Week Number
What week is it? See the current ISO 8601 week number, find the week of any date, and get each week's Monday–Sunday range instantly.
About the Week Number
This tool answers "what week number is it?" at a glance: the current ISO 8601 week and year are shown prominently and can be copied in the standard notation (like 2026-W29). Pick any other date to see its week number, the exact Monday-to-Sunday range that week covers, and whether that year has 52 or 53 weeks.
Week numbers follow ISO 8601, the international standard used across European business, manufacturing schedules, project plans and school calendars. ISO weeks always start on Monday, and week 1 is defined as the week containing the year's first Thursday — which means a few days at the start of January can still belong to the previous year's final week.
That subtlety is why different calendars sometimes disagree: US conventions often start weeks on Sunday and call January 1st week 1, while ISO does not. This tool applies the strict ISO rules, so its numbers match what European colleagues, ERP systems and standards-compliant software expect.
How to Use the Week Number
- 1Read the current ISO week number displayed at the top — it's live for today.
- 2Copy it in standard ISO notation (e.g. 2026-W29) with one click.
- 3Pick any date to see its week number and that week's Monday–Sunday range.
- 4Check the total weeks figure to know if the year has 52 or 53 ISO weeks.
Frequently Asked Questions
How are ISO week numbers calculated?
ISO 8601 weeks run Monday through Sunday, and week 1 is the week containing the year's first Thursday — equivalently, the week that includes January 4. Every subsequent week increments from there. This means January 1–3 can fall in week 52 or 53 of the previous ISO year, and December 29–31 can already be week 1 of the next.
Why does my calendar show a different week number?
Probably a different convention. North American calendars often start weeks on Sunday and simply label the week containing January 1 as week 1, which can differ from ISO by one. Outlook, Google Calendar and Excel all have settings choosing between these systems. This tool uses strict ISO 8601, the international standard.
How many weeks are in a year?
An ISO year has 52 weeks usually, and 53 in years that start on a Thursday — or leap years that start on a Wednesday. That happens about every 5–6 years (2026 has 53, for example, while 2025 has 52). The tool shows the exact count for whatever year your chosen date falls in.
Can a date in January belong to last year's week 53?
Yes. If January 1 lands on, say, a Friday, then January 1–3 belong to the final week (52 or 53) of the previous ISO year, because their week's Thursday is still in the old year. The ISO week-year can therefore differ from the calendar year — which is why the standard notation, like 2026-W01, always names both.
What is week number notation like 2026-W15?
It's the compact ISO 8601 format: the week-based year, a W, and the two-digit week number — optionally with a day suffix, so 2026-W15-3 means Wednesday of week 15. It's used in logistics, manufacturing planning and software date libraries because it's unambiguous and sorts naturally as text.