How to find a week number
- 1
Pick a date
It starts on today. Choose any date to see its week.
- 2
Step through weeks
Use Prev and Next to move a week at a time, or This week to jump back.
- 3
Open the full list
Expand “All weeks” to see every Monday–Sunday range for the year, with the current week highlighted.
The ISO week rules
- Weeks run from Monday to Sunday.
- Week 1 is the week that contains the year's first Thursday, which is also the week that contains January 4.
- A year has 52 or 53 weeks. There is no week 0.
- Days at the very start or end of a year can belong to the neighbouring year's week.
How to calculate the ISO week number by hand
There's a short formula that works for most dates. Take the date's day-of-year number (January 1 is 1, December 31 is 365 or 366) and its weekday number, where Monday is 1 and Sunday is 7. Then:
Example: October 21, 2026 is day 294 of the year and a Wednesday (3).
(294 − 3 + 10) ÷ 7 = 301 ÷ 7 = 43 → week 43
If the result is 0, the date belongs to the last week of the previous year (52 or 53). If the result is 53 but the year only has 52 weeks, the date is in week 1 of the next year. The calculator handles both edge cases for you.
Weeks per year, 2024 to 2034
| Year | Weeks | Week 1 starts | Last week ends |
|---|---|---|---|
| 2024 | 52 | Mon, Jan 1, 2024 | Dec 29, 2024 |
| 2025 | 52 | Mon, Dec 30, 2024 | Dec 28, 2025 |
| 2026 | 53 | Mon, Dec 29, 2025 | Jan 3, 2027 |
| 2027 | 52 | Mon, Jan 4, 2027 | Jan 2, 2028 |
| 2028 | 52 | Mon, Jan 3, 2028 | Dec 31, 2028 |
| 2029 | 52 | Mon, Jan 1, 2029 | Dec 30, 2029 |
| 2030 | 52 | Mon, Dec 31, 2029 | Dec 29, 2030 |
| 2031 | 52 | Mon, Dec 30, 2030 | Dec 28, 2031 |
| 2032 | 53 | Mon, Dec 29, 2031 | Jan 2, 2033 |
| 2033 | 52 | Mon, Jan 3, 2033 | Jan 1, 2034 |
| 2034 | 52 | Mon, Jan 2, 2034 | Dec 31, 2034 |
Week numbers by month in 2026
The ISO weeks that each month of 2026 touches. A week that spans two months appears in both.
| Month | First week | Last week |
|---|---|---|
| January | Week 1 | Week 5 |
| February | Week 5 | Week 9 |
| March | Week 9 | Week 14 |
| April | Week 14 | Week 18 |
| May | Week 18 | Week 22 |
| June | Week 23 | Week 27 |
| July | Week 27 | Week 31 |
| August | Week 31 | Week 36 |
| September | Week 36 | Week 40 |
| October | Week 40 | Week 44 |
| November | Week 44 | Week 49 |
| December | Week 49 | Week 53 |
Which week are the holidays in 2026?
| Holiday | Date | ISO week |
|---|---|---|
| New Year's Day | Jan 1, 2026 | Week 1 |
| Valentine's Day | Feb 14, 2026 | Week 7 |
| Easter | Apr 5, 2026 | Week 14 |
| Memorial Day | May 25, 2026 | Week 22 |
| 4th of July | Jul 4, 2026 | Week 27 |
| Labor Day | Sep 7, 2026 | Week 37 |
| Halloween | Oct 31, 2026 | Week 44 |
| Thanksgiving | Nov 26, 2026 | Week 48 |
| Black Friday | Nov 27, 2026 | Week 48 |
| Christmas | Dec 25, 2026 | Week 52 |
Writing dates as week numbers
ISO 8601 also defines a week date format: the year, the letter W, the week number and the weekday number (Monday is 1). For example, Wednesday, October 21, 2026 is written 2026-W43-3. The short form 2026-W43 means the whole week. You'll see this format in European logistics, manufacturing schedules and some calendar apps.
Week of the year vs. week of the month
Week of the year runs from 1 to 52 or 53 and is the number this calculator shows. Week of the month (1 to 5 or 6) has no single standard: some count the partial week containing the 1st as week 1, others start at the first full week. If a schedule says “the second week of March”, check which rule is meant. Payroll uses its own system too: in the UK, tax week 1 starts on April 6, the first day of the tax year.
ISO weeks vs. US weeks
| ISO 8601 | Common US style | |
|---|---|---|
| First day of the week | Monday | Sunday |
| Week 1 | Week with the first Thursday | Week containing January 1 |
| Weeks per year | 52 or 53 | 53 or 54 (partial weeks) |
| Used by | Europe, ISO standards, most software | US calendars, some payroll systems |
| Excel function | ISOWEEKNUM() | WEEKNUM() |
Why some years have 53 weeks
52 weeks is 364 days, one or two days short of a year. Those spare days build up, and roughly every five or six years they add up to a whole extra ISO week. Over the 400-year Gregorian cycle, 71 years have 53 weeks. In a 53-week year, payroll systems that pay weekly or biweekly may run an extra pay period, and retailers that report by week compare 53 weeks against 52, which is why you sometimes see “on a 52-week basis” in annual results.
Week numbers in code and spreadsheets
| Tool | ISO week of a date |
|---|---|
| Excel | =ISOWEEKNUM(A1) or =WEEKNUM(A1, 21) |
| Google Sheets | =ISOWEEKNUM(A1) |
| Python | date.isocalendar().week |
| SQL (PostgreSQL) | EXTRACT(WEEK FROM date) |
| PHP | date('W', $timestamp) |
JavaScript has no built-in ISO week function, which is a common source of bugs on websites. If you're building something, test the dates around New Year carefully.
Weeks, quarters and the retail calendar
A quarter is about 13 weeks, so weeks 1–13 roughly make up Q1, 14–26 Q2, 27–39 Q3 and 40–52 Q4. Many retailers use a 4-4-5 calendar, where each quarter has two 4-week months and one 5-week month, so every period starts on the same weekday and sales can be compared week for week. The US National Retail Federation publishes a 4-5-4 calendar that many American retailers follow.
Who uses week numbers
Business planning
Retail, manufacturing and logistics plan in weeks: “ships in week 47” is common in Europe.
Payroll and timesheets
Weekly and biweekly pay cycles are often labeled by week number.
Software teams
Sprints and release trains are tracked by ISO week.
Schools and sports
Term planners and training plans are often laid out week by week.