Week Number Calculator
Today’s week number, the week of any date and the dates of any week.
Week number of any date
Dates of a week number
Every week of the year
| Week | Dates | Days |
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About the Week Number Calculator
Week numbers are not the same everywhere. Most of Europe, ISO-based business software and many project plans use ISO 8601 weeks, which start on Monday and count week 1 from the week that contains the year’s first Thursday. The US convention starts weeks on Sunday and makes the week that contains 1 January week 1 — the default of the spreadsheet function WEEKNUM. Calendars in Iran, Egypt and several Arab countries start the week on Saturday.
This calculator shows the current week at the top, the week of any date in all of these systems side by side, the dates of any week number, and every week of a year as a table you can copy or download as CSV. It works out everything in your browser from the calendar rules alone, so it works offline and for any year from 1 to 9999.
How to use it
- The box at the top shows this week — the ISO week number, the dates of the week and the ISO week date — worked out from your device’s date.
- Under Week number of any date, pick a date to see its week in every system, with the matching Excel or Google Sheets formula.
- Under Dates of a week number, choose the numbering system, type a year and a week number, and use the arrows to step through the weeks.
- Under Every week of, choose a year and a system to list all 52, 53 or 54 weeks, then Copy them or Download CSV for a spreadsheet.
Examples
ISO week 40 (2026-W40-6) · US week 40 · Saturday-start week 41
ISO week 53 of 2026 · US week 1 of 2027
ISO week 1 of 2027 begins on Monday 4 January, because 1–3 January are part of a week whose Thursday is 31 December 2026.
Monday 29 December 2025 – Sunday 4 January 2026
54 weeks
2028 is a leap year and starts on a Saturday, so US week 1 is just 1 January and week 54 is just 31 December.
Common uses
- Writing timesheets, sprint plans and delivery schedules that refer to “week 40”.
- Matching week numbers between colleagues in Europe (ISO) and the US (Sunday weeks).
- Checking which week numbering a spreadsheet, report or ERP system uses.
- Building a list of all weeks of a year for a planner or a payroll calendar.
How ISO 8601 week numbers work
ISO 8601 weeks run from Monday to Sunday. Week 1 is the week that contains the first Thursday of January — equivalently, the week that contains 4 January — so at least four of its days are in the new year.
- 1, 2 or 3 January can belong to the last week of the previous year, and 29, 30 or 31 December to week 1 of the next. The year a week belongs to is called its ISO week-year.
- A year has 53 ISO weeks when it starts on a Thursday, or when it is a leap year that starts on a Wednesday. 2026 and 2032 have 53 weeks; 2027 has 52.
- The ISO week date writes a day as year, week and weekday (1 = Monday): Saturday 3 October 2026 is
2026-W40-6.
Weeks that contain 1 January (US, Saturday-start and Excel type 2)
In this family of systems — the “System 1” of the spreadsheet function WEEKNUM — week 1 is always the week that contains 1 January, so every date belongs to its own calendar year. The first and last weeks are usually shorter than seven days, and a year has 53 weeks, or 54 in a leap year whose 1 January is the last day of a week (in the US system: 2000 and 2028).
The start day decides the rest: Sunday is the US convention (WEEKNUM default), Saturday is WEEKNUM(date,16), and Monday is WEEKNUM(date,2). The Monday version is not the same as ISO: in 2027, for example, 1 January is in week 1 under WEEKNUM(date,2) but in week 53 of 2026 under ISO.
Spreadsheet formulas
Each card shows the formula that gives the same number in Excel or Google Sheets, with the date in cell A1:
- ISO 8601:
=ISOWEEKNUM(A1)or=WEEKNUM(A1,21) - US (Sunday start):
=WEEKNUM(A1)or=WEEKNUM(A1,1) - Saturday start:
=WEEKNUM(A1,16) - Monday start, week containing 1 January:
=WEEKNUM(A1,2)
Limitations
- Dates use the Gregorian calendar for every year from 1 to 9999, including years before it was introduced in 1582.
- “This week” comes from your device’s date and time zone; if the device clock is wrong, the current week will be too.
- Fiscal and retail calendars (4-4-5 quarters, broadcast weeks, company years that start in April or July) number their weeks differently and are not covered.
Privacy
Everything happens in your browser. What you enter or open here is not uploaded or stored by MySmartCoPilot.
Frequently asked questions
What week is it today?
The box at the top of the page shows the current week number, worked out from your device’s date: the ISO 8601 week first, then the US and Saturday-start numbers, which can differ by one.
Why is 1 January 2027 in week 53?
Under ISO 8601, week 1 is the week with the year’s first Thursday. 1 January 2027 is a Friday in the week that runs from Monday 28 December 2026 to Sunday 3 January 2027, whose Thursday is in 2026, so it is week 53 of 2026. In the US system the same day is week 1 of 2027.
How many weeks are there in a year?
Every year has 52 full weeks and one or two extra days. ISO years therefore have 52 or 53 weeks. In systems where week 1 contains 1 January, the extra days start a 53rd week, and in some leap years a 54th.
Does the week start on Sunday or Monday?
Both are common. ISO 8601 and most of Europe start on Monday; the US, Canada, Japan, India and many other countries print calendars that start on Sunday; Iran, Egypt and several Arab countries start on Saturday. That is why the same date can have different week numbers.
How do I get the ISO week number in Excel?
Use =ISOWEEKNUM(A1) (Excel 2013 and later, and Google Sheets) or =WEEKNUM(A1,21). Plain =WEEKNUM(A1) uses US Sunday weeks and can give a different number — around New Year very different: 1 January 2027 is ISO week 53 (of 2026) but WEEKNUM week 1.