The short answer
Fifty-two weeks are 364 days, one short of a common year and two short of a leap year. Under the Gregorian leap-year rule, 97 of every 400 years are leap years, so the average year is 365.2425 days, or 52.1775 weeks. Every year starts one weekday later than the year before, or two after a leap year. That drift is what makes some years "long" in week-numbering systems. The rule behind leap years is in Leap Year Rules Explained.
ISO 8601 week numbers
The international standard numbers weeks like this: every week starts on a Monday, and week 1 of a year is the week that contains that year's first Thursday. Equivalently, week 1 is the week with January 4 in it, and it always has at least four days of the new year. A week belongs to the year that its Thursday falls in.
Because of that, the first days of January can belong to the last week of the previous year, and the last days of December to week 1 of the next. January 1, 2027 is a Friday, so it sits in week 53 of 2026. December 31, 2029 is a Monday, so it is already in week 1 of 2030. The week number tool shows the ISO week for any date.
Which years have 53 weeks
An ISO year has 53 weeks when it has 53 Thursdays. That happens in two cases: January 1 is a Thursday, or the year is a leap year and January 1 is a Wednesday. All other years have 52 weeks. Over a full 400-year cycle, 71 years have 53 weeks, which averages to 52.1775 weeks per year, matching the 52.1775 above.
| Year | Jan 1 | Days | ISO weeks | Week 1 starts | US weeks | Weekdays |
|---|---|---|---|---|---|---|
| 2020 | Wed | 366 | 53 | Dec 30, 2019 | 53 | 262 |
| 2021 | Fri | 365 | 52 | Jan 4, 2021 | 53 | 261 |
| 2022 | Sat | 365 | 52 | Jan 3, 2022 | 53 | 260 |
| 2023 | Sun | 365 | 52 | Jan 2, 2023 | 53 | 260 |
| 2024 | Mon | 366 | 52 | Jan 1, 2024 | 53 | 262 |
| 2025 | Wed | 365 | 52 | Dec 30, 2024 | 53 | 261 |
| 2026 | Thu | 365 | 53 | Dec 29, 2025 | 53 | 261 |
| 2027 | Fri | 365 | 52 | Jan 4, 2027 | 53 | 261 |
| 2028 | Sat | 366 | 52 | Jan 3, 2028 | 54 | 260 |
| 2029 | Mon | 365 | 52 | Jan 1, 2029 | 53 | 261 |
| 2030 | Tue | 365 | 52 | Dec 31, 2029 | 53 | 261 |
| 2031 | Wed | 365 | 52 | Dec 30, 2030 | 53 | 261 |
| 2032 | Thu | 366 | 53 | Dec 29, 2031 | 53 | 262 |
| 2033 | Sat | 365 | 52 | Jan 3, 2033 | 53 | 260 |
| 2034 | Sun | 365 | 52 | Jan 2, 2034 | 53 | 260 |
| 2035 | Mon | 365 | 52 | Jan 1, 2035 | 53 | 261 |
| 2036 | Tue | 366 | 52 | Dec 31, 2035 | 53 | 262 |
| 2037 | Thu | 365 | 53 | Dec 29, 2036 | 53 | 261 |
| 2038 | Fri | 365 | 52 | Jan 4, 2038 | 53 | 261 |
| 2039 | Sat | 365 | 52 | Jan 3, 2039 | 53 | 260 |
| 2040 | Sun | 366 | 52 | Jan 2, 2040 | 53 | 261 |
From 2020 to 2040 the 53-week ISO years are 2020, 2026, 2032, 2037. Long years come five or six years apart: over a 400-year cycle there are 27 gaps of five years, 42 of six and 1 of seven, which spans 2300, a century year that is not a leap year. "Week 1 starts" is the Monday that begins ISO week 1, which can fall in late December of the year before.
How US week numbering differs
US week numbers usually follow a different rule. The Unicode CLDR locale data lists Sunday as the first day of the week in the United States and lets the first week of the year have as little as one day, and spreadsheet WEEKNUM functions use the same rule by default: weeks start on Sunday, and the week that contains January 1 is week 1. The last week is the one with December 31.
