Week Number Calculator
Pick a date to get its ISO 8601 week number, ISO week-year and week date, the US week number, and the day of the year. Look up the first and last day of any week, or list every week of a year in one table.
Pick a date to see its week number.
Dates of a week number
Enter a year and a week number.
Every week of a year
Enter a year from 1583 to 9999.
How to Use the Week Number Calculator
- 1. Choose the numbering. ISO 8601 is the international standard: weeks start on Monday, and it is what most European calendars and business software use. The US numbering starts weeks on Sunday and is what most American calendars print.
- 2. Pick a date. The calculator opens on today. It shows both week numbers, the ISO week-year and week date, the day of the year and how many days are left.
- 3. Look up a week. Under Dates of a week number, enter a year and a week number to get the first and last day of that week.
- 4. Scan the whole year. The table lists every week with its start and end dates and its length in days. The week holding your date is highlighted.
How ISO 8601 Week Numbers Work
ISO 8601 weeks run from Monday to Sunday, and week 1 is the week that contains the year's first Thursday — which is the same as the week that contains 4 January. So every week has at least four of its days in the year that numbers it, and every week belongs to exactly one year: the year its Thursday falls in.
That rule creates the year-boundary cases people trip over. 1 January 2027 is a Friday; the Thursday of its week is 31 December 2026, so it belongs to week 53 of 2026, and ISO week 1 of 2027 starts on Monday 4 January. In the other direction, Monday 30 December 2024 was already in week 1 of 2025. The year in an ISO week date — 2026-W53-5 for 1 January 2027 — is the ISO week-year, and it can differ from the calendar year for up to three days at either end.
A year has 53 ISO weeks when it starts on a Thursday, or when it is a leap year that starts on a Wednesday. Between 2000 and 2050 that is 2004, 2009, 2015, 2020, 2026, 2032, 2037, 2043 and 2048; every other year has 52.
ISO and US Week Numbers Compared
| ISO 8601 | US numbering | |
|---|---|---|
| Weeks start on | Monday | Sunday |
| Week 1 is the week that contains | the first Thursday (4 January) | 1 January |
| Weeks in a year | 52 or 53 | 53, or 54 in a leap year that starts on a Saturday |
| Can 1 January fall in the previous year's last week? | Yes, up to 3 January | No — 1 January is always in week 1 |
| Are all weeks seven days long? | Yes | No — the first and last weeks are usually cut short at New Year |
| Spreadsheet function | ISOWEEKNUM(date) or WEEKNUM(date, 21) | WEEKNUM(date) |
The US numbering counts days within their own calendar year, so the Sunday–Saturday week that contains New Year is split in two: 27–31 December 2026 are week 53 of 2026, and 1–2 January 2027 are week 1 of 2027. That is why a US year always reaches week 53 — and why 2000 and 2028, leap years that begin on a Saturday, reach week 54. The table shows those short weeks with their real length.
The two numberings often differ by one week even mid-year, and around New Year by much more, so a deadline given as "week 14" is only unambiguous once you know which system the person setting it uses.
Day of the Year and Days Left
The day of the year counts 1 January as day 1, so 31 December is day 365, or 366 in a leap year. Days left counts the days after the chosen date up to and including 31 December — on 1 January 2027 that is 364. Leap years are the years divisible by 4, except century years not divisible by 400: 2000 was a leap year and 2100 will not be.
Frequently Asked Questions
- What week number is it today?
- The calculator opens on today's date and shows its week number under both conventions — ISO 8601, which starts weeks on Monday, and the US numbering, which starts them on Sunday. The two often differ by one, so check which one your school, employer or software uses before quoting a week number.
- What is ISO week 1?
- The Monday-to-Sunday week that contains the year's first Thursday — equivalently, the week that contains 4 January. It can start as early as 29 December of the previous year or as late as 4 January.
- Why is 1 January sometimes in week 52 or 53?
- Because an ISO week belongs to the year its Thursday falls in. When 1 January is a Friday, Saturday or Sunday, the Thursday of that week is still in December, so those days belong to the last week of the previous year. 1 January 2027, a Friday, is in week 53 of 2026, and 1 January 2023, a Sunday, was in week 52 of 2022.
- How many weeks are there in a year?
- 52 weeks and one day, or two days in a leap year. In ISO numbering most years therefore have 52 weeks and some have 53 — 2020 and 2026 do, and the next is 2032. US numbering always reaches week 53, and 54 in a leap year that starts on a Saturday, because it counts the short weeks at both ends.
- How do I get the ISO week number in Excel or Google Sheets?
- Use
=ISOWEEKNUM(A1), or=WEEKNUM(A1, 21), which Microsoft documents as the ISO 8601 system. A plain=WEEKNUM(A1)gives the US numbering instead, with weeks starting on Sunday and week 1 containing 1 January. - What does 2026-W53-5 mean?
- It is an ISO 8601 week date: ISO week-year 2026, week 53, day 5, counting Monday as day 1 — so a Friday. It names 1 January 2027. The compact form of the same date drops the hyphens: 2026W535.