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Week Number Calculator (Week Number of a Date)
Choose a date and the calculator gives its week number in two systems. The ISO week starts on Monday. It is the one most of Europe uses, and many payroll systems use it too. The US week of the year starts on Sunday instead. Friday, October 9, 2026, for example, sits in ISO week 41. The what week is it page shows only the current week, and the week numbers by year pages list every week of a year.
Quick answer: Monday starts the week in ISO numbering, and week 1 is whichever week contains January 4. The US way starts weeks on Sunday and makes the week of January 1 week 1. On New Year's Day 2027 they split, with ISO week 53 of 2026 against US week 1 of 2027.
ISO week vs US week
In ISO 8601, the first Thursday of the year decides week 1. So the first few days of January can still belong to week 52 or 53 of the year before. The US column always puts January 1 in week 1 and begins each week on a Sunday. Use whichever one your timesheet or planner follows.
Worked example: New Year's Day 2027
Friday, January 1, 2027 is a good test, because the two systems disagree on it. Start with the Monday-to-Sunday week that holds the date. That week runs from Monday, December 28, 2026 to Sunday, January 3, 2027.
In ISO numbering, the Thursday of a week decides which year the week belongs to. Here the Thursday is December 31, 2026. So the whole week, New Year's Day included, counts as part of 2026.
December 31, 2026 is day 365 and a Thursday, ISO weekday 4. Take the weekday from the day number, add 10 and divide by 7, dropping the remainder: (365 − 4 + 10) ÷ 7 gives 53, so it is ISO week 53. The rule works for any date: a result of 0 means the last week of the year before, and 53 in a year that has only 52 weeks means week 1 of the next. US numbering works more simply and always puts January 1 in week 1 of its own year.
Result: ISO week 53 of 2026, but US week 1 of 2027.
ISO week-numbering years reference table
| ISO year | January 1 falls on | Week 1 starts (Monday) | Last week ends (Sunday) | ISO weeks |
|---|---|---|---|---|
| 2024 | Monday | January 1, 2024 | December 29, 2024 | 52 |
| 2025 | Wednesday | December 30, 2024 | December 28, 2025 | 52 |
| 2026 | Thursday | December 29, 2025 | January 3, 2027 | 53 |
| 2027 | Friday | January 4, 2027 | January 2, 2028 | 52 |
| 2028 | Saturday | January 3, 2028 | December 31, 2028 | 52 |
| 2029 | Monday | January 1, 2029 | December 30, 2029 | 52 |
| 2030 | Tuesday | December 31, 2029 | December 29, 2030 | 52 |
| 2031 | Wednesday | December 30, 2030 | December 28, 2031 | 52 |
| 2032 | Thursday | December 29, 2031 | January 2, 2033 | 53 |
A year has 53 ISO weeks when January 1 is a Thursday, or a Wednesday in a leap year. In this range that happens in 2026 and 2032.
Week number in Excel or Google Sheets
Put the date in A2. Both Excel and Google Sheets have a function for the ISO week and one for the US week. The second argument of WEEKNUM picks the day a week starts on.
| What you want | Formula |
|---|---|
| ISO week number | =ISOWEEKNUM(A2) |
| ISO week, older Excel | =WEEKNUM(A2,21) |
| US week (Sunday start, January 1 in week 1) | =WEEKNUM(A2) |
| Monday start, January 1 in week 1 | =WEEKNUM(A2,2) |
| ISO year of the date | =YEAR(A2-WEEKDAY(A2,2)+4) |
| Monday of ISO week N in year Y | =DATE(Y,1,4)-WEEKDAY(DATE(Y,1,4),2)+1+7*(N-1) |
In the last formula, swap Y and N for the year and week, or for cell references. For 2026 and week 1, it returns Monday, December 29, 2025.
Common mistakes
The usual mix-up is sharing a week number without saying which system it uses. Monday, December 29, 2025 is ISO week 1 of 2026, yet in the US system it sits in week 53 of 2025. A client in Germany and a payroll team in Ohio could read "week 1" as two different weeks.
Write the ISO year next to the week to avoid this, as in 2026-W53. Remember too that some years have 53 ISO weeks. 2026 is one of them, and 2032 is the next. A planner that only loops over 52 weeks will drop the last one.
In a spreadsheet, plain WEEKNUM uses Sunday weeks that start on January 1. Use ISOWEEKNUM for the ISO number. In code, the strftime codes %V and %G give the ISO week and the ISO year.
Week number questions
How are week numbers calculated?
In the ISO system, find the Monday-to-Sunday week that holds January 4 and call it week 1. Count on from there one week at a time. The US system calls the week with January 1 week 1 and starts each week on Sunday.
What is an ISO week number?
It is the week number set out in the ISO 8601 standard. Its weeks start on Monday, and a year has either 52 or 53 of them. For 2026, week 1 began on Monday, December 29, 2025.
Why do ISO and US week numbers differ?
The two systems start the week on different days and pick week 1 by different rules. So a date in early January can be week 1 in the US system but week 52 or 53 in the ISO one. Check which your planner or payroll uses.
How do I find the week number in Excel?
Put the date in A2. =ISOWEEKNUM(A2) returns 41 for October 9, 2026, and =WEEKNUM(A2) returns the Sunday-based US week. Google Sheets has both.
How many weeks are in 2026?
There are 53 ISO weeks in 2026, because the year starts on a Thursday. The first runs from Monday, December 29, 2025 and the last ends on Sunday, January 3, 2027.
Which years have 53 weeks?
Any year that starts on a Thursday has 53 ISO weeks, and so does a leap year that starts on a Wednesday. 2020 and 2026 are two of them, and 2032 comes next.
What week number is January 1?
In ISO numbering, January 1 is in week 1 when it falls Monday through Thursday. When it falls Friday through Sunday, it belongs to week 52 or 53 of the year before. In US numbering it is always week 1.
Does the ISO week start on Sunday or Monday?
It starts on Monday. ISO 8601 numbers the days of the week from Monday as 1 to Sunday as 7.
How do I write an ISO week date?
Write the ISO year, the letter W, the week number and, if you like, the weekday from 1 to 7. So 2026-W53-5 means Friday, January 1, 2027.
What dates are in a week number?
An ISO week runs Monday to Sunday. Week 41 of 2026 runs from Monday, October 5 to Sunday, October 11, 2026. The week number pages list the dates of every week.
Can I subtract week numbers to count weeks?
Only inside one ISO year. Week 41 to week 52 of 2026 is 11 weeks. But week 50 of 2026 to week 3 of 2027 is 6 weeks, not 5, because 2026 has 53 weeks. The weeks until calculator avoids this problem.
Cite this page
APA: ClockTotals. (2026, October 16). Week Number Calculator (Week Number of a Date). https://clocktotals.com/week-number-calculator/
MLA: “Week Number Calculator (Week Number of a Date).” ClockTotals, 16 Oct. 2026, clocktotals.com/week-number-calculator/.
Link (HTML): <a href="https://clocktotals.com/week-number-calculator/">Week Number Calculator (Week Number of a Date)</a>