ISO Week Number Calculator
Find the week number of any date, or the dates in any week, under ISO 8601, the US convention or a Saturday start — with the month drawn as a calendar to check.
2026-W45
45
2026
The year the week belongs to, which is not always the calendar year: the days either side of New Year can fall in a week that mostly sits in the other year.
307
2026-11-02 → 2026-11-08
53
Under each rule
- ISO: week 45 of 2026 · 2026-11-02 → 2026-11-08
- US: week 45 of 2026 · 2026-11-01 → 2026-11-07
- Sat: week 45 of 2026 · 2026-10-31 → 2026-11-06
Where it sits in the year
- Weeks left in the week-year
- 8 of 53
- Week of the month
- 2 (of November, by calendar rows)
- Quarter
- Q4
- Days left in the year
- 58 of 365 (84.1% of the year gone)
How this week number is worked out
- The week runs from Monday 2026-11-02 to Sunday 2026-11-08; its Thursday is 2026-11-05.
- An ISO week belongs to the year its Thursday falls in, so this one belongs to 2026. Week 1 of 2026 is the week holding its first Thursday, Thu 2026-01-01.
- Thu 2026-11-05 is 44 weeks after Thu 2026-01-01, so this is week 45.
Where that week falls
| ISO | Mon | Tue | Wed | Thu | Fri | Sat | Sun |
|---|---|---|---|---|---|---|---|
| 44 | 1 | ||||||
| 45 | 2 | 3 | 4 | 5 | 6 | 7 | 8 |
| 46 | 9 | 10 | 11 | 12 | 13 | 14 | 15 |
| 47 | 16 | 17 | 18 | 19 | 20 | 21 | 22 |
| 48 | 23 | 24 | 25 | 26 | 27 | 28 | 29 |
| 49 | 30 |
Which week numbering do you mean?
There is more than one. ISO 8601 starts weeks on Monday and gives week 1 to the week containing the first Thursday. The convention most spreadsheets use starts weeks on Sunday and gives week 1 to whatever week contains 1 January. A third starts the week on Saturday. They agree for most of the year and disagree by a whole year at the turn of it: 1 January 2027 is week 53 of 2026 under ISO and week 1 of 2027 under the US rule. Pick the one you mean; each option spells out its own rule.
The month, so the answer can be checked
Under the numbers the page draws the month as a calendar, with the week number down the left of each row — two of them, written 53/1, on the row where the Sunday- and Saturday-start rules restart the count on 1 January — the grid starting on whichever day your convention starts on, and the date you asked about picked out. A bare week number gives you nothing to check against, and every case that surprises people is at a year boundary, where the calendar shows which week each day falls in.
Both directions, and what a week-year is
Give a date to get its week, or give a year and a week number to get the seven dates it covers, starting on that convention's first day — under the same convention either way. The week-year shown beside the week is the year the week belongs to, which is not always the calendar year: the days either side of New Year can sit in a week that mostly belongs to the other one, which is exactly why the week-year exists as a separate number.
Getting there, and showing the working
The Jump to row under the date goes to Today, the Previous week, the Next week, or New Year’s Day and New Year’s Eve of the date’s year — the two days where the rules part company. “Where it sits in the year” gives the weeks left in the week-year, the week of the month, the quarter and the days left in the year. “How this week number is worked out” applies the chosen rule to the date on screen: under ISO the week’s Monday to Sunday, its Thursday, the week-year that Thursday gives and that year’s first Thursday; under the Sunday and Saturday rules the start of the week holding 1 January. Code format writes the week string in ISO form, with the weekday, without the hyphens, as year and week, or as two-digit year and week (YYWW), and the Week string row and its Copy follow it.
Frequently asked questions
Why is 1 January sometimes week 53 of the year before?
Under ISO 8601 a week belongs to the year holding most of it — formally, the year of its Thursday. So a New Year’s Day falling on a Friday, Saturday or Sunday is in the last week of the previous year. Under the US convention it is always week 1 instead. Switch conventions and watch the answer change; that is the difference, not a bug.
Which convention do spreadsheets use?
It depends on the function. The plain WEEKNUM defaults to the Sunday-start, 1-January rule; ISOWEEKNUM uses ISO 8601. If your sheet and this page disagree, that almost always comes down to which of the two each is using.
What is a week-year?
The year the week belongs to, which can differ from the calendar year of the date you looked up. 31 December 2024 is in ISO week 1 of 2025, so its week-year is 2025 even though the date is in 2024. It is shown separately so the week number is never ambiguous.
How many weeks does a year have?
Under ISO 8601, 52 or 53. The Sunday- and Saturday-start rules restart the count on 1 January, so a short week at either end still gets a number: under those rules a year has 53 weeks, or 54 when a leap year begins on the last day of that rule's week — a Saturday under the US rule, a Friday under the Saturday-start rule (next in 2028 and 2044). The tool works this out per year and refuses a week number that year does not have.
Which dates does a week cover, and what if the rules disagree?
The answer gives the week's first and last dates and how many weeks its year has. The same date is also shown under each week rule, with a warning when they give different numbers.
Can I jump to the year’s end, see how the week number is worked out, or get a YYWW code?
Yes. Jump to, under the date, has Today, Previous week, Next week, and New Year’s Day and New Year’s Eve of the date’s year — the two days where the rules part company. “How this week number is worked out” applies the chosen rule to the date on screen: under ISO the week’s Monday to Sunday, its Thursday, the week-year that Thursday gives, that year’s first Thursday and the weeks between; under the Sunday and Saturday rules the week’s start, the start of the week holding 1 January, and the weeks between. Code format writes the Week string in ISO form, with the weekday, without hyphens, as year and week, or as two-digit year and week (YYWW), and the link keeps that choice.
Can I save the answer or the calendar as an image or a PDF?
Not as an image. The answers are lines of text with Copy buttons beside them, which drop into a plan more easily than a picture would, and Copy link reopens the same date under the same convention. For a PDF, the browser’s own Print saves the page, the month’s calendar included.