What Is a Week Number?
A week number counts the weeks of a year from 1 upwards. It lets a plan say "ships in Week 15" and mean the same week in every year, with no date to retype when the calendar rolls over. The date finder at the top of the page turns any date into its week number.
The catch is that there are two common ways to number weeks. The grid on this page uses ISO 8601, the international standard, which is widely used in Europe and in project tools. Spreadsheets such as Excel default to a different system, so the current-week card also shows that number.
How Are Week Numbers Calculated? (ISO 8601)
Three rules: weeks run Monday to Sunday, Week 1 is the week that contains the year's first Thursday, and a week belongs to the year that holds its Thursday. The second rule is the same as saying Week 1 is the week with January 4th in it.
Two real cases show how this plays out. January 1, 2026 is a Thursday, so Week 1 of 2026 starts on Monday, December 29, 2025, and the first three days of that week are still in 2025. January 1, 2027 is a Friday, so it is not in Week 1 of 2027; it is part of Week 53 of 2026, and Week 1 of 2027 starts on Monday, January 4.
That is why the grid below marks weeks that start in the previous year or end in the next one. The week number and the calendar year stop lining up for a few days at every New Year.
Same Date, Two Week Numbers
The US/Excel system starts the week on Sunday and makes Week 1 the week that contains January 1st, even if that is only a day or two long. This is what Excel's WEEKNUM returns by default. These examples were calculated with the same code as the tools on this site:
| Date | ISO 8601 | US / Excel |
|---|
| Mon, Dec 29, 2025 | Week 1 of 2026 | Week 53 |
| Thu, Jan 1, 2026 | Week 1 of 2026 | Week 1 |
| Sat, Oct 17, 2026 | Week 42 of 2026 | Week 42 |
| Sun, Oct 18, 2026 | Week 42 of 2026 | Week 43 |
| Thu, Dec 31, 2026 | Week 53 of 2026 | Week 53 |
| Fri, Jan 1, 2027 | Week 53 of 2026 | Week 1 |
| Mon, Jan 4, 2027 | Week 1 of 2027 | Week 2 |
Which dates differ depends on the weekday of January 1st, so the pattern changes from year to year. Through most of 2026 the two systems agree from Monday to Saturday and split on Sundays, because a Sunday starts a new US week but ends an ISO week. The days around New Year are the exception, as the first and sixth rows above show. If you share a week number with someone, say which system it is, or give the start date of the week as well.
Years With 53 Weeks
An ISO year has 53 weeks when January 1st is a Thursday, or when it is a leap year starting on a Wednesday. Between 2020 and 2050 those years are 2020, 2026, 2032, 2037, 2043, 2048.
Week 53 is a full seven-day week, not a short leftover. In 2026 it runs from December 28, 2026 to January 3, 2027, so it spans the New Year. Anything that assumes weeks 1 to 52, such as a template with 52 columns, will lose a week in those years.
Using Week Numbers at Work
Teams use week numbers to plan sprints, set delivery windows and compare one week with the same week last year. Two habits prevent most mix-ups: write the week with its year (2026-W42 is the standard way to write it), and put the Monday date next to the number in shared spreadsheets.
Around New Year, check the week-year rather than the calendar year. December 29, 2025 belongs to Week 1 of 2026, and January 1, 2027 to Week 53 of 2026. If a report groups by calendar year and by week number at the same time, those few days will land in the wrong bucket. For counting working days between two dates, the business days calculator is usually the better tool.
Source for the rules: ISO 8601 defines the Monday start, the first-Thursday rule and the week-year notation used above. The US/Excel numbering described here follows Excel's default WEEKNUM setting; other US organisations may define Week 1 differently.
Frequently Asked Questions
What week number is it right now?
The card at the top of the page shows it, using your device's date, and the same week is highlighted in blue in the grid below. The number follows ISO 8601, and the US/Excel number is shown under it because the two often differ.
How do I find the week number of a specific date?
Use the date finder at the top of the page: pick the date and it shows the ISO week and week-year, the US/Excel week number, the weekday, the Monday to Sunday dates of that week, and the day of the year. The grid below jumps to that year and outlines the week. Dates near New Year can belong to a different week-year, so a date in early January may show a week from the previous year.
Does week 1 always start on January 1st?
Not in the ISO system. Week 1 is the week that contains the year's first Thursday, which is the same as the week containing January 4th. If January 1st is a Monday, Tuesday, Wednesday or Thursday it belongs to Week 1, and Week 1 can start in late December. If January 1st is a Friday, Saturday or Sunday it belongs to the last week of the previous year, and Week 1 starts on January 4th, 3rd or 2nd.
Why do the US and Europe sometimes show different week numbers for the same date?
The US/Excel system starts weeks on Sunday and makes Week 1 the week that contains January 1st, so Week 1 can be a short, partial week. ISO weeks start on Monday and Week 1 is the week with the first Thursday. Which dates differ depends on the weekday of January 1st; in 2026, for example, the two agree from Monday to Saturday but differ on Sundays (October 18, 2026 is ISO Week 42 and US/Excel Week 43).
What is ISO 8601 week numbering?
ISO 8601 is the international standard for writing dates and times. Its week rules are: weeks run Monday to Sunday, Week 1 is the week with the year's first Thursday, and a week belongs to the year that holds its Thursday. A week can be written as 2026-W42.
How many weeks are in a year, 52 or 53?
ISO years have 52 weeks, except for 53-week years: those where January 1st is a Thursday, or a leap year where January 1st is a Wednesday. 2026 is one. Week 53 is a full Monday to Sunday week; in 2026 it runs from December 28, 2026 to January 3, 2027. A 53-week year comes around every five to six years.
Can the first days of January belong to week 52 or 53?
Yes. When January 1st is a Friday, Saturday or Sunday under ISO rules, it still belongs to the previous year's last week. January 1, 2027 is a Friday, so it is part of Week 53 of 2026, and Week 1 of 2027 starts on Monday, January 4. The week number and the calendar year do not always match at New Year, which is why ISO also has a week-year.
