How to find the week number of a date
A week number identifies a date's position within a week-based year. Under the ISO calendar convention used here, weeks begin on Monday and end on Sunday. Each date belongs to an ISO week number, an ISO week year and a numbered weekday.
The ISO week year is important near New Year's Day. A date in early January can belong to the last week of the previous ISO year, while a date in late December can belong to the first week of the next ISO year.
ISO week numbering rules
An ISO week is a Monday-to-Sunday period. Week 1 is the week containing January 4. The same rule can be expressed as the week containing the year's first Thursday or the first week with at least four days in the new calendar year.
These rules keep weeks intact across year boundaries. Week numbers run from 1 to 52 in most ISO years, and from 1 to 53 in others. They are not calculated by simply dividing a date's day-of-year number by seven.
How to calculate the ISO week number
Find the Monday of the selected date's week. The Thursday in that same week determines its ISO week year. Then count the number of complete seven-day weeks from the Monday of ISO week 1 to the Monday of the selected week, and add one.
For example, a Friday and the Monday immediately before it share the same ISO week number. The following Monday starts the next numbered week. This differs from a calendar system where weeks begin on Sunday.
Why the ISO week year can differ from the calendar year
The calendar year changes on January 1, but an ISO week does not split at that date. Its Thursday determines which ISO year owns the whole week. This means early-January dates may still be in the previous year's final ISO week.
January 1, 2021 was a Friday in week 53 of ISO year 2020. January 4, 2021 was the Monday that began week 1 of ISO year 2021. In the opposite direction, December 30, 2024 was a Monday in week 1 of ISO year 2025.
Always include the ISO week year when recording a week number around New Year. “Week 1” alone is not enough to identify a particular week.
Which years have 53 ISO weeks?
An ISO year has 53 weeks when January 1 is a Thursday, or when it is a leap year beginning on Wednesday. Otherwise, it has 52 weeks. For example, ISO year 2020 had 53 weeks, while ISO year 2021 had 52.
The number of ISO weeks is different from dividing the 365 or 366 days of a calendar year by seven and rounding up. ISO years are made of complete weeks and can begin in the previous December or end in the following January.
Examples of ISO week numbers
October 4, 2026: Sunday in week 40 of ISO year 2026. That week begins on Monday, September 28 and ends on Sunday, October 4.
January 1, 2021: Friday in week 53 of ISO year 2020. The week's Monday is December 28, 2020, and its Sunday is January 3, 2021.
December 30, 2024: Monday in week 1 of ISO year 2025. The week ends on Sunday, January 5, 2025.
Reading ISO week-date notation
ISO week-date notation combines the week year, week number and weekday. The format is YYYY-Www-D, where the weekday is Monday 1 through Sunday 7. The week number uses two digits.
For example, 2020-W53-5 identifies Friday, January 1, 2021. Although that date belongs to calendar year 2021, its week-date notation begins with 2020 because it belongs to ISO week year 2020.
A week number is a calendar position, not an elapsed duration. To measure the time between dates, use the Date Difference Calculator. To find a date's weekday name, use the Day of Week Calculator.
Frequently asked questions about week numbers
Does week 1 always begin on January 1?
No. ISO week 1 begins on the Monday of the week containing January 4. Its first day can fall in late December or early January.
Do ISO weeks start on Monday or Sunday?
Monday. Sunday is the last day of the ISO week. A Sunday-first numbering system may give a different week number.
Can January 1 belong to the previous week year?
Yes. If it falls in a week whose Thursday is in December, that entire week belongs to the previous ISO year.
Why do some years have week 53?
The alignment of calendar weekdays gives some ISO years 53 complete weeks. This occurs in years beginning on Thursday, or leap years beginning on Wednesday.
Are remaining weeks counted after the selected date?
The remaining-week figure here counts complete numbered weeks after the selected week. It does not include the unused days in the selected week.
Can two week-number calculators disagree?
Yes, if they use different conventions for the first weekday or week 1. Compare their numbering rules and include the week year when checking dates near New Year.