Week Number Calculator
Compute ISO 8601 week number + US week number for any date. Plus day-of-year, day-of-week, week-of-month, quarter, and ISO week date format (YYYY-Www).
Week Number Calculator
How to use the Week Number Calculator
Pick any date
The date starts on today in Singapore time. Click the field to use your browser's date picker; every figure below updates as you change it.
Read the ISO 8601 and US week numbers
ISO 8601: weeks run Monday to Sunday and week 1 is the week containing the year's first Thursday. Microsoft's Excel documentation calls this "System 2", commonly known as the European week numbering system. US system: weeks run Sunday to Saturday and week 1 is the week containing 1 January. This is Excel's default WEEKNUM numbering ("System 1").
Check the other date facts
Day of year (1 to 365, or 366 in a leap year), day of the week, the Sunday-start row of the month the date sits in, the quarter, the ISO week date (e.g. 2025-W14), and the Monday and Sunday that bound the ISO week.
Or go the other way
Type an ISO week such as 2026-W53 into the lookup to get its Monday-to-Sunday dates; add a day number (2026-W53-4) for one day in it. For people, the friendly form works too: "Week 14 of 2025, ending 6 April" is 2025-W14.
Week numbering — two systems that disagree at the edges
Week numbers ("Week 14 of 2025") label a seven-day period without naming its dates. The ISO 8601 standard defines them precisely: weeks run Monday to Sunday, and week 1 of each year is the week containing the year's first Thursday — equivalently, the week containing 4 January. That rule gives a year 52 or 53 ISO weeks, and RFC 3339, the internet profile of ISO 8601, writes the week number as 01 to 52 or 01 to 53 accordingly. 2026 has 53. Excel's default WEEKNUM uses a different rule — weeks from Sunday, week 1 the week containing 1 January — and the two numbers for the same date often differ by one, so it pays to say which system a week number is in.
The 53-week year
An ISO year has 53 weeks when 1 January falls on a Thursday, or when a leap year starts on a Wednesday. This century that means 2004, 2009, 2015, 2020, 2026, 2032 and 2037. Over the full 400-year cycle of the Gregorian calendar 71 years are long, usually 5 or 6 years apart and once in each cycle 7 (2296 to 2303 is the next). A simple check: 28 December is always in the last ISO week of its year, so its week number is the year's week count. Any system that assumes exactly 52 weeks a year will meet a week 53 sooner or later, and a weekly report keyed on ISO weeks needs a slot for it.
An ISO week belongs to the year its Thursday falls in. That one rule is why the last days of December can sit in next year's week 1.
Year-boundary edge cases
ISO weeks cross Gregorian year boundaries. 31 December 2024 is a Tuesday in the week that began on Monday 30 December, and that week's Thursday is 2 January 2025, so the date is in 2025-W01. Going the other way, 1 January 2023 was a Sunday ending the week that began on Monday 26 December 2022, so it is in 2022-W52. This is why ISO week dates always carry the year: "2025-W01" is unambiguous, "Week 1" alone is not. The US system never borrows from a neighbouring year, at the price of a short first and last week: because week 1 is whatever part of a Sunday-to-Saturday week contains 1 January, every year has at least 53 US weeks, and a leap year that starts on a Saturday has 54 (2000 and 2028).
Reading week numbers in practice
In Excel, ISOWEEKNUM(date) or WEEKNUM(date, 21) give the ISO week, and WEEKNUM(date) with no second argument gives the US-style week. With 11 to 17 as the second argument the week starts on Monday through Sunday, but week 1 is still the week containing 1 January, so none of them is ISO 8601. When you send a week number across a border or between teams, write the ISO form with its year (2026-W43), and say so if you mean anything else.
ISO weeks against the US convention, the 53-week years, and YYYY-Www
ISO 8601: weeks run Monday to Sunday. Week 1 is the week containing the year's first Thursday, which is also the week containing 4 January.
US system (Excel's default WEEKNUM): weeks run Sunday to Saturday and week 1 is the week containing 1 January, however few days of it are in the new year.
This century's 53-week ISO years so far and next: 2004, 2009, 2015, 2020, 2026, 2032 and 2037.
ISO week date format: YYYY-Www (e.g. 2025-W14), with an optional day number, 2025-W14-7 for its Sunday. It sorts correctly as text.
ISO weeks cross Gregorian year boundaries: 31 December 2024 is in 2025-W01, and 3 January 2021 is in 2020-W53.
28 December is always in the last ISO week of its year, and 4 January always in the first.
A US-system year always has at least 53 weeks, and 54 when a leap year starts on a Saturday — 2000, and next in 2028.
In any 400-year Gregorian cycle, 71 years have 53 ISO weeks.
The same date can carry different week numbers in the two systems: 6 April 2025 is ISO week 14 but US week 15, because the US week turns on Sunday.
Excel offers both: ISOWEEKNUM() or WEEKNUM(date, 21) for ISO weeks, and WEEKNUM(date) for the Sunday-start count.
Frequently Asked Questions
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52 weeks are 364 days, one or two short of a year. ISO 8601 fills whole Monday-to-Sunday weeks and assigns each week to the year its Thursday falls in, so the spare days pile up until a year holds 53 Thursdays' worth of weeks. That happens when 1 January is a Thursday, or a leap year starts on a Wednesday: 2004, 2009, 2015, 2020, 2026, 2032 and 2037 this century. The gap between them is 5 or 6 years, occasionally 7.
