Find the exact ISO 8601 week number for any date, complete with the week's starting and ending boundaries and total weeks in the year.
ISO Week Number
W36
out of 53 weeks in 2026
Week Starts (Monday)
Aug 31st, 2026
Week Ends (Sunday)
Sep 6th, 2026
Last updated: August 2026
The Standard For Business Planning
If you work in manufacturing, logistics, or agile software development, you've likely encountered schedules based on week numbers rather than specific calendar dates. Our calculator uses the strict ISO 8601 standard to ensure your week numbers align perfectly with international business standards.
How the ISO Week Number Is Calculated
Under ISO 8601, every week runs Monday through Sunday, and Week 1 of a year is whichever week contains that year's first Thursday (equivalently, the week that contains January 4th). Counting forward in 7-day blocks from there gives every date in the year a week number from 1 to 52, or 53 in a leap week year.
For example, March 15, 2026 is a Sunday. Counting from the first Thursday of 2026 (January 1st, which is itself a Thursday), that date falls in ISO Week 11 of 2026, running from Monday, March 9 to Sunday, March 15, 2026. Since 2026's January 1st lands on a Thursday, 2026 is also one of the less common 53-week years - its final week, Week 53, runs from December 28, 2026 through January 3, 2027.
Watch Out For the Year-End Edge Case
Because Week 1 is anchored to the first Thursday rather than to January 1st, dates right at the turn of the year can belong to a different ISO week-year than their calendar year suggests. January 1, 2027, for instance, is a Friday - so it actually falls in Week 53 of 2026, not Week 1 of 2027. This calculator always labels results using the correct ISO week-year so the week count stays consistent, even when it looks unintuitive at first glance.
Frequently Asked Questions
Select today's date (already pre-filled) in the calculator above to see the current ISO week number, along with the Monday it started and the Sunday it ends.
The ISO week date system is part of the ISO 8601 international standard used by businesses and governments worldwide. Weeks always start on Monday, and Week 1 of a year is defined as the week containing that year's first Thursday.
Under ISO 8601, weeks start on Monday and end on Sunday. This differs from the everyday US convention, where calendars and some scheduling tools start the week on Sunday. If you're comparing against a Sunday-start calendar, week boundaries and numbers may shift by one.
Most years have 52 ISO weeks. Because a year is about 52.18 weeks long, roughly every 5 to 6 years an extra "leap week" is added, producing a 53-week year. A year has 53 weeks if January 1st falls on a Thursday, or if it's a leap year and January 1st falls on a Wednesday.
Because ISO weeks are defined by the Thursday rule, the first few days of January can belong to the last week of the previous year, and the last few days of December can belong to week 1 of the next year. For example, January 1, 2027 falls on a Friday, so it's actually part of Week 53 of 2026, not Week 1 of 2027 - the calculator's "year" reflects this ISO week-year, not the calendar year.
Week numbers (e.g., W12, W45) are common in manufacturing, logistics, agile software sprints, and retail/fiscal reporting because every week is exactly 7 days, unlike calendar months, which vary from 28 to 31 days. That makes week-based schedules and comparisons more consistent.
Not always. Some US systems (and Excel's default WEEKNUM function) start the week on Sunday and count the week containing January 1st as Week 1, regardless of how many days of that week fall in the new year. ISO 8601, which this calculator uses, starts weeks on Monday and requires at least 4 days of the week to fall within the year. The two methods usually agree in the middle of the year but can differ by one near January.
