Week Number Calculator
Find the ISO 8601 week number and week date for any day, compare it with US and Excel WEEKNUM weeks, and list the dates in any week of the year.
The interactive tool loads here when JavaScript runs.
About this toolWhat is the Week Number Calculator?
Pick a date and the Week Number Calculator shows which week of the year it falls in under every system people actually use: the ISO 8601 week (Monday start, used across Europe and in most business software), the US week (Sunday start, week 1 is the week with 1 January), Excel's WEEKNUM with a Monday start, and the C and Python strftime %U and %W counters. For 29 October 2026 that is ISO week 44 (2026-W44-4), US week 44, and %U/%W week 43.
It also works the other way round. Enter a year and a week number to see the seven dates in that week, and scroll the week calendar to see every week of the year with its first and last day. The week containing your chosen date is highlighted, and Previous week and Next week buttons step through the year.
Dates near New Year get a note when the ISO year differs from the calendar year, which is the usual source of confusion: 31 December 2025 is in ISO week 1 of 2026, and 3 January 2021 is in week 53 of 2020.
- ISO 8601 week number, week-numbering year and week date
- US (Sunday) and Excel WEEKNUM Monday-start week numbers
- strftime %U and %W, day of year, quarter and days left
- Dates for any week number, plus a full-year week calendar
- Exact integer date maths, years 1 to 9999, nothing sent to a server
How to use the Week Number Calculator
- Choose a date with the picker, or press Today.
- Read the ISO week, the ISO week date (YYYY-Www-D) and the Monday-to-Sunday range, then the US, Excel and strftime numbers beside it.
- To see the dates in a particular week, pick ISO or US, enter the year and the week number under "Dates in a week", and click a date to load it.
- Use the week calendar to scan every week of the year, switching between ISO and US weeks.
- Copy the results or share a link that reopens the same date.
Worked examples
A date in late October
Input
29 October 2026
Output
ISO 2026-W44-4 (26 Oct to 1 Nov); US week 44 (25 to 31 Oct); WEEKNUM(,2) 44; %U 43, %W 43; day 302 of 365
The ISO and US numbers agree here, but strftime counts from week 0, so it is one lower. 2026 starts on a Thursday, which is also why it has 53 ISO weeks.
New Year's Eve in the next ISO year
Input
31 December 2025
Output
ISO 2026-W01-3; US week 53 of 2025
The Thursday of that Monday-to-Sunday week is 1 January 2026, so the whole week belongs to ISO year 2026.
Listing a week
Input
ISO year 2026, week 1
Output
Monday 29 December 2025 to Sunday 4 January 2026
Week 1 always contains 4 January, so its Monday can be as early as 29 December.
Background and insight
ISO 8601 defines a week as Monday to Sunday and numbers the weeks of a "week-numbering year" so that week 1 is the week containing the year's first Thursday. Equivalent rules: week 1 contains 4 January, or week 1 is the first week with at least four days in the new year. Because whole weeks never split, a week-numbering year has 52 or 53 weeks, and up to three days at either end of the calendar year can belong to the neighbouring ISO year. A year has 53 ISO weeks exactly when 1 January is a Thursday, or a Wednesday in a leap year; 2020 and 2026 are recent examples.
The US convention used by most American wall calendars, and by Excel's WEEKNUM(date, 1), starts weeks on Sunday and makes week 1 the week that contains 1 January, however short it is. Week numbers therefore restart on 1 January and the first and last weeks are often partial, which gives 53 and sometimes 54 numbered weeks. WEEKNUM(date, 2) uses the same rule with Monday as the first day. Excel's ISOWEEKNUM and WEEKNUM(date, 21) return ISO weeks.
Programmers meet two more counters. C's strftime, and Python's, has %U (weeks starting Sunday, days before the first Sunday are week 0) and %W (the same with Monday). These are not ISO numbers; ISO weeks are %V with %G for the week-numbering year. The calculator does all of this with integer day counts rather than JavaScript Date objects, so time zones and daylight-saving changes cannot move a result.
Common use cases
Payroll and timesheets
Match a pay period or a timesheet labelled "week 44" to real dates, in whichever system your payroll provider uses.
Sprint and release planning
Teams that name sprints or releases by ISO week (2026-W44) can check which dates a label covers.
Logistics and manufacturing
Delivery schedules and production plans in Europe are often quoted in week numbers; translate them into dates.
Spreadsheet and code checks
Confirm what WEEKNUM, ISOWEEKNUM, %U, %W or %V should return before debugging a report.
Tips and limitations
- Always say which system you mean. "Week 1" of 2027 is 4 to 10 January in ISO weeks but 1 to 2 January in US weeks.
- Group ISO weeks by the ISO year, not the calendar year, or the first days of January will land in the wrong bucket.
- In SQL and code, use the ISO week-year field (%G, IYYY, isocalendar().year) together with the ISO week, never the calendar year.
- Dates are Gregorian throughout, extended backwards before 1582; for historical Julian dates use the Day of the Week Calculator.
Frequently asked questions
What is the ISO week number rule?
Weeks run Monday to Sunday and week 1 is the week that contains the first Thursday of the year, which is the same as the week containing 4 January. The year of that Thursday is the ISO week-numbering year.
Why is 31 December sometimes in week 1?
If 31 December falls on a Monday, Tuesday or Wednesday, the Thursday of its week is in January, so ISO 8601 puts that whole week in week 1 of the next year. 31 December 2025, a Wednesday, is 2026-W01-3.
Which years have 53 ISO weeks?
Years that start on a Thursday, and leap years that start on a Wednesday. Recent ones are 2015, 2020 and 2026; the next after that is 2032.
How do US week numbers differ from ISO week numbers?
US weeks start on Sunday and week 1 is whatever week contains 1 January, so the first and last weeks can be just a day or two long. ISO weeks start Monday and are always seven days, so they can begin in late December.
What does Excel WEEKNUM return compared with ISOWEEKNUM?
WEEKNUM(date) or WEEKNUM(date, 1) gives the US Sunday-start number, WEEKNUM(date, 2) the Monday-start version of the same rule, and ISOWEEKNUM(date) or WEEKNUM(date, 21) the ISO 8601 week. The calculator shows the first three side by side.
Why does Python strftime %U give a lower number?
%U and %W count days before the year's first Sunday or Monday as week 0, so they usually read one less than WEEKNUM. For ISO weeks use %V with %G, or date.isocalendar().
What does a week date like 2026-W44-4 mean?
ISO year 2026, week 44, day 4, where Monday is 1 and Sunday is 7. 2026-W44-4 is Thursday 29 October 2026.
Is there an in-depth guide to the Week Number Calculator?
Yes. “ISO Week Numbers Explained: Week 1, Week 53 and the US System” covers the background, how it works, common pitfalls and worked examples. Read the full guide.
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