Two ways to number weeks
There isn't one universal week number, so the calculator offers the two systems in common use:
- ISO 8601, the international standard used across Europe and much of Asia: weeks start on Monday, and week 1 is the week that contains the year's first Thursday (equivalently, the week with 4 January in it). A year has 52 or 53 ISO weeks.
- US system, common in the United States, Canada and Japan: weeks start on Sunday, and week 1 is the week that contains 1 January, even if that week is mostly in December.
The two systems often give different numbers for the same date, especially in the first and last days of the year. When you agree a "week 12" deadline with someone abroad, check which system they mean.
Why 1 January can be in week 53
In the ISO system a week belongs to the year that holds its Thursday. 1 January 2027 is a Friday, so the week from Monday 28 December 2026 to Sunday 3 January 2027 has its Thursday in 2026. That makes 1 January 2027 part of week 53 of 2026. This is why ISO dates are sometimes written with a separate "week-numbering year", like 2026-W53-5.
ISO week = ⌊(day of the year of that week's Thursday − 1) ÷ 7⌋ + 1
Which years have 53 weeks?
An ISO year has 53 weeks when it starts on a Thursday, or on a Wednesday in a leap year. That happens 71 times every 400 years, roughly every five or six years. 2026 has 53 ISO weeks; the next ones are 2032 and 2037.
Week numbers 2026 (US (weeks start Sunday))
| Week | From | To |
|---|---|---|
| 1 | Sun, Dec 28 | Sat, Jan 3 |
| 2 | Sun, Jan 4 | Sat, Jan 10 |
| 3 | Sun, Jan 11 | Sat, Jan 17 |
| 4 | Sun, Jan 18 | Sat, Jan 24 |
| 5 | Sun, Jan 25 | Sat, Jan 31 |
| 6 | Sun, Feb 1 | Sat, Feb 7 |
| 7 | Sun, Feb 8 | Sat, Feb 14 |
| 8 | Sun, Feb 15 | Sat, Feb 21 |
| 9 | Sun, Feb 22 | Sat, Feb 28 |
| 10 | Sun, Mar 1 | Sat, Mar 7 |
| 11 | Sun, Mar 8 | Sat, Mar 14 |
| 12 | Sun, Mar 15 | Sat, Mar 21 |
| 13 | Sun, Mar 22 | Sat, Mar 28 |
| 14 | Sun, Mar 29 | Sat, Apr 4 |
| 15 | Sun, Apr 5 | Sat, Apr 11 |
| 16 | Sun, Apr 12 | Sat, Apr 18 |
| 17 | Sun, Apr 19 | Sat, Apr 25 |
| 18 | Sun, Apr 26 | Sat, May 2 |
| 19 | Sun, May 3 | Sat, May 9 |
| 20 | Sun, May 10 | Sat, May 16 |
| 21 | Sun, May 17 | Sat, May 23 |
| 22 | Sun, May 24 | Sat, May 30 |
| 23 | Sun, May 31 | Sat, Jun 6 |
| 24 | Sun, Jun 7 | Sat, Jun 13 |
| 25 | Sun, Jun 14 | Sat, Jun 20 |
| 26 | Sun, Jun 21 | Sat, Jun 27 |
| 27 | Sun, Jun 28 | Sat, Jul 4 |
| 28 | Sun, Jul 5 | Sat, Jul 11 |
| 29 | Sun, Jul 12 | Sat, Jul 18 |
| 30 | Sun, Jul 19 | Sat, Jul 25 |
| 31 | Sun, Jul 26 | Sat, Aug 1 |
| 32 | Sun, Aug 2 | Sat, Aug 8 |
| 33 | Sun, Aug 9 | Sat, Aug 15 |
| 34 | Sun, Aug 16 | Sat, Aug 22 |
| 35 | Sun, Aug 23 | Sat, Aug 29 |
| 36 | Sun, Aug 30 | Sat, Sep 5 |
| 37 | Sun, Sep 6 | Sat, Sep 12 |
| 38 | Sun, Sep 13 | Sat, Sep 19 |
| 39 | Sun, Sep 20 | Sat, Sep 26 |
| 40 | Sun, Sep 27 | Sat, Oct 3 |
| 41 | Sun, Oct 4 | Sat, Oct 10 |
| 42 | Sun, Oct 11 | Sat, Oct 17 |
| 43 | Sun, Oct 18 | Sat, Oct 24 |
| 44 | Sun, Oct 25 | Sat, Oct 31 |
| 45 | Sun, Nov 1 | Sat, Nov 7 |
| 46 | Sun, Nov 8 | Sat, Nov 14 |
| 47 | Sun, Nov 15 | Sat, Nov 21 |
| 48 | Sun, Nov 22 | Sat, Nov 28 |
| 49 | Sun, Nov 29 | Sat, Dec 5 |
| 50 | Sun, Dec 6 | Sat, Dec 12 |
| 51 | Sun, Dec 13 | Sat, Dec 19 |
| 52 | Sun, Dec 20 | Sat, Dec 26 |
| 53 | Sun, Dec 27 | Sat, Jan 2 |
Where week numbers are used
- Business and manufacturing: production plans, shipping schedules and sprints are often set by week number.
- Schools and payroll: timetables and pay periods may follow week numbers.
- Retail: sales are compared week by week, year over year.
Frequently asked questions
What week number is it today?
The calculator above shows today's week number as soon as the page loads, in your country's usual system. Switch between ISO and US numbering with the menu.
Does the week start on Monday or Sunday?
In the ISO 8601 standard, used in Europe and most of Asia, weeks start on Monday. In the US, Canada, Japan and some other countries, calendars usually start the week on Sunday.
How many weeks are in a year?
52 weeks and 1 day, or 52 weeks and 2 days in a leap year. In ISO numbering, a year has either 52 or 53 numbered weeks.
What is week 1 of the year?
In ISO 8601, week 1 is the week with the year's first Thursday, so it always contains 4 January. In the US system, week 1 is the week that contains 1 January.
Week numbers follow ISO 8601 or the common US convention. Some companies use their own fiscal weeks, which may differ.