Week Number Calculator
Find the ISO 8601 week number and week-based year of any date, the US-style week number (weeks starting on Sunday), the day of the year and the days left in the year.
Last updated: October 2026
ISO week number
43
Formula below · 3 sources (NIST, iso.org, aa.usno.navy.mil) · Updated Oct 2026
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About this calculator
ISO 8601 numbers weeks from Monday to Sunday, and week 1 is the week that contains the year's first Thursday (equivalently, the week containing January 4). The calculator finds the Thursday of the week your date is in, takes that Thursday's year as the week-based year, and counts whole weeks from the Monday of that year's week 1. Dates at the start of January can belong to week 52 or 53 of the previous year, and December 29 to 31 can be week 1 of the next, so the week number is always reported together with its week-based year ("2026-W43"). An ISO year has 53 weeks when January 1 is a Thursday, or a Wednesday in a leap year; otherwise 52.
The US convention, used by Microsoft Excel's WEEKNUM with its default setting and by many wall calendars, starts weeks on Sunday and calls the week containing January 1 week 1. Its week numbers can be one higher or lower than the ISO ones and the last week can be a short week 53.
The day of the year counts January 1 as day 1 (an ordinal date, "2026-296" in ISO notation), and the days left exclude the date itself, so December 31 has 0 days left. Together they give the position in the year: day 296 of 365 is 81% of the way through 2026.
ISO weeks by year: when week 1 starts and how many weeks the year has
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| ISO year | Week 1 begins (Monday) | Weeks | January 1 falls on |
|---|---|---|---|
| 2024 | 2024-01-01 | 52 | Monday |
| 2025 | 2024-12-30 | 52 | Wednesday |
| 2026 | 2025-12-29 | 53 | Thursday |
| 2027 | 2027-01-04 | 52 | Friday |
| 2028 | 2028-01-03 | 52 | Saturday |
| 2029 | 2029-01-01 | 52 | Monday |
| 2030 | 2029-12-31 | 52 | Tuesday |
How to use
Enter the date as YYYY-MM-DD. Example (the default): Friday 2026-10-23. The Thursday of that week is October 22, 2026, so the week-based year is 2026. Week 1 of 2026 began on Monday 2025-12-29 (the week containing January 1, a Thursday). From December 29 to October 22 is 42 full weeks, so the date is in ISO week 43. By the US rule, 2026 began on a Thursday, so US week 1 ran from Sunday December 28, 2025 to Saturday January 3, and October 23 is also in week 43. It is day 296 of the year, with 69 days left. Example: Friday 2027-01-01. Its week's Thursday is December 31, 2026, so it belongs to ISO week 53 of 2026 (2026-W53); by the US rule it is week 1 of 2027. The two conventions disagree by a whole year at the turn of the year.
Frequently asked questions
What week of the year is it?
Enter today's date to see the ISO week number (Monday-start weeks, week 1 holds the first Thursday) and the US week number (Sunday-start, week 1 holds January 1). The two can differ by one, and around New Year by a whole year.
Why does January 1 sometimes fall in week 52 or 53?
Because ISO week 1 is the week with the year's first Thursday. If January 1 is a Friday, Saturday or Sunday, it sits in the last week of the previous ISO year: 2027-01-01, a Friday, is in week 53 of 2026.
How many weeks are in a year?
52 ISO weeks in most years and 53 in years that begin on a Thursday, or on a Wednesday in a leap year: 2026, 2032 and 2037 have 53 weeks. A calendar year is 52 weeks plus 1 day (2 in a leap year).
How does Excel number weeks?
WEEKNUM(date) with the default return type uses the US rule (Sunday start, week 1 contains January 1). WEEKNUM(date, 21) and ISOWEEKNUM(date) give the ISO week. The ISO week-based year is not returned by either, so a date in week 53 of 2026 shows 53 next to the calendar year 2027.
What is the day of the year used for?
Ordinal dates (day 1 to 365 or 366) appear in Julian-day style date codes on food packaging and military and NASA records, and in formulas that need "fraction of the year elapsed": day 296 of 365 is 0.81 of the year.
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Sources & references
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