How Many Days in a Month?

The average month is 30.44 days long. But no month is actually 30.44 days. You get 28, 29, 30, or 31, and the distribution is: seven months with 31 days, four months with 30, and February with 28 (or 29 in a leap year). The reason we cannot just have twelve 30-day months with 5 extra days at the end is that calendar reform has been proposed roughly once per century since the 1600s and has failed every single time.

The French Revolutionary Calendar (1793) tried 12 months of exactly 30 days each, with 5 "complementary days" at the end (6 in a leap year). The months had beautiful names based on nature: Vendémiaire (grape harvest), Brumaire (fog), Frimaire (frost). Napoleon killed it in 1805. The International Fixed Calendar proposed 13 months of 28 days each (plus one extra day). Kodak actually used it internally from 1928 to 1989. The World Calendar proposed four identical 91-day quarters. The United Nations studied it in the 1950s and quietly shelved it.

The problem with every reform is the same: changing the calendar disrupts everything. Payroll, contracts, religious observances, software, historical records. The cost of switching is enormous, and the benefit (slightly neater months) is not enough to justify it. So we keep using a system where February is 10% shorter than March and nobody can remember how long September is without checking.

📅 Month Length Summary

📊 Average Days per Month

💰 Why Does Month Length Matter for Billing?

Subscriptions, rent and salary are usually quoted monthly, but months are not equal. January has 3 more days than February. If you pay the same rent each month, you are paying about 10% more per day in February than in January. Some SaaS companies bill on 30-day cycles instead of calendar months to avoid this, which means your "monthly" bill sometimes hits twice in the same calendar month.

📐 Why Is 30.44 the Number Used in Calculations?

When you need an average month length for financial or scientific calculations, 30.44 days (or more precisely 30.4375) is the standard. This comes from dividing 365.25 (the average year including leap years) by 12. Its used in mortgage calculations, pharmaceutical dosing schedules, and any situation where you need to convert between months and days without referencing a specific calendar month.

📅 Which Months Have 31 Days?

Seven months have 31 days: January, March, May, July, August, October and December. The easiest way to remember is the knuckle trick: make a fist and count across your knuckles and the valleys between them starting from January. Knuckles are 31-day months, valleys are 30 (or 28 for February). When you reach the last knuckle, start over.

📅 February 30 Has Existed Twice in History

Sweden had a February 30, 1712 as part of a botched calendar transition. They had planned to gradually shift from the Julian to the Gregorian calendar by skipping leap days over 40 years, but forgot to skip the leap day in 1704 and 1708. To get back on track, they added an extra day to February 1712, creating a February 30 that exists nowhere else in history. They eventually gave up and switched to the Gregorian calendar in one go in 1753.