Week Number Calculator
The ISO 8601 week number for any date, and the seven dates in any week — both directions, with the week-numbering year that makes the answer complete, the 52-or-53 count for that year, and the US convention reported alongside so you can tell the two apart.
A date
Both directions use ISO 8601. The other convention is reported alongside so you can tell which one a colleague meant.
Starts at today. Clear it to come back to today at any point.
January 1, 2026 is a Thursday, in the week running Monday December 29, 2025 to Sunday January 4, 2026.
- ISO week
- 2026-W01
- Week number
- 1
- Week-numbering year
- 2026
- Week starts, Monday
- December 29, 2025
- Week ends, Sunday
- January 4, 2026
- ISO weeks in 2026
- 53
- Weekday
- Thursday (ISO 4 of 7)
- US convention, Sunday start
- Week 1
| Weekday | Date | ISO date |
|---|---|---|
| Monday | December 29, 2025 | 2025-12-29 |
| Tuesday | December 30, 2025 | 2025-12-30 |
| Wednesday | December 31, 2025 | 2025-12-31 |
| Thursday | January 1, 2026 | 2026-01-01 |
| Friday | January 2, 2026 | 2026-01-02 |
| Saturday | January 3, 2026 | 2026-01-03 |
| Sunday | January 4, 2026 | 2026-01-04 |
- ISO 8601 weeks start on Monday, and week 1 is the week holding the year’s first Thursday — the same rule as “the week containing 4 January”.
- A week belongs entirely to one year, so up to three days either side of New Year sit in a week numbered for the other year. That is why the answer above is written 2026-W01 and not just 1.
- The US convention counts differently — weeks start on Sunday and week 1 is whichever week contains 1 January. Check which one your employer, supplier, or spreadsheet means before quoting a number.
Three rules, one of which everybody forgets
ISO 8601 defines week numbering in three parts, and it is the third that trips people up. First, a week runs Monday to Sunday. Second, week 1 is the week containing the year’s first Thursday — equivalently, the week containing 4 January, or the first week with a majority of its days in the new year; all three phrasings pick out the same week. Third, and this is the one left out of most implementations, a week belongs entirely to one year.
That third rule is why the answer is never just a number. If a week cannot be split across two years, then some days must be numbered for a year they do not fall in — up to three on either side of New Year. So “week 1” is not an answer to anything. 2026-W01 is.
week 1 = the week containing the first Thursday of the year- Week starts
- Monday. Day 1 of 7; Sunday is day 7
- Anchor
- 4 January is always in week 1, and 28 December is always in the last week of its year
- Week-numbering year
- The calendar year of the week’s Thursday — the only day guaranteed to sit in the week’s own year
- Weeks in a year
- 53 when the year starts on a Thursday, or when a leap year starts on a Wednesday; 52 otherwise
Every figure above comes from one move: step from the date to the Thursday of its own week, and read that Thursday’s calendar year. Because week 1 is defined as the week holding the first Thursday, the Thursday is the only day of the week that is guaranteed to be in the week’s own year, so it settles the week-numbering year with no special cases at all. The week number is then whole weeks from 1 January of that year to that Thursday, plus one. Counting the weeks in a year is the same trick aimed at 28 December — always in the last week, so its week number is the count.
The turn of 2026, where every rule fires at once
1 January 2026 is a Thursday. That makes it the first Thursday of 2026, so its week is week 1 — and since weeks start on Monday, that week opens on 29 December 2025 and closes on 4 January 2026. Three days of December 2025 are therefore in 2026-W01. Ask a spreadsheet for the week number of 29 December 2025 with WEEKNUM and it says 53, counting from the far end of 2025. Both numbers describe the same Monday.
Because 2026 begins on a Thursday it is one of the long years: it runs to 53 ISO weeks rather than 52. Check it against the anchor — 28 December 2026 falls in week 53, and that is the count. So 2026-W53 exists, it runs Monday 28 December 2026 to Sunday 3 January 2027, and 1 January 2027 is in it. A Friday, numbered for the previous year.
Follow one date through both conventions to see the size of the gap. For 29 December 2025: ISO 8601 says 2026-W01, Excel’s default says week 53 of 2025. For 1 January 2027: ISO says 2026-W53, Excel’s default says week 1 of 2027. In both cases the two systems disagree about the year andput the number at opposite ends of the range. A delivery promised “week 1” without a convention attached could land a full year and 52 weeks away from what the other party meant.
Name the one you mean before you quote a number
| ISO 8601 | US convention | |
|---|---|---|
| Week starts on | Monday | Sunday |
| Week 1 is | The week containing the first Thursday | The week containing 1 January |
| Weeks in a year | 52 or 53 | 53 or 54 |
| Can a week span two years? | No — it belongs wholly to one | Yes — week 1 may hold days of the old year |
| In a spreadsheet | ISOWEEKNUM | WEEKNUM, default settings |
| Usual home | Europe, and international standards work | United States, Canada, Japan |
Week numbers earn their place because a week is the only calendar unit that never changes length. A month is 28 to 31 days and holds four or five of any given weekday, so a month-on-month comparison is partly a statement about the calendar. Manufacturing, logistics, retail and construction plan in weeks for exactly that reason — and “ship in W38” carries the same meaning in Stuttgart, Shenzhen and Gothenburg, which is more than can be said for a date written 09/08.
