Weeknumber.io

52 or 53 weeks

How many weeks are in a year?

A year has 52 weeks and 1 day (365 days), or 52 weeks and 2 days in a leap year. In the ISO week system that most of the world uses for week numbers, a year has either 52 or 53 weeks.

52 weeks plus one or two days

Divide the days by seven: 365 ÷ 7 = 52.14, and 366 ÷ 7 = 52.29. The leftover day (or two) is why a calendar year never splits into whole weeks, and why the weekday of your birthday moves forward each year.

ISO 8601 solves the leftover by giving every week to exactly one year. A week belongs to the year that contains its Thursday. Most years get 52 weeks this way. Every five or six years, the leftover days add up to a full 53rd week.

When does a year have 53 weeks?

An ISO year has 53 weeks when January 1 is a Thursday, or when it is a leap year that starts on a Wednesday. In both cases the year contains 53 Thursdays. That happens 71 times in every 400 years.

53-week years from 2000 to 2050: 2004, 2009, 2015, 2020, 2026, 2032, 2037, 2043, 2048.

2020–2035

Weeks per year

ISO weeks, days, the weekday of January 1, and the first and last day of each ISO year. Highlighted rows have 53 weeks.

YearISO weeksDaysJanuary 1Week 1 startsLast week ends
202053366Wednesday
202152365Friday
202252365Saturday
202352365Sunday
202452366Monday
202552365Wednesday
202653365Thursday
202752365Friday
202852366Saturday
202952365Monday
203052365Tuesday
203152365Wednesday
203253366Thursday
203352365Saturday
203452365Sunday
203552365Monday

Work weeks in a year

A year has 52 five-day work weeks, plus one or two extra weekdays. Subtract public holidays to get working days. The table counts Monday–Friday dates and the US federal holidays observed on a weekday in that year.

YearWeekdaysFederal holidaysWorking days
202526111250
202626111250
202726112249
202826010250

Most full-time schedules therefore have about 50 working weeks once vacation is included.

Weeks per month

Months have 4 weeks and 0–3 days, about 4.35 weeks on average (52.18 ÷ 12). In ISO week numbering, a week counts toward the month that contains its Thursday, so a month has 4 or 5 ISO weeks. Accounting calendars such as 4-4-5 fiscal weeks use the same idea.

2026DaysWeeksISO weeks
January314 weeks + 3 days5
February284 weeks4
March314 weeks + 3 days4
April304 weeks + 2 days5
May314 weeks + 3 days4
June304 weeks + 2 days4
July314 weeks + 3 days5
August314 weeks + 3 days4
September304 weeks + 2 days4
October314 weeks + 3 days5
November304 weeks + 2 days4
December314 weeks + 3 days5

Quick answers

How many weeks are in a year?
A year has 52 weeks and 1 day, or 52 weeks and 2 days in a leap year. In the ISO week system, a year has either 52 or 53 full Monday-to-Sunday weeks.
Does 2026 have 53 weeks?
Yes. January 1, 2026 is a Thursday, so 2026 has 53 ISO weeks. Week 53 runs from Monday, December 28, 2026 to Sunday, January 3, 2027.
How many weeks are in 2027?
2027 has 52 ISO weeks. Week 1 starts on Monday, January 4, 2027 and week 52 ends on Sunday, January 2, 2028.
How many weeks are in a leap year?
366 days is 52 weeks and 2 days. A leap year has 53 ISO weeks only if it starts on a Wednesday or a Thursday, as 2020 did (Wednesday) and 2032 will (Thursday).
How many work weeks are in a year?
There are 52 work weeks of five days, plus one or two extra weekdays. That gives 260 to 262 weekdays before holidays. With the 11 US federal holidays, most years have about 250 working days.
How many weeks are in a month?
A month has 4 weeks plus 0 to 3 days: 28 days is exactly 4 weeks, 30 days is 4 weeks and 2 days, 31 days is 4 weeks and 3 days. On average a month has about 4.35 weeks.