Week Number Calculator

DateMath gives two week numbers for every date: the ISO 8601 week, which starts on Monday with week 1 holding the year's first Thursday, and the US week, which starts on Sunday with week 1 holding January 1. Near New Year the ISO week-year can differ from the calendar year, and DateMath shows which year the week belongs to.

• ISO 8601 week dates. DateMath numbers weeks by ISO 8601: weeks start on Monday, week 1 is the week with the year's first Thursday, and a year has 52 or 53 weeks. Source: ISO 8601.
• The Gregorian calendar. DateMath treats every date as a Gregorian date, with its leap-year rule: a year divisible by 4 is a leap year, except century years not divisible by 400, so 2000 had a February 29 and 2100 will not. Source: Gregorian calendar.

DateMath's date math is tested against more than 100 cases worked out by hand. Every worked example and table on the site is computed with the same code the calculators run, so a page and its calculator can't disagree.

The calculator works in both directions. Pick a date to get its week number, or type a year and a week number to get that week’s dates. Every result uses ISO 8601 weeks (Monday to Sunday) unless it says “US”.

Week number of a date

The date field opens on today’s date from your device, so it answers “what week of the year is it?” straight away. Pick any other date to see:

• The ISO week and its year, for example “ISO week 53 of 2026” for January 1, 2027. The year shown is the ISO week-year, which differs from the calendar year for a few days around New Year.
• That week: its Monday and Sunday, so you can see which days share the number.
• The US week number, with weeks starting on Sunday and January 1 always in week 1.
• The day of the year, from 1 on January 1 to 365, or 366 in a leap year.
• The number of ISO weeks in that ISO year, 52 or 53.

Dates of a week number

Switch to “Dates of a week” and enter the ISO year and the week number. The calculator shows the Monday the week starts on, the Sunday it ends on and how many weeks that year has. Week 1 of 2027 runs from Monday, January 4, 2027 to Sunday, January 10, 2027. If you ask for a week the year does not have, such as week 53 of 2027, it tells you the year has 52 weeks instead of guessing.

ISO 8601, the international date standard, numbers weeks with three rules:

1. Every week runs from Monday to Sunday.
2. Week 1 is the week that contains the year’s first Thursday. That is the same as the week with January 4 in it, or the first week with at least four days in the new year.
3. The weeks are numbered on from there, with no partial weeks, until the next year’s week 1.

Because weeks are never split, the ISO year does not start on January 1. Week 1 starts on the Monday between December 29 and January 4, so two edge cases follow:

• January 1 to 3 can be in the previous year. When the year starts on a Friday, Saturday or Sunday, those days belong to the last week of the year before. January 1, 2027 is a Friday, so it falls in week 53 of 2026.
• December 29 to 31 can be in the next year. When December 31 is a Monday, Tuesday or Wednesday, the last days of December belong to week 1 of the next year. December 31, 2029 is a Monday, so it falls in week 1 of 2030.

Every ISO week has exactly one Thursday, and the week takes the number and the year of that Thursday. This is the rule DateMath uses, and it is also the quickest way to work a week out by hand.

Use the Thursday rule. Take Monday, March 15, 2027:

1. Find the Thursday of the date’s Monday-to-Sunday week. March 15, 2027 is a Monday, so the Thursday is March 18, 2027.
2. The Thursday’s year is the ISO year: 2027.
3. Find the Thursday’s day of the year: 77.
4. Subtract 1, divide by 7 and drop the fraction: (77 − 1) ÷ 7 = 10.86, so 10.
5. Add 1. March 15, 2027 is in ISO week 11 of 2027.

Near New Year

The same steps handle the edge cases on their own. January 1, 2027 is a Friday; its Thursday is December 31, 2026, day 365 of 2026, so the date is in week 53 of 2026. December 31, 2029 is a Monday; its Thursday is January 3, 2030, day 3 of 2030, so the date is in week 1 of 2030.

The formula version

Without finding the Thursday: week = ⌊(day of the year − weekday + 10) ÷ 7⌋, with the weekday counted from 1 for Monday to 7 for Sunday. For March 15, 2027: (74 − 1 + 10) ÷ 7, rounded down, is 11. Two results need a correction:

• 0 means the last week of the year before. For January 1, 2027 the formula gives 0, and 2026 has 53 weeks, so it is week 53 of 2026.
• A number above the year’s week count means week 1 of the next year. For December 31, 2029 the formula gives 53, but 2029 has only 52 weeks, so it is week 1 of 2030.

ISO 8601 also writes a day as its week-year, week and weekday: YYYY-Www-D, with the week padded to two digits and the day running from 1 for Monday to 7 for Sunday. The compact form drops the hyphens, and a whole week is written without the day.

