Enter any date to get its ISO 8601 week number, day of the year, and weekday.
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.