you exactly how many calendar days are in any given month and year. While most people know the rough pattern — "30 days has September, April, June, and November; all the rest have 31, except February" — February's count varies between 28 and 29 depending on whether the year is a leap year. This calculator removes all guesswork by giving you the precise day count for any month of any year.

Month lengths in the Gregorian calendar follow a fixed pattern for 11 of the 12 months: January (31), March (31), April (30), May (31), June (30), July (31), August (31), September (30), October (31), November (30), and December (31). The only variable month is February, which has 28 days in common years and 29 days in leap years. The irregular pattern of month lengths was inherited from Roman calendar reforms and has been unchanged since the Gregorian reform of 1582.

Knowing the exact number of days in a month is essential for date arithmetic, billing and payroll calculations, scheduling recurring events, computing monthly averages in data analysis, and financial modeling where daily rates must be multiplied by the correct number of days in each month. This calculator also shows how the current month fits in the year — what fraction of the year the month represents and how many days remain in the year after the month ends.

February's day count is determined by the Gregorian leap year rule, which balances the solar year's 365.2425-day length by adding leap days on a regular schedule.

The irregular month lengths in the Gregorian calendar trace to Roman calendar history. The original Roman calendar had 10 months (304 days), starting in March. Around 713 BC, King Numa Pompilius added January and February at the beginning, extending the year to 355 days (a lunar year). Julius Caesar reformed the calendar in 46 BC to align with the solar year, creating a 365-day year with a leap year every 4 years — this Julian calendar established most of the month lengths we use today.

The modern month name sequence and the 30-31 day pattern were codified in Caesar's reform, though Augustus Caesar later renamed "Sextilis" to August and is often (though possibly incorrectly) credited with rearranging days so August has 31 days rather than 30, taking the extra day from February. The Gregorian reform of 1582 adjusted only the leap year rule (removing some century leap years), leaving month lengths unchanged from the Julian calendar.

Daily rate calculations in payroll, rent, and interest frequently multiply a monthly rate by the actual days in each month. Dividing an annual salary by 365.25 (average days per year) gives the daily rate; multiplying by the days in a specific month gives that month's gross pay. February's short month means lower total pay in that period, while months with 31 days yield higher pay — a consideration in daily-rate contracting and project billing.

Data analysts computing monthly averages or rates must account for varying month lengths. "Monthly revenue" comparison between February ($950,000 / 28 days = $33,928/day) and March ($975,000 / 31 days = $31,451/day) shows March actually had a lower daily rate despite higher total revenue. Normalizing to a per-day rate requires knowing the exact days in each month. This calculator makes that reference instantly available for any historical or future month.

Each month represents a slightly different fraction of the annual year, ranging from February's shortest share (28/365 = 7.67% in common years) to January, March, May, July, August, October, and December's 31/365 = 8.49%. These fractions are used in interest accrual calculations, particularly in the "actual/365" day count convention where each month's interest is the principal × rate × (days in month / 365).

The "actual/actual" interest day count convention uses the actual days in each month and the actual days in each year (365 or 366) — maximally precise but requiring knowledge of both the specific month's length and whether the year is a leap year. This calculator provides both values, making it ideal as a reference for such calculations.