Week Number Calculator
See the ISO and US week number of any date, or the dates and working days of any week.
Wednesday is day 3 of ISO week 44, which runs from Oct 26, 2026 to Nov 1, 2026. The US count also gives week 44.
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October 2026
| Wk | Mo | Tu | We | Th | Fr | Sa | Su |
|---|---|---|---|---|---|---|---|
| ISO week 40 | |||||||
| ISO week 41 | |||||||
| ISO week 42 | |||||||
| ISO week 43 | |||||||
| ISO week 44 |
October 2026 spans ISO weeks 40 to 44. Holidays: Columbus Day (Oct 12). Tap a day to see its week; pick a week in the year strip for its dates.
All 53 ISO weeks of 2026
Rows are quarters of 13 weeks; a brass dot marks a week with a holiday.
- Excel or Sheets, ISO
=ISOWEEKNUM(A1) → 44- Excel or Sheets, US
=WEEKNUM(A1) → 44- Weeks from Monday
=WEEKNUM(A1,2) → 44
Find the dates of a week
Switch to Dates of a week above, or pick a week here.
Week 47 of 2026 opens when you type here.
ISO weeks of 2026
| Monday | Sunday | Holidays | ||
|---|---|---|---|---|
| 1 | Dec 29 | Jan 4 | 4 | New Year's Day |
| 2 | Jan 5 | Jan 11 | 5 | |
| 3 | Jan 12 | Jan 18 | 5 | |
| 4 | Jan 19 | Jan 25 | 4 | Birthday of Martin Luther King, Jr. |
| 5 | Jan 26 | Feb 1 | 5 | |
| 6 | Feb 2 | Feb 8 | 5 |
Work days are Monday to Friday minus US federal holidays (5 U.S.C. 6103, observed dates).
Week numbers for a list of dates
Paste a column of dates from a spreadsheet, as 2026-11-26 or 11/26/2026.
| # | Date |
|---|---|
| 1 | |
| 2 | |
| 3 | |
| 4 |
Weeks per date
| Date | ISO week | ISO date | US week |
|---|---|---|---|
| Thu, Jan 1, 2026 | 1 | 2026-W01-4 | 1 |
| Thu, Dec 31, 2026 | 53 | 2026-W53-4 | 53 |
| Fri, Jan 1, 2027 | 53 | 2026-W53-5 | 1 |
| Thu, Nov 26, 2026 | 48 | 2026-W48-4 | 48 |
How it's worked out
The ISO 8601 standard numbers weeks from Monday to Sunday. Week 1 is the week that holds the year's first Thursday, which is the same as the week holding January 4. Every week then belongs to exactly one year, the year its Thursday falls in. To find a date's week, the tool finds the Thursday of that week and counts:
T is the day number of the Thursday in the date's week and J is the day number of January 1 of the Thursday's year. The brackets mean round down. Because the count follows the Thursday, December 29 to 31 can land in week 1 of the next year, and January 1 to 3 in week 52 or 53 of the last one.
The US system is different. Weeks start on Sunday, and week 1 is the week that holds January 1, however few days of it are in the new year. This is what Excel's WEEKNUM returns by default. Some planners use weeks that start on Monday with the same January 1 rule, which is WEEKNUM(date, 2).
A year has 53 ISO weeks when it starts on a Thursday, or when it is a leap year that starts on a Wednesday. That gives 52 weeks in 2025, 53 weeks in 2026, 52 weeks in 2027, 52 weeks in 2028, 53 weeks in 2032.
Worked examples
A delivery in KW 47
A supplier in Germany confirms delivery "in KW 47" (Kalenderwoche, calendar week). In 2026 that week runs from Mon, Nov 16, 2026 to Sun, Nov 22, 2026. It has 5 working days and no US federal holiday, so the goods can arrive any weekday. The week after, week 48, has only 4 because of Thanksgiving.
New Year's Day 2027
Friday, January 1, 2027 is in ISO week 53 of 2026. Its week runs from Mon, Dec 28, 2026 to Sun, Jan 3, 2027, and that week's Thursday, December 31, is in 2026. 2026 is one of the years with 53 weeks because it starts on a Thursday. In the US count the same day is week 1 of 2027.
Why Excel disagrees
Lena types November 1, 2026 into a sheet. =ISOWEEKNUM(A1) gives 44, because that Sunday ends an ISO week. =WEEKNUM(A1) gives 45, because the US week starts on that Sunday. =WEEKNUM(A1,2) gives 44. Match the formula to the system your team uses before sharing a plan.
Reading your result
The big number is the week in the system you chose. The line under it gives the first and last day of that week and the ISO week date, written as year, week and day: 2026-W44-2 is the Tuesday of week 44. That form sorts correctly as text and is used in logistics and many European calendars.
The tiles give the other numbers side by side: ISO, the US count from WEEKNUM, and the Monday-start count. They agree for most of the year and differ near New Year, or by one when the date is a Sunday.
Working days in the weeks table are Monday to Friday minus the holidays you pick. US federal holidays follow 5 U.S.C. 6103, with Saturday holidays observed on Friday and Sunday ones on Monday. England and Wales bank holidays move to the next weekday when they fall on a weekend.
Common mistakes
- Assuming week 1 starts on January 1. In ISO numbering, week 1 of 2025 began on Dec 30, 2024. Dates in the last days of December can belong to the next year's week 1.
- Mixing systems in one plan. In a year that starts on a Friday, Saturday or Sunday, such as 2027, a US colleague using
WEEKNUMand a European partner using ISO are one week apart for most of the year. - Expecting 52 weeks every year. Some years have 53 ISO weeks. A plan that runs to week 52 misses a week in 2026.
- Writing the year from the date. January 1, 2027 is 2026-W53, not 2027-W53. Use the ISO year that the tool shows.
Questions people ask
Sources
- ISO 8601-1:2019 Date and time, Part 1: Basic rules (week dates)
- Microsoft, ISOWEEKNUM and WEEKNUM function documentation
Maintained by Anil ShahUpdated October 28, 2026Checked against 16 reference values