Enter any date to get its ISO 8601 week number, day of the year, and weekday.
ISO 8601 defines a week as running Monday to Sunday, and assigns week 1 as whichever week contains the year’s first Thursday — equivalently, the week containing 4 January. This calculator finds the Thursday of your date’s own week, then counts how many Mondays separate that from the first Thursday of its year, so a date and "its" year are decided by where the middle of the week falls, not by the calendar page it happens to sit on.
Worked example: 15 June 2026 is a Monday. Its own week runs to Sunday 21 June, its midweek Thursday is 18 June, and counting Thursdays back to the year’s first (8 January) gives week 25 — also day 166 of 365.
Take 1 January 2027. It is a Friday. Under a naive "week 1 starts on 1 January" rule you would call it week 1 of 2027. The ISO rule disagrees: that Friday's own Monday–Sunday week runs from 28 December 2026 to 3 January 2027, its Thursday is 31 December 2026, and that Thursday belongs to 2026 — so 1 January 2027 is week 53 of 2026, and the calculator correctly reports the ISO year as 2026, not 2027. ISO week 1 of 2027 does not begin until Monday 4 January 2027.
This is exactly why the calculator reports an ISO year separately from the calendar year of the date you entered — for the first days of January and the last days of December, the two frequently disagree.
Most years have 52 ISO weeks; a year gets a 53rd whenever 1 January falls on a Thursday, or in a leap year, when it falls on a Wednesday. 1 January 2026 is a Thursday, which is why 2026 runs long — its week 53 covers 28 December 2026 through 3 January 2027, spanning the New Year boundary shown above.
The United States commonly uses a Sunday-start week numbered from 1 January regardless of what day that lands on, which routinely disagrees with the ISO figure by one week and sometimes assigns week 1 to what ISO still calls the final week of the prior year. Software defaults vary too — spreadsheet WEEKNUM functions, for instance, default to the US convention unless told otherwise. If a week number from this tool does not match one you see elsewhere, mismatched conventions are the most likely reason, not an error in either source.
ISO 8601 defines week 1 of a year as the week containing that year's first Thursday, with weeks running Monday to Sunday. This means the ISO week-numbering year can differ from the calendar year for a few days at the very start or end of January/December — for example, 31 December can fall in week 1 of the following ISO year, and 1 January can fall in week 52 or 53 of the previous one.
Because ISO weeks always run Monday–Sunday and week 1 is defined by containing the first Thursday of the year, not by containing January 1st. If January 1st falls on a Friday, Saturday, or Sunday, it actually belongs to the last week (52 or 53) of the previous ISO year rather than week 1 of the new one — this is the most common source of "why does this say week 52 for a January date" confusion.
No — ISO 8601 (Monday-start weeks, week 1 contains the first Thursday) is the standard across most of Europe and in many business, engineering, and payroll contexts worldwide, but the US commonly uses a Sunday-start week where week 1 simply contains January 1st. The same calendar date can therefore have a different week number depending on which system is used — this calculator uses the ISO 8601 standard.
52 weeks × 7 days = 364 days, one short of a normal 365-day year (and two short of a 366-day leap year), so the leftover days have to go somewhere. A year has 53 ISO weeks whenever 1 January falls on a Thursday, or in a leap year, when it falls on a Wednesday — roughly 1 year in 5 or 6.
Worked example: 1 January 2027 is a Friday, so it belongs to week 53 of the 2026 ISO year, not week 1 of 2027 — week 1 of 2027 only begins on Monday, 4 January. Enter both dates above to see the ISO week-numbering year shift.
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