ISO Week Number Finder

Week numbers sound simple until two people use different ones. One person counts weeks from 1 January, another starts counting on the first Monday, and a third counts the first full week, so the same date gets three different week numbers and every plan built on them quietly disagrees. The ISO-8601 standard ends that confusion with one rule used across business, payroll and project planning in much of the world, and this tool applies it to any date you enter.

The ISO rule has two parts, and both matter. Weeks start on Monday, not Sunday, so a week runs Monday to Sunday. Week 1 of a year is the week containing the first Thursday of that year, which is the same as saying week 1 is the week containing 4 January. Most years have 52 ISO weeks, but years whose 1 January falls on a Thursday, or leap years starting on a Wednesday, have 53. This tool returns the week number, the week year, the Monday and Sunday of that week, and how many weeks the week year contains.

Here is a worked example. The date 2026-10-27 falls in ISO week 44 of 2026, and that week runs from Monday 26 October 2026 to Sunday 1 November 2026. The year 2026 has 53 ISO weeks, because its 1 January falls on a Thursday, so week 44 sits comfortably in the middle of the year with no edge-case surprise. A more surprising example is 2021-01-01, which belongs to ISO week 53 of 2020, not week 1 of 2021, because the week containing that Friday started on Monday 28 December 2020 and its Thursday fell in 2020.

The week year is the detail that catches most people out. Early January dates often belong to the previous week year, and late December dates often belong to the next one, because ISO weeks never split across the year boundary: a week belongs entirely to the year containing its Thursday. That means a date in early January can carry a week number like 52 or 53 with last year as its week year, and a date in late December can carry week 1 with next year as its week year. Reporting the week without the week year is therefore incomplete, and this tool always gives both.

ISO weeks matter most wherever people coordinate across teams, countries or systems. Payroll periods, sprint plans, school terms, shipping schedules and financial reporting often run on ISO weeks precisely because the standard removes local arguments about when week 1 starts. If a supplier says delivery in week 44, a factory in another country reads the same Monday-to-Sunday span, with no need to ask which counting method was meant. That shared meaning is the whole value of a standard.

A common mistake is mixing ISO weeks with other week counts. Calendar apps and spreadsheets sometimes default to weeks starting on Sunday, or to counting week 1 from 1 January regardless of the weekday, and those counts can differ from ISO by one for weeks at a time. If your tool and your colleague disagree by one week, check the week-start day and the week-1 rule first: nine times out of ten, one of you is not using ISO. This finder uses the ISO rule only, so its answers match any other correct ISO implementation.

Use this finder whenever a week number appears in a plan, a report, or a deadline. Enter the date in year-month-day order, read the week number together with its week year, and use the Monday-to-Sunday range to book work inside that week. For year-end planning, check late December and early January dates individually rather than assuming they belong to the year on the calendar page: the ISO week year at the edges of the year is exactly where costly off-by-one mistakes hide. One wrong week at year end can shift payroll, reporting and deliveries together, so a thirty-second check here is time very well spent.

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