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-W38
38
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.
260
Where that week falls
| ISO | Mon | Tue | Wed | Thu | Fri | Sat | Sun |
|---|---|---|---|---|---|---|---|
| 36 | 1 | 2 | 3 | 4 | 5 | 6 | |
| 37 | 7 | 8 | 9 | 10 | 11 | 12 | 13 |
| 38 | 14 | 15 | 16 | 17 | 18 | 19 | 20 |
| 39 | 21 | 22 | 23 | 24 | 25 | 26 | 27 |
| 40 | 28 | 29 | 30 |
🔒 Calculated in your browser — nothing is uploaded.
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 — exactly where seeing it laid out is the difference between believing the answer and not.
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.
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 is almost always which of the two you are each 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, rather than answering for one that does not exist.
Are my dates uploaded?
No. Everything is calculated in your browser with JavaScript. Nothing you enter is sent anywhere or stored.