Week number calculator for ISO 8601 and US calendars
Offices, factories, schools and software teams often plan in weeks rather than dates: “the release is in week 23”, “invoices close in KW 48”, “the shipment arrives in week 7”. This week number calculator tells you which week any date belongs to, which dates make up any week, and lists every week of a year with its start and end dates.
It supports the two systems people actually use. ISO 8601 weeks run Monday to Sunday and are the standard across Europe, much of Asia and in most business software. US week numbering runs Sunday to Saturday and starts week 1 on the week containing 1 January. The current week is highlighted so you can answer “what week is it?” at a glance.
How to use the Week Number Calculator
- Choose the numbering system at the top: “ISO 8601 · Monday start” or “US · Sunday start”.
- Pick a date, or press “Today”. The result shows the week number, its date range and whether it is this week, next week or further away.
- To go the other way, enter a year and a week number under “Find the dates of a week” to see all seven days of that week.
- Under “All weeks of a year”, type a year to list every week with its first and last day. The current week is highlighted.
- Press “Download CSV” to take the week list into Excel, Google Sheets or a planning tool.
Week Number Calculator features
Correct at year boundaries
Dates such as 1 January 2027 (week 53 of 2026) and 30 December 2024 (week 1 of 2025) get the right week and week-year.
Two numbering systems
Switch between ISO 8601 and the Sunday-based US system used by Excel’s default WEEKNUM function.
Week to date lookup
Turn “2026-W14” into the exact Monday-to-Sunday dates, with days outside the year shown faded.
Full year table
All 52 or 53 weeks of any year from 1 to 9999, exportable as CSV.
Exact calendar maths
Uses integer day counts rather than clock times, so daylight-saving changes can never shift a week.
How ISO 8601 week numbers work
ISO 8601 defines week 1 as the week that contains the year’s first Thursday — which is the same as the week containing 4 January. Weeks always start on Monday, and every week belongs entirely to one week-year. Because of that rule, 1–3 January can belong to the last week of the previous year, and 29–31 December can belong to week 1 of the next year.
A quick way to find the week yourself: take the Thursday of the week your date falls in. The year of that Thursday is the week-year, and the week number is that Thursday’s day of the year, plus 6, divided by 7 and rounded down. For example, Wednesday 7 October 2026 belongs to the week whose Thursday is 8 October, day 281 of 2026, so the week is (281 + 6) ÷ 7 = 41.
ISO weeks are written as 2026-W41, or 2026-W41-3 to name a specific day (3 = Wednesday). In German-speaking countries the same number is called the Kalenderwoche or KW.
Years with 53 weeks
A year has 53 ISO weeks when 1 January is a Thursday, or when it is a leap year that starts on a Wednesday. That happens 71 times every 400 years — roughly every five or six years. Recent and upcoming long years are 2015, 2020, 2026, 2032, 2037, 2043 and 2048. Payroll, retail and manufacturing calendars that assume 52 weeks need an extra period in these years.
In the US system every year has at least 53 numbered weeks, because the first or last week is cut short by the year boundary. A leap year that starts on a Saturday — 2000, 2028 and 2056 — even reaches week 54, which contains only 31 December.
ISO and US week numbers compared
The two systems often differ by one week, especially in January. Check which one your company, school or software expects before you share a week number.
| ISO 8601 | US (Sunday start) | |
|---|---|---|
| First day of the week | Monday | Sunday |
| Week 1 is… | the week with the first Thursday | the week containing 1 January |
| Weeks per year | 52 or 53 | 53, or 54 in a leap year starting on Saturday |
| Week that crosses New Year | Belongs wholly to one week-year | Last week of one year and week 1 of the next |
| Excel / Google Sheets | ISOWEEKNUM(A1) or WEEKNUM(A1,21) | WEEKNUM(A1) or WEEKNUM(A1,1) |
| Used in | Europe, ISO standards, most of Asia | United States, Canada, some payroll systems |
Tips for planning by week
- Always write the week-year with the week (
2026-W53, not just “week 53”) so nobody reads it as the wrong year. - To count working days inside a run of weeks, use the Business Days Calculator; to print a wall planner with week numbers, use the Calendar Generator.
- If you only need to know which weekday a date falls on, the Day of the Week Calculator also explains the maths.
Week Number Calculator FAQ
What week number is it today?
Open this page and the date field is set to today, so the result shows the current week number straight away, in ISO 8601 by default. Switch to “US · Sunday start” if you need the US number.
How many weeks are there in a year?
Most years have 52 ISO weeks; some have 53. A year has 53 ISO weeks when it begins on a Thursday, or when it is a leap year beginning on a Wednesday. 2026 is one of them.
Why is 1 January sometimes in week 52 or 53?
Under ISO 8601, week 1 must contain the first Thursday of the year. If 1 January falls on a Friday, Saturday or Sunday, those days still belong to the last week of the previous year.
Why does Excel give a different week number?
Excel’s WEEKNUM uses the US system by default. Use ISOWEEKNUM, or WEEKNUM with 21 as the second argument, to get ISO 8601 week numbers that match this week number calculator in ISO mode.
How do I find the dates of a week number?
Enter the year and week number under “Find the dates of a week”. The calculator shows the first and last day and all seven dates in between.
What is an ISO week-year?
It is the year a week belongs to under ISO 8601. It matches the calendar year except for a few days around New Year — for example, 1 January 2027 is in week-year 2026.