Week number: turn a date into a week and back
Pick a date and you see its ISO week number. Or enter a week and a year, and you get the Monday and Sunday of that week.
Calculator
Both ways: from a date to the week number, and from a week number back to the Monday and the Sunday.
Week number and year
Pick a date, or enter a week number and a year.
What this calculator does and does not do
- These are ISO 8601 weeks: they start on Monday, and week 1 is the week containing 4 January
- A week can belong to a different year than the date itself, and the calculator says so when it does
- Not the same as WEEKNUM() in Excel without a second argument: that counts differently around the new year
- No school years, financial years or four-week periods: those depend on an agreement, not on the calendar
Why week 1 often starts in December
Because an ISO week hangs on its Thursday. The first week is the week containing 4 January, and it starts on the Monday on or before that date. For example, 4 January 2026 is a Sunday, so the first week of 2026 already starts on Monday 29 December 2025. That day is still in the old year but belongs to week 1 of 2026. Only when 1 January is itself a Monday do the calendar year and week 1 start on the same day.
- Week 1 is the week with 4 January, not the one with 1 January
- A week runs from Monday up to and including Sunday
- A week belongs to the year in which its Thursday falls
How many weeks in a year? 52 or 53
A year has 53 weeks when 1 January is a Thursday, or when it is a leap year that starts on a Wednesday. In all other cases it has 52. The reason is a remainder: 52 weeks are 364 days, but a year has 365 or 366. One example is 2026, which starts on a Thursday and so has 53 weeks. Its week 53 runs from Monday 28 December 2026 to Sunday 3 January 2027. So 1 January 2027 falls in week 53 of 2026, and that is exactly the warning about the year that the calculator gives.
- ISO weeks, the numbering this calculator uses
- Week 53 always reaches into the next calendar year
- A yearly plan with a fixed 52 columns then does not add up
Calendars and spreadsheets do not always count the same
Excel does not number weeks by ISO automatically, and around New Year that shows. WEEKNUM without a second argument starts the week on Sunday and counts the week containing 1 January as week 1. That is not ISO, and it is also the numbering many US calendars use, so week numbers can differ between countries and tools. You get ISO numbering with ISOWEEKNUM, or with WEEKNUM and 21 as the second argument. If someone sends you a plan, first check which numbering it uses.
- You get ISO with ISOWEEKNUM, or WEEKNUM with 21
- WEEKNUM without a second argument starts the week on Sunday
- The difference mostly shows around New Year
Where this comes up in practice
In schedules and timesheets. They are kept per week, approved per week and processed per period, and then the week number is the key everything hangs on. A weekly total that groups by week number alone throws week 1 of two different years together as soon as the year is missing.
- Group by year and week, never by the week alone
- A week across New Year belongs to one of the two years
- Check the first and last week of the year by hand
Week numbers in daily work
The week number organizes shifts, leave and deadlines. How many hours add up in a week is worked out by add hours, and how long a single shift lasted is worked out by the hours calculator.
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