- Week 1 can be a single day. If January 1 is a Saturday, week 1 is just that Saturday, and week 2 starts the next day.
- A year always spans at least 53 such weeks, since 365 days do not fit in 52. A leap year that starts on a Saturday spans 54: in this range that happens in 2028.
- Week numbers can differ from ISO by one for the whole year. In 2026 both systems put January 1 in week 1, but in a year starting on a Friday, ISO week 1 starts the following Monday while the US week 1 is the Sunday-to-Saturday week with January 1.
When you share a week number with someone abroad, say which system you use, or give the date of the Monday or Sunday that starts the week. Spreadsheets usually offer both: in the WEEKNUM function, return type 21 gives the ISO number.
Weeks in a month and a quarter
A month of 28 to 31 days is 4 weeks plus 0 to 3 days. Spread over a year, the average month is 4.348 weeks, which is why a monthly budget is not simply four times a weekly one: multiply a weekly amount by 52 and divide by 12, or by 4.33, to get a fair monthly figure. A quarter of 13 weeks is 91 days, while calendar quarters in a common year have 90, 91, 92, 92 days. That one-day mismatch is how a 13-week plan slowly slides against the calendar.
Pay periods and the 53rd week
Weekly pay follows the same arithmetic. A year has 52 weeks plus one or two days, so the weekday of a given date moves one day later each year, two after a leap day, and in some years a weekly payroll gets a 53rd payday in the calendar year. Biweekly pay, every 14 days, gets 27 paydays in some years for the same reason. The years and what it does to a salary are in Years With 27 Biweekly Pay Periods.
Workdays in a year
The weekday column counts Monday to Friday only. A common year has 52 full weeks, which is 260 weekdays, plus one extra day; if that day is a weekday the year has 261. A leap year has two extra days, so 260, 261 or 262. Public holidays then come off that count, which differs by country and employer. The business days calculator counts working days between two dates and can skip holidays, and the days between dates calculator gives the plain number of days, or weeks and days.
Rules of thumb
- 52 weeks is 364 days. A year is always a little more.
- ISO weeks start on Monday; week 1 has the first Thursday.
- A year starting on a Thursday always has 53 ISO weeks.
- Late December and early January are where week numbers disagree, so check those dates twice.
- Quote the start date of the week along with its number when the reader may use a different system.
Frequently asked questions
How many weeks are in a year?
A common year of 365 days has 52 weeks and 1 day; a leap year of 366 days has 52 weeks and 2 days. On average a Gregorian year is 365.2425 days, or about 52.18 weeks. Week-numbering systems round this to 52 or 53 numbered weeks.
Which years have 53 weeks?
In the ISO 8601 system, a year has 53 weeks when January 1 is a Thursday, or when it is a Wednesday in a leap year. Between 2020 and 2040 that is 2020, 2026, 2032, 2037. Over a 400-year cycle, 71 years have 53 ISO weeks.
Does 2026 have 53 weeks?
Yes, in ISO numbering. January 1, 2026 is a Thursday, so the ISO year 2026 has 53 weeks and its week 53 ends on Sunday, January 3, 2027. The next 53-week ISO year after 2026 is 2032.
Why does my calendar show a different week number?
Many US calendars and spreadsheets count Sunday-start weeks and call the week containing January 1 week 1. ISO 8601 starts weeks on Monday and calls the week with the first Thursday week 1. Around New Year the two can differ by one, and the US count can reach 53 or even 54.
How many workdays are in a year?
Counting Monday to Friday only, a year has 260, 261 or 262 weekdays, depending on which days it starts and ends on. Between 2020 and 2040 the count ranges from 260 to 262. Subtract public holidays and leave to get working days.
Sources
- WHATWG HTML Living Standard, Weeks (a week starts on a Monday; week 1 contains the first Thursday of the year; 53 weeks when January 1 is a Thursday, or a Wednesday in a leap year; Gregorian leap-year rule)
- Unicode CLDR supplemental data, week data (United States: first day of the week Sunday, minimum days in the first week 1)
- Microsoft Support, WEEKNUM function (system 1: the week containing January 1 is week 1, weeks starting Sunday by default; system 2: the ISO 8601 week with the first Thursday)