Week Number
The current ISO 8601 week number is shown below; pick any date to see which week it falls in.
—
Loading current week…
ISO 8601: weeks start on Monday; week 1 contains the year’s first Thursday.
How it works
- Read the current week number and its Monday–Sunday date range shown at the top of the page.
- Use the date picker to select any other date, past or future — the week number updates as soon as you change it.
- Open the all-weeks list to see every week of the selected year with its start and end dates.
Frequently asked questions
- How is the ISO 8601 week number calculated?
- ISO weeks always start on Monday, and week 1 is the week that contains the year’s first Thursday — equivalently, the week containing January 4. This tool applies that rule directly, so a date in late December or early January can belong to a week of the neighboring ISO year.
- Why does my calendar app show a different week number?
- Some calendars, notably in the United States, count weeks starting from January 1 with Sunday as the first day, which can differ from ISO by one week. This tool uses the ISO 8601 standard, which is what European businesses, logistics schedules, and most planners mean by week numbers.
- Can a year have 53 weeks?
- Yes. An ISO year has 53 weeks when January 1 falls on a Thursday, or on a Wednesday in a leap year — this happens roughly every five to six years. The all-weeks list in this tool shows exactly 52 or 53 rows for the selected year, so you can check at a glance.
- Is the date I pick sent anywhere?
- No. The week number is computed with plain date arithmetic in your browser, and nothing is uploaded, logged, or stored on a server. Once the page has loaded, the tool keeps working even if you go offline.
- What is the week number used for in practice?
- Week numbers are standard in manufacturing lead times, freight and delivery schedules, project plans, and payroll periods across much of Europe. Saying “delivery in week 38” is unambiguous once both sides use ISO weeks, and this tool translates that back into concrete Monday–Sunday dates.
About this tool
This tool answers a simple but surprisingly common question: what week number is it? On load it shows the current ISO 8601 week number in large type together with the exact Monday-to-Sunday date range it covers. A date picker below lets you check any other date — a delivery promised for week 42, the week a project milestone lands, or the week number of a birthday — and the result updates the moment you change the date, with no button to press.
The calculation follows the ISO 8601 standard used across Europe and in most business scheduling. Weeks run Monday through Sunday, and week 1 of a year is defined as the week containing the first Thursday of January. That definition has two consequences worth knowing: December 29–31 can belong to week 1 of the following year, and January 1–3 can belong to week 52 or 53 of the previous one. The tool handles both cases correctly and labels the ISO year alongside the week number so there is never any ambiguity at year boundaries.
Everything runs client-side in your browser using plain JavaScript date arithmetic — no date is transmitted or stored anywhere, and the page works offline after it loads. That also means results are instant regardless of connection speed.
For planning a whole year, the collapsible list shows every week of the selected year with its number, start date, and end date, and highlights the week you have picked. Years with 53 ISO weeks — which occur when January 1 falls on a Thursday, or on a Wednesday in a leap year — show all 53 rows. A practical tip: when someone quotes a week number, always confirm they mean ISO weeks, because North American calendars that start weeks on Sunday can be off by one.