Week Number Calculator
Find the ISO week number of any date — plus which days that week covers.
The Week Number Calculator tells you which week of the year any date falls in, using the ISO 8601 standard most of Europe and the business world rely on. Enter a date and you get its week number, its week-year (which can differ from the calendar year around New Year), the Monday-to-Sunday span of that week, and where the date sits within the year.
ISO weeks follow strict rules: weeks run Monday to Sunday, and week 1 is the week containing the first Thursday of the year. The result is that January 1 can belong to week 52 or 53 of the previous year — for example, January 1, 2027 falls in week 53 of 2026. This tool handles those boundary cases exactly, which matters for manufacturing schedules, school terms, payroll periods and any reporting that keys on week numbers.
The calculation is exact ISO 8601 math, runs locally in your browser, and defaults to today's date for quick checks.
ISO week number
Week 36
of ISO week-year 2026
Week starts Monday
Aug 31
ends Sunday
Day of year
249 / 365
calendar days used
Weekday
Sunday
ISO 8601 weeks run Monday–Sunday; week 1 is the week containing the first Thursday of the year, which is why January 1 can still belong to week 52/53 of the previous year.
How to use the Week Number Calculator
- Pick a date (it defaults to today).
- Read the ISO week number and its week-year.
- See the exact Monday-to-Sunday span that week covers.
- Check the day-of-year counter and the weekday.
- Use it to cross-check planning documents, spreadsheets or production schedules that reference week numbers.
Frequently asked questions
How is the ISO week number calculated?
ISO 8601 defines weeks as Monday–Sunday, with week 1 as the week that contains the first Thursday of the Gregorian year. Equivalently, week 1 contains January 4. This tool implements that definition exactly, so results match spreadsheet formulas like ISOWEEKNUM.
Why is my date sometimes in week 52 or 53 of the previous year?
Because early January days before the first Thursday belong to the last week of the previous ISO year. For example, January 1, 2026 is a Thursday (so it starts week 1 of 2026), but January 1, 2027 falls on a Friday and belongs to week 53 of ISO year 2026.
What is the difference between week-year and calendar year?
The week-year is the year the weeks belong to for numbering purposes. Around New Year it can be one less than the calendar year — December 29, 2026 belongs to week 53 of ISO year 2026, while January 1, 2027 still belongs to week 53 of ISO year 2026 as well.
Does every year have 52 weeks?
No — ISO years have 52 or 53 weeks. A 53-week ISO year occurs when January 1 is a Thursday (or a leap year starting on Wednesday). 2026 has 53 weeks; 2025 has 52. The calculator reports whichever applies to your date.
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