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Use whichever the people reading your number use, and say which it is. ISO 8601 has the advantage of a standard written form (YYYY-Www) and weeks that never split across a year end; the US system has the advantage that week 1 always starts the calendar year. If you are unsure, write the ISO form with its year, such as 2026-W43, which no one can misread.
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Two ways. Pick any date and the "ISO week boundaries" row shows the Monday and Sunday of its ISO week. Or type the week itself, such as 2026-W53, into the lookup box, and the tool gives its Monday and Sunday — 28 December 2026 to 3 January 2027 for that one. A week that does not exist, such as 2025-W53, is refused with the year's week count.
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If 31 December falls on a Monday, Tuesday or Wednesday, the Thursday of its week is in January, so the whole week is week 1 of the next ISO year. In the same way, if 1 January falls on a Friday, Saturday or Sunday, it belongs to week 52 or 53 of the previous ISO year. That is why ISO week dates always carry their year.
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For a fiscal calendar built on ISO weeks, yes. Many companies define their own fiscal weeks instead — for example a 4-4-5 calendar, where each quarter is 13 weeks split into periods of 4, 4 and 5 weeks, which makes a 364-day year and so needs a 53rd week every few years. Those calendars depend on where the company starts its year, so check your own finance calendar; this tool covers the ISO and US systems only.
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Microsoft documents
WEEKNUM(date, return_type)with two systems. Return type 1 or omitted: weeks begin on Sunday and week 1 contains 1 January (this tool's "US System Week"). Return type 2: weeks begin on Monday, still with week 1 containing 1 January. 11 to 17: weeks begin on Monday through Sunday respectively, again with week 1 containing 1 January. Type 21: ISO 8601 (this tool's "ISO 8601 Week"), the same asISOWEEKNUM(date). Microsoft's own example: 9 March 2012 is week 10 by WEEKNUM and by ISOWEEKNUM, and week 11 by WEEKNUM(date, 2). -
Today's date in Singapore (UTC+8), whatever your own clock says. A reader in New York on Sunday evening is already on Monday here, which can put the default date in the next ISO week. Change the date to the one you mean; the week numbers depend only on the date you pick, not on your time zone.
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The row of a Sunday-start month calendar the date sits on: the 1st is always in row 1, and a new row begins each Sunday. A month can run to six rows — March 2025 did, with the 31st on a Monday. There is no standard definition of a week of the month, so treat this one as a calendar-row count.
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No. All calculations run in your browser. The date stays on your device.
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Pair with: Date Difference Calculator for between-dates math; Working Days Calculator for business-day arithmetic; Date Add/Subtract Calculator for date arithmetic; Time Duration Calculator for hour/minute math. In a spreadsheet, Excel's ISOWEEKNUM() gives the same ISO week as this page.
Method & sources
How it computes
ISO 8601: weeks run Monday to Sunday and week 1 holds the year's first Thursday (equivalently 4 January); a date belongs to the ISO year of its week's Thursday. US (Excel WEEKNUM type 1): weeks run Sunday to Saturday and week 1 holds 1 January. All counts are on whole calendar days, never local-time milliseconds; ISO weeks also look up to their Monday-Sunday dates (WeekNumberKernel).
What this tool implements
- ISO 8601 week dates, profiled for the internet by RFC 3339: YYYY-Www, Monday-start, week numbers 01-52 or 01-53 by year; week 1 holds the first Thursday.
- Microsoft documents both systems for WEEKNUM: System 1 (week containing 1 January is week 1; return types 1, 2 and 11-17) and System 2 (week containing the first Thursday; return type 21, the ISO 8601 method), and ISOWEEKNUM for the ISO week. Its worked example, 9 March 2012 = week 10 by WEEKNUM and by ISOWEEKNUM, is a test vector.
- A year has 53 ISO weeks when 1 January is a Thursday, or a leap year starts on a Wednesday — 2004, 2009, 2015, 2020, 2026, 2032 and 2037 this century; 71 years in each 400-year cycle, usually 5 or 6 years apart and once 7 (2296 to 2303 next).
- A US-system year always has at least 53 weeks, and 54 when a leap year starts on a Saturday (2000, 2028).
- Every day of 2000-2399 is cross-checked against an independent 4-January routine, and the answers are checked to be identical under daylight-saving time zones.
- Corrected 2026-10-19: day of year and US week one day short after a daylight-saving change (local-time millisecond arithmetic); "always 52-53 weeks" for the US system (53 or 54); unsourced Malaysian weekend and broadcast-calendar claims and a wrong 4-4-5 quarter sum (removed); week 53 gaps "every 5-6 years" (5, 6 and once per cycle 7).
Sources
- Klyne G, Newman C. RFC 3339, Date and Time on the Internet: Timestamps (2002), Appendix A — an internet profile of ISO 8601. https://www.rfc-editor.org/rfc/rfc3339.txt
- Microsoft Support. WEEKNUM function (System 1 and System 2; return_type table). https://support.microsoft.com/en-us/office/weeknum-function-e5c43…
- Microsoft Support. ISOWEEKNUM function. https://support.microsoft.com/en-us/office/isoweeknum-function-1c…
What can make this go out of date
- None at runtime; the default date is the Singapore business day (window.RecaBusinessDay) and nothing is sent anywhere.
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