How the ISO week number is calculated
ISO 8601 weeks always run Monday to Sunday. Week 01 of a year is defined as the week containing that year's first Thursday — equivalently, the week containing January 4th.
- week 01 — the Mon–Sun week containing Jan 4 (the year's first Thursday)
- week-numbering year — the year that owns the week's Thursday — can differ from the calendar year
More detail
Why the week-numbering year can differ from the calendar year
A date near December 31 or January 1 can belong to a week whose year label differs from the calendar year. For example, December 29, 2008 (a Monday) starts the week 2008-12-29 to 2009-01-04 — since that week contains January 4, 2009, it's counted as week 2009-W01, not 2008-W53.
Not the same as a plain calendar week
Some calendars and spreadsheet functions start the week on Sunday and number week 1 as whatever week contains January 1st, which can produce a different result. ISO 8601 always starts weeks on Monday and anchors week 1 to January 4th, so it stays consistent across systems that follow the standard — payroll, manufacturing, and project-reporting tools that cite "week number" usually mean this one.
Tip. If a date near a year boundary shows a week-numbering year that looks off by one, check the week's Monday–Sunday range above — that's the definitive check, since the label follows whichever year owns the week's Thursday.
Frequently asked questions
What does a result like 2026-W05 mean?
It means the date falls in the 5th ISO 8601 week of the 2026 week-numbering year. Enter any date above to see its own week number in this format, along with the Monday–Sunday dates that make up that week.
Why does my date near January 1 show last year's week number, or vice versa?
ISO 8601 assigns week 01 of a year to the week containing that year's January 4th. A date like January 1–3 can still fall in the previous year's last week (e.g. 2010-01-03 is week 2009-W53), and a late-December Monday can already start next year's week 01 (e.g. 2008-12-29 starts 2009-W01).
Does the ISO week always start on Monday?
Yes. Every ISO 8601 week runs Monday through Sunday, unlike some calendar conventions that start the week on Sunday. The Week starts (Mon) and Week ends (Sun) outputs above show the exact range for your input date.
Can a year have 53 weeks instead of 52?
Yes — years where January 1 falls on a Thursday (or, in a leap year, on a Wednesday) have a week 53. 2015, 2020, and 2026 all have one (e.g. December 28–31, 2026 falls in 2026-W53), while a year like 2027 tops out at week 52; enter a late-December date above to check any specific year.