Pick a date and get its ISO week number, the Monday to Sunday it belongs to, the day of the year, and the US Sunday-start week too.
I built this week number calculator for the moment your boss says “we ship in week 44” and you nod like that means anything. Loads of offices, warehouses and school districts plan this way. Which is fine, right up until you need actual dates. So pick a day. You’ll get its ISO 8601 week, the Monday and Sunday on either side, and the day of the year. I also tossed in the US-style number (weeks starting Sunday) because the two don’t always agree, and that little mismatch has wrecked more than one shared calendar I’ve been on.
What week number is it?
ISO 8601 says weeks start Monday, and week 1 is whichever week holds the year’s first Thursday. The week number calculator opens on today, so that big number up there is this week. Want a fixed example? Monday, October 5, 2026 lands in ISO week 41, running October 5 to 11, and it’s day 278 of the year.
How ISO week numbers work
Honestly there are only three rules. Monday to Sunday, that’s a week. Week 1 is the one with January’s first Thursday in it (same thing as the week with January 4, if that’s easier to remember). And weeks never get chopped between two years, so sometimes a couple of January days end up in week 52 or 53 of the old year. Looks like a bug the first time. It isn’t.
where weekday runs Monday = 1 to Sunday = 7
Quick test with October 5, 2026: day 278, a Monday. (278 − 1 + 10) ÷ 7 = 41, drop the decimals if you get any. Ended up with 0? Then the date belongs to last year’s final week. And a 53 in a year that only has 52 means you’re actually in week 1 of next year. Weird, but that’s the deal.
Years with 53 ISO weeks
| Year | January 1 falls on | ISO weeks |
|---|---|---|
| 2026 | Thursday | 53 |
| 2027 | Friday | 52 |
| 2032 | Thursday (leap year) | 53 |
| 2037 | Thursday | 53 |
| 2043 | Thursday | 53 |
You get a 53rd week when the year kicks off on a Thursday, or when it’s a leap year starting on a Wednesday. 2026 qualifies. Which is why Friday, January 1, 2027 still counts as week 53 of 2026, and yeah, payroll people usually learn that one the hard way.
When ISO and US week numbers disagree
| Date | ISO week | US week (Sunday start) |
|---|---|---|
| Sun, Jan 4, 2026 | 1 | 2 |
| Mon, Oct 5, 2026 | 41 | 41 |
| Sun, Dec 27, 2026 | 52 | 53 |
| Fri, Jan 1, 2027 | 53 (of 2026) | 1 |
| Sun, Jan 3, 2027 | 53 (of 2026) | 2 |
Heads up on Excel. Plain WEEKNUM goes US-style unless you tell it not to, so type =ISOWEEKNUM(A2) or =WEEKNUM(A2,21) for the ISO one. Google Sheets has both too. Python folks, it’s date.isocalendar().week. Sending a plan to a team in Germany or wherever? Peek at which formula your sheet used first. Saves a confused email.
How to use the Week Number Calculator
Pick a date
Today’s already filled in. Pick some other day if you like, and if you wander off, the Today button drags you back.
Read the ISO week number
That big number? That’s your week. The Monday and Sunday that box it in sit right below.
Check the extras
Further down there’s the ISO code (looks like 2026-W41), the day of the year, how many weeks this year has, and the US Sunday-start number for comparison.
How do I get the week number in Excel, Google Sheets or Python?
Spreadsheets and programming languages ship with more than one week rule, so pick the function that matches the system you need.
- Excel:
=ISOWEEKNUM(A1)returns the ISO 8601 week (added in Excel 2013).=WEEKNUM(A1)on its own uses return type 1, where weeks start on Sunday and week 1 is the week containing January 1.=WEEKNUM(A1,21)gives the ISO result too. - Google Sheets:
ISOWEEKNUMandWEEKNUMbehave the same way as in Excel. - Python:
date.isocalendar()returns the ISO year, week and weekday. Instrftime,%Vis the ISO week,%Ucounts Sunday-first weeks and%Wcounts Monday-first weeks, both starting at week 00. - JavaScript: the built-in
Dateobject has no week number method, so you compute it yourself or use a date library.
A quick test: January 1, 2026 is a Thursday, so %V gives 01 while %U and %W give 00. If you also need the weekday name for any date, the day of the week calculator shows it directly.
What does an ISO week date like 2026-W01-1 mean?
ISO 8601 defines a full week date notation in the form YYYY-Www-D. The first part is the ISO week-numbering year, then the letter W and a two-digit week, then the day of the week from 1 (Monday) to 7 (Sunday). The compact form drops the hyphens, as in 2026W011.
The catch is that the ISO year is not always the calendar year. Because week 1 is the week that holds the first Thursday, a few days at the edges of the year belong to the neighboring ISO year:
| Calendar date | ISO week date |
|---|---|
| Monday, December 29, 2025 | 2026-W01-1 |
| Thursday, January 1, 2026 | 2026-W01-4 |
| Friday, January 1, 2027 | 2026-W53-5 |
That is why a report labeled week 53 can carry dates from the next January. When you sort or group data by week, store the ISO year next to the week number, not the calendar year. To count the exact span between two such dates, use the days between dates calculator.
Frequently asked questions
What week of the year is it?
Just look at the big number, the page opens on today. If you’re reading this later, for reference, October 5, 2026 sits in week 41.
Does 2026 have 53 weeks?
Yep. January 1, 2026 fell on a Thursday, and that’s the trigger. Week 53 starts Monday, December 28, 2026 and wraps up Sunday, January 3, 2027.
Why is January 1 sometimes in week 52 or 53?
ISO gives each week to whichever year owns its Thursday. So if New Year’s Day falls on a Friday, Saturday or Sunday, that week’s Thursday was still in December, and the day gets lumped in with last year. Feels backwards, I know.
What's the difference between ISO and US week numbers?
ISO starts on Monday and counts from the first-Thursday week. The US habit starts Sunday and calls the week with January 1 week 1, even if it’s a one-day stub. Most of the year they match, or they’re one apart.
How do I get the ISO week number in Excel?
=ISOWEEKNUM(A2) does it, so does =WEEKNUM(A2,21). Plain =WEEKNUM(A2) gives you the Sunday-start US number, and that’s the one that bites people around Christmas.
Which dates are in week 1 of 2027?
Monday, January 4 to Sunday, January 10, 2027. Those first three days of January? Still week 53 of 2026.
What does 2026-W41 mean?
ISO shorthand. Year, then a W, then the week in two digits. It sorts nicely in file names, which is probably why planning tools are so fond of it.
Sources and further reading
Related tools
Last reviewed: October 2026