Week Number Calculator
Find the ISO week number for any date, and see which dates a given week covers.
How the week number is worked out
Pick a date and the tool returns its ISO 8601 week number. ISO weeks run from Monday to Sunday, and week 1 of a year is the first week that contains a Thursday. That is always the week containing 4 January.
Monday 19 October 2026 is in week 43, and so is every day up to Sunday 25 October. Monday 26 October starts week 44.
The date box opens on 1 January 2000, and the result there is 52, not 1. That date was a Saturday, and the Thursday of its week fell in 1999, so it belongs to the last week of 1999. Change the date to the one you need before reading the number.
Dates at the turn of the year
Because weeks are kept whole, a few days at the start or end of December and January belong to the neighbouring year's numbering. The tool shows the week number only, not which year it belongs to, so these are the dates to watch:
| Date | Tool shows | Full ISO week |
|---|---|---|
| Mon 29 Dec 2025 | 1 | 2026-W01 |
| Thu 1 Jan 2026 | 1 | 2026-W01 |
| Mon 28 Dec 2026 | 53 | 2026-W53 |
| Fri 1 Jan 2027 | 53 | 2026-W53 |
| Sun 3 Jan 2027 | 53 | 2026-W53 |
| Mon 4 Jan 2027 | 1 | 2027-W01 |
So 29 December 2025 is in week 1 of 2026, and 1 January 2027 is in week 53 of 2026. When you write a week reference for a report or a delivery schedule, add the ISO year, as in 2026-W53, so nobody reads it as week 53 of 2027.
Finding the dates that a week covers
The tool goes from a date to a week number, not the other way round. To find the days in week n, start from the Monday of week 1 and add seven days for every week after it:
Monday of week n = Monday of week 1 + (n − 1) × 7 days
For 2026, week 1 starts on Monday 29 December 2025. Week 43 therefore starts 42 × 7 = 294 days later, on Monday 19 October 2026, and ends on Sunday 25 October. The Add or Subtract Days Calculator on this site does the adding for you.
To find the Monday of week 1 for any year, look up the week containing 4 January and take its Monday. For 2027 that is Monday 4 January itself.
When someone else's week number doesn't match
Not every calendar uses ISO numbering. Some start weeks on Sunday or count whichever week holds 1 January as week 1, and then the same date can carry a different number.
If a colleague's planner says week 44 and yours says week 43, check which system each one uses before rescheduling anything. Many calendar apps and spreadsheet week functions let you choose the system. Agreeing to use ISO weeks, written with the year, removes the ambiguity.
Frequently asked questions
How many weeks are there in 2026?+
53. An ISO year has 52 or 53 weeks, and 2026 is a long one: week 53 runs from Monday 28 December 2026 to Sunday 3 January 2027.
What week number is 19 October 2027?+
Week 42, one lower than in 2026. The same calendar date can fall in a different week from one year to the next, because each year starts on a different weekday.
Does the week number depend on my time zone?+
No. It depends only on the calendar date you pick, so the same date gives the same week number everywhere. The one time zone effect is on what today's date is: just after midnight in Tokyo it can still be the previous day in New York.
Embed this tool on your website
Free to use on blogs, school sites and intranets. Paste this code into your page's HTML; please keep the credit line.
<iframe src="https://utilityapp.lat/embed/datetime/week-number-calculator" title="Week Number Calculator" width="100%" height="600" style="border:0;max-width:760px" loading="lazy"></iframe> <p style="font-size:13px"><a href="https://utilityapp.lat/datetime/week-number-calculator">Week Number Calculator</a> by UtilityApp</p>