Week Number
The ISO 8601 week number for today or any date you pick, with the day of the year, the weeks left in it, and that week's Monday and Sunday.
ISO week
Choose a date to begin.
This week straddles two years.
The weeks of the year
Two conventions on one date
How to use
- Nothing to do for today. Your device's date is filled in when the page loads and the number is on screen.
- Type or pick another day in the Date box, backwards or forwards, and the whole readout moves with it.
- Glance at the week-year under the big number. Around the new year it will not always match the year in the date box, and that is the point of printing it.
- Press any row in the week list to jump to that week's Monday.
- Press Copy share link for a link ending in ?date=2026-08-12, which reopens on exactly that day.
How the numbering works
Load the page and the week number for today is already the biggest thing on it. For 12 August 2026 that reads 33 of 2026, the year beside the number being the week-year. Change the Date box and every figure follows the new day.
Four cards fill in underneath. Day of year is the plain ordinal count, 224 for that same August date. Weeks remaining takes the current week off the total the ISO year holds. The last two are the Monday and the Sunday that bracket the week, written short in your locale's own wording, like Mon, Aug 10 and Sun, Aug 16.
ISO 8601 puts week 1 wherever the year's first Thursday lands, which is another way of saying the week that contains 4 January. That single rule is why 29 December 2025 already belongs to week 1 of 2026, and why 2026 is one of the long years carrying 53 weeks instead of the usual 52.
A week number cannot be typed into the Date box, but every week of the year sits in the list under the answer, opened at the current one, and pressing a row moves the page to that week's Monday. Clearing the box holds the last answer, dimmed, under a line naming the date it belongs to. The address bar carries the date as you go and Copy share link copies it, and a shared link whose date is not a real day opens on today and says what the link held.
A supplier in Hamburg says KW 41, and the warehouse needs a Monday
The confirmation email promises delivery in KW 41, which is how much of Europe schedules a week rather than a day. Type 2026-10-05 as a first guess at early October and the answer reads 41 of 2026. The cards fill in around that: day 278, Weeks remaining at 12, Monday 5 October for the start and Sunday 11 October for the end. Nudge the box back one day to 4 October and the number drops to 40, so the pallet is due somewhere between the fifth and the eleventh, and the Monday is the one to plan the loading bay around.
The same page settles the year-end payroll question. Type 2026-12-28 and it says 53 of 2026, running Monday 28 December to Sunday 3 January, with Weeks remaining at 0. Step forward to 1 January 2027 and the big number stays put, still 53 of 2026, while Day of year resets to 1 and says it is counted in 2027. Only 4 January 2027, the first Monday, turns the readout to 1 of 2027, and because that year holds 52 weeks the remaining card reads 51.
Good to know
- The answer is ISO 8601 with a Monday start. Excel's plain WEEKNUM counts from Sunday and forces week 1 onto 1 January; the page shows that count beside the ISO one, and the two disagree on Sundays and around the turn of the year.
- Day of year is counted inside the calendar year you typed, while the week number can belong to the year next door. Enter 29 December 2025 and you get day 363 sitting next to Week 1 of 2026. Both figures are correct, they are just counting different years.
- There is no box for typing a week number and getting its dates back. The week list does that by sight instead, one row per week with its Monday and Sunday, so a quarter's sprint boundaries are thirteen rows to read.
Frequently asked questions
What week of the year is it today?
Whatever number the page is showing when it loads, because it fills the Date box with your own current date first. On 12 August 2026 it says Week 33.
Why does the week start on Monday here?
ISO 8601 defines it that way, and that standard is what payroll systems and shared calendars are built on. It is also why the last two cards are labelled Monday and Sunday rather than Sunday and Saturday.
Why is 1 January sometimes in week 52 of the previous year?
A week belongs to whichever year holds its Thursday. 1 January 2023 was a Sunday, so the Thursday of its week was still back in December, and the date lands in week 52 of 2022. The year beside the number always says which week-year you are reading.
How can a year have 53 weeks?
The leftover days at the ends of the calendar year sometimes stretch far enough to carry an extra Thursday. 2020 had 53 weeks, 2026 has 53, and 2027 drops back to 52. The Weeks remaining card uses the right total for whichever week-year you are on.
Does this agree with Excel?
It matches ISOWEEKNUM exactly, and it matches WEEKNUM when you pass the ISO return type of 21. Left at its default, WEEKNUM counts from Sunday with week 1 on 1 January, and the page shows that count too, under Two conventions on one date, so you can see where the two part.
A colleague abroad says we're in a different week. Who's right?
Probably both of you. The opening date comes from your own clock and time zone, so for a few hours either side of midnight two people genuinely sit on different dates. Typing an explicit date settles the argument, and the copied link pins that same day for whoever opens it.
What is Weeks remaining actually counting?
Whole ISO weeks after the one you are looking at, running to the end of the week-year. In week 33 of a 53-week year the card reads 20. During the final week it shows 0 rather than going negative.
Can I look up a date years ahead?
Any date the box will accept works, and the arithmetic is worked out from the calendar rather than looked up in a table, so leap years and 53-week years both stay right. Planning next year's sprints is the usual reason people go that far out.
What is the day-of-year number good for?
Ordinal dates turn up in aviation logs, satellite data, batch codes and Julian-day style filenames, where 224 is easier to sort than 2026-08-12. It is also a quick sanity check on how much of the year is gone.
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