By the Age-Finder team · Last updated 7 November 2026
Quick answer
Today, Saturday 7 November 2026, is in ISO week 45 of 2026, Monday 2 November to Sunday 8 November. Leave the date empty to see the current week, or enter any date to see its week number, its Monday to Sunday dates, and the day of the year.
The Week Number Calculator shows the ISO week number of any date, and of today if you leave the date empty. It also shows the Monday-to-Sunday dates of that week, how many weeks the year has and the day of the year.
How to Use the Week Number Calculator
Type a date to see its week, or leave the box empty for today. The result gives the ISO week number, the Monday to Sunday dates, how many weeks the year holds, and the day of the year.
To find the weekday of a date instead, use the Day of the Week Calculator. To move a date forward or back by a number of weeks, use the Date Calculator.
This Week
Today, Saturday 7 November 2026, is in ISO week 45 of 2026, written 2026-W45-6: Monday 2 November to Sunday 8 November. It is day 311 of 365, so 85.2% of the year is complete, with 8 weeks remaining after this one (weeks 46 to 53) and 54 days left in 2026.
The US week number agrees today: week 45, Sunday 1 November to Saturday 7 November. The two systems disagree at the year boundary, as the table in the next section shows.
How ISO Week Numbers Work
The calculator uses ISO week numbering, defined by the international standard ISO 8601 and used across Europe, much of Asia and most business software. The rules are short: Weeks start on Monday and end on Sunday.
The first week is the one holding the year’s first Thursday. Equivalently, it is the week that contains 4 January. A year has 52 or 53 weeks.
Because a week is counted in the year that holds its Thursday, the first days of January can belong to the last week of the previous year, and the last days of December can belong to week 1 of the next year.
| Date | ISO week | Week-year |
|---|---|---|
| 29 December 2025 | Week 1 | 2026 |
| 6 November 2026 | Week 45 | 2026 |
| 31 December 2026 | Week 53 | 2026 |
| 1 January 2027 | Week 53 | 2026 |
| 4 January 2027 | Week 1 | 2027 |
When Week 1 Starts
Week 1 of 2026 started on Monday, 29 December 2025, because the first Thursday of 2026 was 1 January. Week 1 of 2027 starts on Monday, 4 January 2027, and week 1 of 2028 on Monday, 3 January 2028.
This is why a calendar that simply calls 1 to 7 January “week 1” can disagree with ISO week numbers by one week.
Years With 53 Weeks
Every year has 52 full weeks plus one or two extra days, so in ISO numbering some years need a week 53. A year has 53 weeks when it starts on a Thursday, or when it is a leap year that starts on a Wednesday.
Between 2015 and 2045, the years with 53 ISO weeks are 2015, 2020, 2026, 2032, 2037 and 2043.
If your planning, payroll or reporting runs on week numbers, check for week 53 before copying last year’s schedule. Anyone paid weekly gets 53 pay packets in a 53-week year rather than 52, and anyone paid fortnightly can get 27 rather than 26.
ISO vs US, Worked Out
The tool shows the ISO week number only. The US system, used by many calendars in the United States, starts weeks on Sunday and counts week 1 as the week containing 1 January. The two agree for most of the year and disagree at the boundaries:
| Date | ISO week | US week | Why they differ |
|---|---|---|---|
| 7 November 2026 | 45 of 2026 | 45 of 2026 | They agree |
| 31 December 2026 | 53 of 2026 | 53 of 2026 | They agree |
| 1 January 2027 | 53 of 2026 | 1 of 2027 | ISO keeps the week in 2026; the US system starts a new year at 1 January |
| 31 December 2028 | 52 of 2028 | 54 of 2028 | 31 December 2028 is a Sunday, so it starts a 54th US week of its own |
US week 53 of 2026 has only five days, Sunday 27 December to Thursday 31 December, because the US system cuts weeks at the year boundary while ISO weeks never split across years.
The US system has 53 weeks in 2026, 53 in 2027 and 54 in 2028. In Excel, ISOWEEKNUM gives the ISO number, while WEEKNUM without options does not.
The Weeks Around This One
Week 45 of 2026 runs Monday 2 November to Sunday 8 November. The weeks around it:
| ISO week | Monday | Sunday |
|---|---|---|
| Week 43 | 19 October 2026 | 25 October 2026 |
| Week 44 | 26 October 2026 | 1 November 2026 |
| Week 45 (this week) | 2 November 2026 | 8 November 2026 |
| Week 46 | 9 November 2026 | 15 November 2026 |
| Week 47 | 16 November 2026 | 22 November 2026 |
| Week 48 | 23 November 2026 | 29 November 2026 |
| Week 53 | 28 December 2026 | 3 January 2027 |
The Week You Were Born
Enter your date of birth to see the week you were born in. Someone born on Monday 14 May 1990 was born in ISO week 20 of 1990, written 1990-W20-1, on day 134 of the year.
