Week Number Calculator
Find the week number of any date, or the dates of any week. Uses ISO 8601 weeks (Monday start, the standard in Europe and most of the world), with US and Middle East weeks in Advanced.
Used for the big answer, the week dates and the year list.
31 December, unless you change it.
How your week number is worked out
The ISO 8601 rule: every week belongs to the year its Thursday is in.
- 1
Weekday number
Mon 19 Oct 2026
Mon = 1 … Mon = 1 - 2
Find the Thursday
19 Oct − 1 + 4
Thu 22 Oct 2026 - 3
Its day of the year
22 Oct is day
295 of 2026 - 4
Count whole weeks
⌊(295−1)÷7⌋+1 = 43
2026-W43-1
Read more: the formula, a worked example, the other week systems and 53-week years
ISO 8601 is the international standard for week numbers, used for business, school terms, shipping and government across Europe and much of the world. Weeks start on Monday, and each week belongs to the year its Thursday falls in. That one rule decides week 1 and whether a year has 52 or 53 weeks.
The formula
ISO week (Monday = 1 … Sunday = 7):
1. weekday number of your date: Monday 1, Tuesday 2 … Sunday 7
2. Thursday of the same week = your date − weekday number + 4
3. the ISO year = the calendar year of that Thursday
4. week number = ⌊(day of the year of that Thursday − 1) ÷ 7⌋ + 1
Dates of a week (reverse):
Monday of week 1 = 4 January − (weekday number of 4 January − 1)
Monday of week W = Monday of week 1 + (W − 1) × 7 days; Sunday = Monday + 6 days
US style (Sunday start) and Middle East style (Saturday start):
days before = days of week 1 that fall in the old year (0 to 6)
week = ⌊(day of the year − 1 + days before) ÷ 7⌋ + 1
Simple week = ⌊(day of the year − 1) ÷ 7⌋ + 1
Worked example
What week is Tuesday 13 October 2026? Tuesday is weekday 2. The Thursday of that week is 13 − 2 + 4 = 15 October 2026, so the week belongs to 2026. 15 October is day 288 of the year (31 + 28 + 31 + 30 + 31 + 30 + 31 + 31 + 30 = 273 days to the end of September, plus 15). ⌊(288 − 1) ÷ 7⌋ + 1 = ⌊287 ÷ 7⌋ + 1 = 41 + 1 = week 42. Written the ISO way that is 2026-W42-2, and the week runs from Monday 12 to Sunday 18 October.
13 October itself is day 286 of 365, so 365 − 286 = 79 days are left in 2026. 1 January 2026 was a Thursday, so 2026 has 53 ISO weeks. In the US system 4 days of week 1 fell in December 2025 (Sunday 28 to Wednesday 31), so the week is ⌊(286 − 1 + 4) ÷ 7⌋ + 1 = ⌊289 ÷ 7⌋ + 1 = 42 (Sunday 11 to Saturday 17 October). In the Middle East system 5 days fell in 2025 (Saturday 27 to Wednesday 31 December), giving ⌊290 ÷ 7⌋ + 1 = 42 (Saturday 10 to Friday 16 October). The simple week is ⌊285 ÷ 7⌋ + 1 = 41. It is in quarter 4.
The four week systems
| System | Week starts | Week 1 is | Weeks a year | Excel / Google Sheets |
|---|---|---|---|---|
| ISO 8601 | Monday | The week with the first Thursday (and 4 January) | 52 or 53 | =ISOWEEKNUM(A1) |
| US style | Sunday | The week with 1 January | 53 or 54 | =WEEKNUM(A1) |
| Middle East style | Saturday | The week with 1 January | 53 or 54 | =WEEKNUM(A1,16) |
| Simple | Same weekday as 1 January | 1 to 7 January | 53 (the last is 1 or 2 days) | =INT((A1-DATE(YEAR(A1),1,1))/7)+1 |
In the US and Middle East systems week 1 and the last week are often short: the calculator still shows all 7 days of those weeks, including the days that fall in the year next door. When someone gives you a week number, ask which system they mean; in Europe and in most software settings for business it will be ISO.
Years with 53 ISO weeks
A year has 53 ISO weeks when it starts on a Thursday, or on a Wednesday in a leap year. That happens 71 times in every 400 years, about once every 5 or 6 years. From 2015 to 2050: 2015, 2020, 2026, 2032, 2037, 2043, 2048.
Common questions
What week number is it today?
The calculator opens on today's date and shows its ISO week number straight away, with the Monday to Sunday dates of the week. On 13 October 2026, for example, it is week 42 (Monday 12 to Sunday 18 October).
How are ISO week numbers worked out?
ISO 8601 weeks start on Monday, and week 1 is the week that has the year's first Thursday in it (which is also the week with 4 January). Each week belongs to the year its Thursday is in, so a few days at the start or end of a year can belong to the week-year next door.
Does a year have 52 or 53 weeks?
An ISO year has 53 weeks when 1 January is a Thursday, or a Wednesday in a leap year; every other year has 52. Between 2015 and 2050 the 53-week years are 2015, 2020, 2026, 2032, 2037, 2043, 2048.
Why is 1 January in week 53?
If 1 January falls on a Friday, Saturday or Sunday, that week's Thursday is still in December, so the days belong to the last week of the year before. 1 January 2027 is a Friday, so it is in week 53 of 2026 (2026-W53-5).
Why do US week numbers sometimes differ?
In the US system weeks start on Sunday and week 1 is simply the week with 1 January in it, however short. This matches Excel's plain WEEKNUM(). The answer can be one week higher than the ISO number, and a year can have 53 or even 54 numbered weeks.
How do I get the week number in Excel or Google Sheets?
Use =ISOWEEKNUM(A1) for the ISO week, =WEEKNUM(A1) for the US Sunday-start week and =WEEKNUM(A1,16) for weeks starting on Saturday. The calculator shows all three side by side in Advanced.
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