So 2027-W01-1 is Monday, January 4, 2027, the first day of ISO week 2027-W01. The year in the notation is the ISO week-year, which is why January 1, 2027 is written 2026-W53-5. To write any date this way, read the week and year from the calculator and add the weekday number.

The calculator shows both numbers because they are not interchangeable. ISO weeks are the international standard; the US system is the default of Excel’s WEEKNUM function.

Away from the turn of the year, the gap follows a pattern. In years that start on Sunday to Thursday the two numbers match from Monday to Saturday, and the US number is one higher on Sundays. In years that start on a Friday or Saturday the US number is one higher from Monday to Saturday and two higher on Sundays. 2027 starts on a Friday: July 1, 2027 is ISO week 26 and US week 27, and July 4, 2027 is ISO week 26 and US week 28.

Because the US system counts partial weeks, its last week number can be higher than any ISO week. In 2028, a leap year that starts on a Saturday, December 31 is in US week 54.

A year is 52 weeks and 1 day, or 52 weeks and 2 days in a leap year. Week numbering rounds that to whole weeks, so an ISO year has 52 or 53 weeks. A year has week 53 when it has 53 Thursdays: when January 1 is a Thursday, or when it is a leap year and January 1 is a Wednesday. Over the 400-year cycle of the Gregorian calendar, 71 years have 53 weeks, an average of 52.1775 weeks a year.

Leap years matter only through the weekday. 2020 was a leap year that started on a Wednesday, so it had 53 weeks. 2024 was a leap year too, but it started on a Monday and had 52. From 2020 to 2035, the years with week 53 are 2020, 2026, and 2032.

ISO week 1 starts on the Monday closest to January 1, which can be in the old year. January 1 itself is in week 1 only when it falls on a Monday to Thursday; the US system always puts it in week 1.

2027 has 52 ISO weeks. Week 1 starts on Monday, January 4, 2027; the days before it belong to week 53 of 2026. Each week below runs from Monday to Sunday.

Week numbers by month

To find the week from a month and a day, look up the month, then use the calculator for the exact date. A year in brackets means the week belongs to another ISO year.

The calculator also shows the day of the year: the date’s position counted from January 1 as day 1. It is the number behind the week calculation, and it shifts by one after February in a leap year:

• March 1, 2027 is day 60.
• March 1, 2028 is day 61 (leap year).
• December 31, 2027 is day 365.
• December 31, 2028 is day 366 (leap year).

To count the days until or since a date instead, use the date calculator.

Excel and Google Sheets

• =ISOWEEKNUM(A1) returns the ISO week. For January 1, 2027 it returns 53.
• =WEEKNUM(A1) returns the US week (Sunday start, January 1 in week 1); for January 1, 2027 it returns 1. =WEEKNUM(A1, 21) returns the ISO week, while =WEEKNUM(A1, 2) starts weeks on Monday but still puts January 1 in week 1, so it is not ISO.
• =YEAR(A1-WEEKDAY(A1,2)+4) returns the ISO year, the year of the week’s Thursday: 2026 for January 1, 2027.
• =DATE(Y,1,4)-WEEKDAY(DATE(Y,1,4),2)+1+7*(W-1) returns the Monday of ISO week W of year Y: January 4, 2027 for week 1 of 2027.

Python

date.isocalendar() returns the ISO year, week and weekday: for January 1, 2027, 2026, 53 and 5. date.fromisocalendar(2027, 1, 1) goes the other way and returns January 4, 2027, and strftime("%G-W%V-%u") writes the ISO week date. The %U and %W codes count the days before the year’s first Sunday or Monday as week 0, so they match neither system on this page.

SQL

PostgreSQL: EXTRACT(WEEK FROM d) is the ISO week and EXTRACT(ISOYEAR FROM d) its year. MySQL: WEEK(d, 3) is the ISO week; other modes use other systems. SQL Server: DATEPART(ISO_WEEK, d), while DATEPART(WEEK, d) depends on the server’s DATEFIRST setting.

A week number names a whole working week in a few characters, which is why it is common in planning, especially in Europe:

• Delivery and production schedules, such as “ships in week 11”.
• Sprints, release plans and project timelines.
• Timesheets, rotas and payroll periods.
• Reports that compare a week with the same week last year, which need a plan for week 53 in years such as 2020, 2026, and 2032.

Related calculations: add or subtract weeks from a date, or count the weeks between two dates, with the date calculator; measure an exact span with the date duration calculator; count the working days in a week or a project with the business days calculator; add hours to a shift start with the time calculator; and see an age in total weeks with the age calculator.