A caution on the retail calendars: 4-4-5 and 4-5-4 fiscal years, broadcast weeks, and the US CDC’s MMWR epidemiological weeks are all week-based systems that agree with neither column above. This tool does not implement them. If a number came out of a retail-planning system, check its own definition rather than assuming either convention here.
What this calculator assumes
- ISO 8601 throughout. The US convention is reported for comparison but is never used to derive the Monday, the Sunday, or the week count.
- The Gregorian calendar, and dates from 1583 onward only. The reform of 1582 deleted ten days, and applying modern rules to earlier dates would answer confidently about a calendar nobody was using.
- Plain calendar dates with no time and no time zone. A date is in the same ISO week everywhere it is that date.
- The two extremes of the supported range report a week-numbering year one step outside it — 1 January 1583 is in 1582-W52, and the last three days of 2200 are in 2201-W01. That is ISO 8601 working correctly, not the tool overflowing, but the reverse direction will not accept those years.
- No fiscal, broadcast, retail or epidemiological week systems. Nor any locale’s own first-day-of-week preference — that setting changes what a calendar app highlights, not what ISO 8601 says.
- Today’s date is read from your device in UTC. Late in the evening a long way west of Greenwich it can already show tomorrow; change the field if it does.
Week number FAQ
What week number is it this week?
The calculator opens on today's date and reports it as something like 2026-W31 — week 31 of the week-numbering year 2026. Switch the selector to the other direction and it will hand back the seven dates in that week instead, Monday through Sunday. Both directions use ISO 8601, and the US convention is reported alongside so you can see when the two disagree.
Why does my spreadsheet give a different week number?
Almost certainly because you used Excel's WEEKNUM rather than ISOWEEKNUM. WEEKNUM defaults to the US convention: weeks start on Sunday, and week 1 is whichever week contains 1 January even if that is a single day of it. ISOWEEKNUM follows ISO 8601: weeks start on Monday, and week 1 is the week containing the first Thursday. The two can differ by a whole week for months at a time. Google Sheets behaves the same way, and LibreOffice adds further variants. Neither is wrong — they are different systems, and the only real mistake is not knowing which one a number came from.
Can a year have 53 weeks?
Yes, and it happens more often than most people expect — roughly 71 years in every 400. A year has 53 ISO weeks when it starts on a Thursday, or when it is a leap year that starts on a Wednesday; otherwise it has 52. 2020 had 53, 2026 has 53, and 2032 will. This is the field that breaks naive planning spreadsheets: a template with 52 columns silently loses a week of data in those years.
Why is 31 December sometimes in week 1 of the next year?
Because an ISO week belongs entirely to one year and never splits across two. The week containing 31 December 2025 also contains 1 January 2026, and since it holds the first Thursday of 2026 it is week 1 of 2026 — so 29, 30 and 31 December 2025 are all in 2026-W01. The mirror case happens too: 1 January 2027 is a Friday, and its week belongs to 2026, making it part of 2026-W53. Up to three days on either side of New Year can be numbered for the neighbouring year.
What is a “week-numbering year” and why does this tool insist on it?
It is the year the week belongs to, which is not always the calendar year its dates fall in. A result of “week 1” is genuinely ambiguous: week 1 of which year? Say 29 December 2025 is in week 1 and someone will file it under 2025, a year out. Writing it 2026-W01 removes the ambiguity, and that is the form ISO 8601 defines. It is also the field naive implementations leave out, which is why two week-number calculators can both be “right” and still disagree.
Why do businesses use week numbers at all?
Because a week is the only calendar unit that never changes length. Months run 28 to 31 days and contain four or five of any given weekday, so comparing March to February tells you as much about the calendar as about the business. Manufacturing, logistics, retail and construction plan in weeks for exactly that reason: “ship in W38” is unambiguous about duration in a way that “ship in September” is not. Week numbers are also how a lead time crosses a language barrier — W38 reads the same in Stuttgart, Shenzhen and Gothenburg.
Do ISO weeks start on Monday or Sunday?
Monday, always, under ISO 8601. Sunday-start weeks are the norm in the United States, Canada, Japan and much of the Middle East, and that is the convention Excel's WEEKNUM defaults to. Both are shown above for a date so you can tell which one your employer, supplier, or spreadsheet means before you quote a number back at them.
Sources and review notes
- ISO 8601-1:2019 — the standard that defines the week-numbering year, the Monday start, the first-Thursday rule, and the
YYYY-Wwwwritten form - ISO’s own summary of 8601 — the short account of why the format exists and what it is meant to make unambiguous
- RFC 3339 — the internet profile of ISO 8601, and the reason the
YYYY-MM-DDdates in the table above sort correctly as plain text
There is no dataset behind this page to go stale — ISO 8601 week numbering has not changed since the standard was first published in 1988. The test suite behind it round-trips every date from 1900 to 2200 through its week number and back, and fails if any of those 109,935 days lands outside the week it was numbered into.