How to do it?
- 1.
Read the current week
The top card shows today’s week number and ISO year based on your device’s date.
- 2.
Check any date
Pick a date to see its week number, the ISO year the week belongs to, the Monday-Sunday range, the day of the week and the day of the year.
- 3.
Find a week’s dates
Enter a year and week number to get the Monday and Sunday of that week; the hint shows whether the year has 52 or 53 weeks.
- 4.
Browse the whole year
Scroll the table of all weeks of the chosen year; the current week is marked “now”.
How ISO week numbers work
Under ISO 8601 weeks start on Monday, and week 1 is the week containing the first Thursday of the year (always the week with 4 January). A year has 52 or 53 weeks, so early January days can belong to the previous year’s last week and late December days to the next year’s week 1. For example, 1 January 2027 is in week 53 of 2026, and 29 December 2025 is in week 1 of 2026.
Formula: take the Thursday of the same Monday-Sunday week; week = ⌊(day of year of that Thursday − 1) ÷ 7⌋ + 1. Wednesday 15 July 2026 → Thursday 16 July is day 197 → ⌊196 ÷ 7⌋ + 1 = week 29.
Years with 53 weeks
A year has 53 ISO weeks when it starts on a Thursday, or on a Wednesday in a leap year - roughly 71 years out of 400. Recent and upcoming examples: 2015, 2020, 2026, 2032, 2037. In 2026 week 1 runs from 29 December 2025 to 4 January 2026 and week 53 from 28 December 2026 to 3 January 2027.
| Week 2026 | Monday | Sunday |
|---|---|---|
| 1 | 29 Dec 2025 | 4 Jan 2026 |
| 14 | 30 Mar 2026 | 5 Apr 2026 |
| 27 | 29 Jun 2026 | 5 Jul 2026 |
| 40 | 28 Sep 2026 | 4 Oct 2026 |
| 53 | 28 Dec 2026 | 3 Jan 2027 |
A quick way to find any Monday: Monday of week n = Monday of week 1 + 7 × (n − 1) days.
ISO weeks vs US week numbers
Europe, most of Asia and international business use ISO weeks (in German “KW”, Kalenderwoche). In the US and Canada calendars often start weeks on Sunday and call the week containing 1 January week 1, regardless of the weekday. The two systems can differ by one week:
- Sunday 11 January 2026 is ISO week 2 but US week 3, because the US week 3 starts that Sunday.
- Thursday 1 January 2026 is week 1 in both systems, but the US week 1 is only three days long (1-3 January).
When scheduling with partners abroad, say “ISO week” or give the Monday date to avoid confusion. This tool always uses ISO 8601.
Week numbers in spreadsheets and code
| Tool | ISO week | Note |
|---|---|---|
| Excel, Google Sheets | =ISOWEEKNUM(A1) | plain WEEKNUM counts from Sunday and 1 January |
| Python | date.isocalendar().week | also returns the ISO year |
| PHP | date('W') | use 'o' for the ISO year |
| PostgreSQL | EXTRACT(WEEK FROM d) | ISO by default |
| MySQL | WEEK(d, 3) | mode 3 = ISO |
| JavaScript | no built-in | use a library such as date-fns getISOWeek |
Common mistake: pairing an ISO week with the calendar year. For 1 January 2027 the correct label is 2026-W53, not 2027-W53 - always use the ISO year (Python’s isocalendar, PHP’s “o”) together with the ISO week.
Frequently asked questions
How many weeks are in 2026?
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When does week 1 start?
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Does the week start on Monday or Sunday?
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How many weeks are in a year?
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How do I get the ISO week number in Excel?
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Why does 1 January show week 53 or 52?
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