Week Number Calculator
Select a date to find its ISO‑8601 week number (Monday‑first week).
Select a date to find its ISO‑8601 week number (Monday‑first week).
The date 27 July 2026, which is a Monday.
ISO week 31 of 2026.
The Thursday rule means the first days of January can fall in the last week of the previous ISO year, and the last days of December can fall in week 1 of the next. 1 January 2027 is a Friday, and the Thursday of its week is 31 December 2026, so that date sits in week 53 of 2026 rather than week 1 of 2027. This is correct ISO behaviour, not an error — which is exactly why the tool prints the ISO year alongside the week rather than assuming it matches the date.
A year gets 53 ISO weeks when it starts on a Thursday, or when it is a leap year starting on a Wednesday. Everything else gets 52. Reporting systems that assume a fixed 52 produce a phantom period or silently drop one every few years, which is a familiar source of year-end discrepancies in weekly dashboards.
Some regions and some spreadsheet defaults start weeks on Sunday, and some define week 1 as the week containing 1 January regardless of weekday. Those systems return a different number for the same date, often off by one. If you are reconciling a week number against another system, confirm which convention it uses before assuming one of them is broken.
Manufacturing and logistics schedules, payroll and shift rotas, retail planning calendars and project plans frequently run on ISO weeks because a week is a constant length and a month is not. That makes week-over-week comparison honest in a way month-over-month is not.
Enter any date and the page returns its ISO 8601 week and the ISO year that week belongs to. Weeks run Monday to Sunday and week 1 is the one containing the first Thursday of January.
Because ISO assigns a week to whichever year contains its Thursday. If 1 January lands on a Friday, Saturday or Sunday, that week’s Thursday was in December, so those days belong to the final week of the previous ISO year.
Fifty-two in most years and fifty-three in some — a year has 53 ISO weeks when it begins on a Thursday, or when it is a leap year beginning on a Wednesday. Any system that hardcodes 52 will eventually be wrong.
Most likely because it is not using ISO by default. Some week functions start the week on Sunday, and some define week 1 as whichever week contains 1 January. Both conventions commonly differ from ISO by one, particularly around the turn of the year.
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