What an ISO 8601 week number is
A week number labels each week of the year with a figure from 1 to 53 instead of a date. Manufacturing, logistics, accounting and software teams lean on it every day: “shipping in week 28” is shorter and less ambiguous than “the week of July 6 to 12”. The international rule that defines how those weeks are numbered is ISO 8601, and this tool follows it to the letter.
Two rules set the ISO scheme apart from casual week counts:
- Weeks start on Monday and end on Sunday.
- Week 1 is the week that contains the year’s first Thursday — equivalently, the week holding January 4, or the first week with most of its days in the new year.
How to use the tool
- Pick a date, or press Today to use the current day.
- Read the ISO week code, such as
2026-W28. - Check the ISO year, the day of the week, and the Monday-to-Sunday range for that week.
- Press Copy to put the
YEAR-Wweekcode on your clipboard.
The ISO formula
The math runs on UTC calendar dates so no time-zone change can nudge the result:
dayNr = weekday, with Monday = 0 ... Sunday = 6
thursday = date + (3 - dayNr) days // the Thursday of that week
isoYear = year of that Thursday
firstThursday = first Thursday of isoYear
week = 1 + round((thursday - firstThursday) / 7 days)Because every week is anchored to its Thursday, a late-December or early-January date can belong to a different ISO year than the calendar year. That is why the ISO year is shown on its own.
Years with 53 weeks
Most years hold 52 ISO weeks, but some hold 53. A year has 53 weeks when its January 1 is a Thursday, or when it is a leap year that starts on a Wednesday.
| Year | Weeks | Reason |
|---|---|---|
| 2015 | 53 | January 1 is a Thursday |
| 2020 | 53 | leap year starting on a Wednesday |
| 2026 | 53 | January 1 is a Thursday |
| 2032 | 53 | January 1 is a Thursday |
| 2037 | 53 | January 1 is a Thursday |
Worked example
Take January 1, 2021. It is a Friday, so the Thursday of its week is December 31, 2020 — which means the date belongs to the ISO year 2020, not 2021. The first Thursday of 2020 is January 2 (2020 opens on a Wednesday). Between those two Thursdays there are 364 days, and 364 ÷ 7 = 52 whole weeks, so the week is 52 + 1 = 53. The week runs from Monday, December 28, 2020 to Sunday, January 3, 2021, and the code is 2020-W53 even though the calendar already reads 2021.
For a plain case, July 9, 2026 is itself a Thursday. The first Thursday of 2026 is January 1, and 189 days separate them, which is exactly 27 weeks, so the week is 28. Its code is 2026-W28, ISO year 2026, spanning Monday July 6 to Sunday July 12.
Frequently asked questions
Why can January 1 read as week 52 or 53 of the previous year?
Because a week is assigned to the year that holds its Thursday. When January 1 falls on a Friday, Saturday or Sunday, that week’s Thursday is still back in December, so the date inherits the previous year’s final week. That mismatch is exactly what the separate ISO-year field is there to flag.
How does this differ from US week numbering?
The common US scheme starts weeks on Sunday and treats week 1 as simply the week containing January 1, with no “first Thursday” test. That shifts its numbers away from ISO on many dates. This tool computes only the ISO 8601 standard, the one used across Europe and most enterprise systems.
How many weeks does a year have?
Almost always 52. It has 53 when January 1 is a Thursday, or when the year is a leap year that begins on a Wednesday. Over a full cycle roughly 71% of years have 52 weeks and 29% have 53.
Does my time zone change the answer?
No. The date is treated as a UTC calendar day with no clock time, so the week number is the same wherever you are. Only the day, month and year you choose matter.