That week ran Monday 14 May to Sunday 20 May 1990, and its Thursday was 17 May 1990, day 137 of the year. The US week number agrees for that date: week 20 as well.
How to Work Out a Week Number by Hand
You can check any ISO week number with a calendar and three steps: Find the Thursday of the date’s week (weeks run Monday to Sunday). Find that Thursday’s day of the year, where 1 January is day 1.
Subtract 1, divide by 7, round down and add 1. The year of that Thursday is the week-year. For Friday, 6 November 2026, the Thursday of the week is 5 November 2026, which is day 309 of the year.
308 divided by 7 is 44, plus 1 makes week 45 of 2026. As a formula: week = floor((day of year of the Thursday − 1) ÷ 7) + 1.
ISO Weeks and Other Week Systems
ISO weeks are not the only system in use: Many calendars in the United States start weeks on Sunday and count week 1 as the week containing 1 January. Some spreadsheet functions use that system by default.
In Excel, ISOWEEKNUM gives the ISO number, while WEEKNUM without options does not. Some companies run their own fiscal weeks that start in a different month. So “week 1” can mean different dates depending on who is counting.
When dates matter, share the date range of the week as well as its number. The calculator always shows the Monday and Sunday of the week for that reason. A third system starts weeks on Saturday: in the Middle East, weeks often run Saturday to Friday, with week 1 holding 1 January.
For 7 November 2026 that system gives week 46, running Saturday 7 November to Friday 13 November, one more than the ISO number, because 7 November is a Saturday and starts a new week there.
ISO Day Numbers
In ISO 8601 each weekday also has a number: Monday is 1 and Sunday is 7. Combined with the week number, any date can be written as a week date, such as 2026-W45-5 for Friday, 6 November 2026.
Saturday 7 November 2026 is 2026-W45-6, and Monday 14 May 1990 is 1990-W20-1. A compact code used in manufacturing and logistics is YYWW, two digits of the year and two of the week: week 45 of 2026 is 2645.
Common Uses
Production schedules, sprints and delivery plans that run by week number. Payroll and shift rotas organised in weekly cycles. School and university calendars that number teaching weeks. Reports that compare the same week across different years.
What the Tool Does Not Show
The tool shows the ISO week number, the Monday to Sunday dates, how many weeks the year has and the day of the year.
It does not show the US week number, the dates of a week number you type in, the weeks around the one you asked for, how far through the year you are, or the compact YYWW week code. The table at the end of this guide marks which of those the other calculators add.
What other week number calculators offer
Most week number calculators on the first page give the same ISO week this one does. The difference is whether they also give the US week and the dates of a week number. The table below puts the three I checked side by side.
| Feature | Age-Finder | devoven | weekof.today | unitjuggler |
|---|---|---|---|---|
| ISO week number | Yes | Yes | Yes | Yes |
| Monday to Sunday dates | Yes | Yes | Yes | Yes |
| Weeks in the year | Yes | Yes | Yes | Yes |
| Day of the year | Yes | Yes | Not shown | Yes |
| US week number | No | Yes | Not shown | Yes |
| Dates of a week number | No | Not shown | Not shown | Not shown |
| Surrounding weeks | No | Not shown | Not shown | Not shown |
| Year progress and weeks remaining | No | Not shown | Not shown | Not shown |
| Middle Eastern Saturday start | No | No | No | Yes |
| Free to use, no download | Yes | Yes | Yes | Yes |
What is missing from the tool: the tool shows no US week number, no dates for a week number you type in, no surrounding weeks, no year progress and no YYWW code.
What it adds is the week-year note, which appears only when a date belongs to a week of a different year, and the ISO day number for the date.
Frequently Asked Questions
What week number is it today?
Leave the date empty and select Calculate. The calculator shows today’s ISO week number and the Monday-to-Sunday dates of the current week.
Why does 1 January sometimes fall in week 52 or 53?
ISO weeks belong to the year that contains their Thursday. When 1 January falls on a Friday, Saturday or Sunday, its week’s Thursday is in December, so the day belongs to the last week of the previous year.
How many weeks are in a year?
52 in most years and 53 in some. 2026 has 53 ISO weeks; 2027 and 2028 have 52.
Does the week start on Monday or Sunday?
ISO weeks start on Monday. Some calendars, especially in the United States, start on Sunday, which can change the week number of a Sunday date.
What is a week-year?
It is the year an ISO week belongs to: the year that contains the week’s Thursday. It usually matches the calendar year, but not for a few days around 1 January. For example, 1 January 2027 is in week 53 of week-year 2026.
Is the ISO week number the same as Excel’s WEEKNUM?
Not by default. Excel’s ISOWEEKNUM function matches ISO 8601 and this calculator, while WEEKNUM starts weeks on Sunday unless you give it a different return type.
How do I write a date as an ISO week date?
Write the week-year, the letter W, the week number and the ISO day number, where Monday is 1 and Sunday is 7. Friday, 6 November 2026 is 2026-W45-5, and Monday, 4 January 2027 is 2027-W01-1.