What this calculator does
This week calculator answers two related questions. Given a date, which numbered week of the year does it fall in? And given a year and a week number, which actual dates does that week cover, from Monday to Sunday? Switch the direction field above to move between the two.
Week numbering follows the ISO 8601 standard used on calendars, timesheets and project schedules across most of the world: weeks start on Monday, and week 1 of any year is defined as the week containing that year's first Thursday. That definition, rather than simply counting from 1 January, is why the first few days of January sometimes belong to the final week of the previous year, and why some years run to 53 weeks instead of the usual 52.
The formula
For a date, the calculator finds the Thursday of that date's Monday-to-Sunday week and reads off which ISO year and week number that Thursday belongs to. For a year and week number, it finds 4 January (which is always in week 1 by definition), steps back to that week's Monday, then adds on seven days for each week past the first.
| Term | Meaning |
|---|---|
| ISO week | A Monday-to-Sunday week numbered 1 to 52 or 53, defined by ISO 8601, the international date and time standard. |
| ISO week-year | The year a week belongs to by this numbering, which can differ from the calendar year for a few days at the very start or end of December. |
| 53-week year | A year has 53 ISO weeks whenever 1 January falls on a Thursday, or on a Wednesday in a leap year. |
The inputs explained
| Field | What to enter |
|---|---|
| Direction | Whether you are looking up a week number from a date, or a date range from a week number. |
| Date | The date to find the week number for, used in "Date → week number" mode. |
| Year | The calendar year to look a week number up in, used in "Week number → date range" mode. |
| Week number | The week number, 1 to 53, used in "Week number → date range" mode. |
When to use it
Filling in a timesheet or project plan
Many workplace systems, especially in Europe, reference "week 24" or "week 41" rather than a calendar date, so converting a normal date to its week number is a routine translation.
Reading a "week on week" report
Sales, traffic and other recurring reports are often broken down by ISO week rather than calendar month, so knowing exactly which dates week 33 covers ties the report back to real events.
Planning around a numbered school or rostering week
Rosters, school terms and some billing cycles are built around week numbers rather than dates, so working out the date range of week 10 or week 45 in advance helps with planning.
Worked examples
Every figure in the tables below is produced by this page’s own calculator at build time, so the numbers and the tool always agree. Select any row to load that scenario.
Which week number does a date in 2026 fall in?
A spread of dates across the year, each converted to its ISO week number and week-year.
| Date | ISO week number |
|---|---|
| 2026-01-01 | Week 1 of 2026 |
| 2026-03-15 | Week 11 of 2026 |
| 2026-06-01 | Week 23 of 2026 |
| 2026-08-13 | Week 33 of 2026 |
| 2026-10-31 | Week 44 of 2026 |
| 2026-12-31 | Week 53 of 2026 |
What dates does each week of 2026 cover?
A spread of week numbers across 2026, each converted to its Monday-to-Sunday date range.
| Week number | Date range |
|---|---|
| Week 1 | 29 Dec 2025 to 4 Jan 2026 |
| Week 10 | 2 Mar 2026 to 8 Mar 2026 |
| Week 20 | 11 May 2026 to 17 May 2026 |
| Week 30 | 20 Jul 2026 to 26 Jul 2026 |
| Week 40 | 28 Sep 2026 to 4 Oct 2026 |
| Week 52 | 21 Dec 2026 to 27 Dec 2026 |
Questions
Why does week 1 sometimes start in late December?
Because ISO week 1 is defined as the week containing the year's first Thursday, not the week containing 1 January. If 1 January falls on a Friday, Saturday or Sunday, the days before it belong to week 1, and that week's Monday sits in the previous December.
Do all years have 53 weeks?
No, most years have 52. A year gets a 53rd ISO week only when 1 January falls on a Thursday, or falls on a Wednesday in a leap year. Roughly one year in five or six qualifies.
Does the week start on Monday or Sunday?
ISO 8601 weeks always start on Monday and end on Sunday. This is different from the US convention used on some calendars, where the week is shown starting on Sunday, though the underlying seven days covered are the same.
Is the ISO week-year always the same as the calendar year?
Almost always, but not for every date. The final days of December can belong to week 1 of the following ISO year, and the first days of January can belong to the final week of the previous ISO year. The calculator reports the ISO week-year separately for exactly this reason.
For the number of weeks a specific month spans rather than a week number, see the weeks in a month calculator. To find the weekday, quarter and day-of-year for a single date instead, use day of the week & week number.