There are 52 ISO week numbers in 2027, with week 1 running from Monday 4 January to Sunday 10 January 2027. Week 52 starts on Monday 27 December 2027 and ends on Sunday 2 January 2028. The calculator above finds the week for any date, and the table below lists all 52 weeks.
2027 week number finder
How many weeks are in 2027
There are 52 ISO weeks in 2027. Week 1 runs from Monday 4 January 2027 to Sunday 10 January 2027, and week 52 runs from Monday 27 December 2027 to Sunday 2 January 2028.
That gives 2027 one week fewer than 2026, which has 53 ISO weeks. The extra week in 2026 is the reason the opening days of January 2027 still carry a 2026 week number. The table below lists every week from 1 to 52 with its Monday and Sunday dates.
Each week in the list is a full Monday to Sunday block. No week is split between two numbers, even when it starts in one month and ends in the next. Week 13, for example, runs from 29 Mar to 4 Apr 2027, and week 22 runs from 31 May to 6 Jun 2027.
All 52 Week Numbers in 2027
ISO 8601 week numbers, weeks starting on Monday.
| Week | Monday | Sunday |
|---|---|---|
| Week 1 | 4 Jan 2027 | 10 Jan 2027 |
| Week 2 | 11 Jan 2027 | 17 Jan 2027 |
| Week 3 | 18 Jan 2027 | 24 Jan 2027 |
| Week 4 | 25 Jan 2027 | 31 Jan 2027 |
| Week 5 | 1 Feb 2027 | 7 Feb 2027 |
| Week 6 | 8 Feb 2027 | 14 Feb 2027 |
| Week 7 | 15 Feb 2027 | 21 Feb 2027 |
| Week 8 | 22 Feb 2027 | 28 Feb 2027 |
| Week 9 | 1 Mar 2027 | 7 Mar 2027 |
| Week 10 | 8 Mar 2027 | 14 Mar 2027 |
| Week 11 | 15 Mar 2027 | 21 Mar 2027 |
| Week 12 | 22 Mar 2027 | 28 Mar 2027 |
| Week 13 | 29 Mar 2027 | 4 Apr 2027 |
| Week 14 | 5 Apr 2027 | 11 Apr 2027 |
| Week 15 | 12 Apr 2027 | 18 Apr 2027 |
| Week 16 | 19 Apr 2027 | 25 Apr 2027 |
| Week 17 | 26 Apr 2027 | 2 May 2027 |
| Week 18 | 3 May 2027 | 9 May 2027 |
| Week 19 | 10 May 2027 | 16 May 2027 |
| Week 20 | 17 May 2027 | 23 May 2027 |
| Week 21 | 24 May 2027 | 30 May 2027 |
| Week 22 | 31 May 2027 | 6 Jun 2027 |
| Week 23 | 7 Jun 2027 | 13 Jun 2027 |
| Week 24 | 14 Jun 2027 | 20 Jun 2027 |
| Week 25 | 21 Jun 2027 | 27 Jun 2027 |
| Week 26 | 28 Jun 2027 | 4 Jul 2027 |
| Week 27 | 5 Jul 2027 | 11 Jul 2027 |
| Week 28 | 12 Jul 2027 | 18 Jul 2027 |
| Week 29 | 19 Jul 2027 | 25 Jul 2027 |
| Week 30 | 26 Jul 2027 | 1 Aug 2027 |
| Week 31 | 2 Aug 2027 | 8 Aug 2027 |
| Week 32 | 9 Aug 2027 | 15 Aug 2027 |
| Week 33 | 16 Aug 2027 | 22 Aug 2027 |
| Week 34 | 23 Aug 2027 | 29 Aug 2027 |
| Week 35 | 30 Aug 2027 | 5 Sep 2027 |
| Week 36 | 6 Sep 2027 | 12 Sep 2027 |
| Week 37 | 13 Sep 2027 | 19 Sep 2027 |
| Week 38 | 20 Sep 2027 | 26 Sep 2027 |
| Week 39 | 27 Sep 2027 | 3 Oct 2027 |
| Week 40 | 4 Oct 2027 | 10 Oct 2027 |
| Week 41 | 11 Oct 2027 | 17 Oct 2027 |
| Week 42 | 18 Oct 2027 | 24 Oct 2027 |
| Week 43 | 25 Oct 2027 | 31 Oct 2027 |
| Week 44 | 1 Nov 2027 | 7 Nov 2027 |
| Week 45 | 8 Nov 2027 | 14 Nov 2027 |
| Week 46 | 15 Nov 2027 | 21 Nov 2027 |
| Week 47 | 22 Nov 2027 | 28 Nov 2027 |
| Week 48 | 29 Nov 2027 | 5 Dec 2027 |
| Week 49 | 6 Dec 2027 | 12 Dec 2027 |
| Week 50 | 13 Dec 2027 | 19 Dec 2027 |
| Week 51 | 20 Dec 2027 | 26 Dec 2027 |
| Week 52 | 27 Dec 2027 | 2 Jan 2028 |
How ISO week numbers are worked out
ISO week numbers follow the ISO 8601 standard, where every week starts on a Monday. Week 1 is the week that contains the first Thursday of the year.
There is a simpler way to put the same rule. Week 1 is the week that contains 4 January. In 2027 that week begins on Monday 4 January itself, so the first ISO week and the first Monday of the year line up.
Because the rule counts whole weeks from a Monday, the ISO year does not always begin on 1 January. In 2027 it begins on Monday 4 January 2027 and ends on Sunday 2 January 2028. Dates outside that span belong to the ISO year before or after.
Why 1 January 2027 is in week 53 of 2026
1 January 2027 belongs to week 53 of 2026, not week 1 of 2027. It falls on a Friday, which is day 5 of that ISO week.
This follows straight from the rule. Week 1 of 2027 does not start until Monday 4 January 2027, so any January date before that Monday stays in the last week of the previous ISO year. 2026 has 53 weeks, so that last week is week 53.
The effect shows up in anything filed by week number. A delivery note or timesheet dated Friday 1 January 2027 sits under 2026 week 53 in an ISO week system. The calculator above shows which ISO year a date in early January belongs to.
When week 52 of 2027 ends
Week 52 is the last ISO week of 2027 and it ends on Sunday 2 January 2028. It starts on Monday 27 December 2027.
31 December 2027 is a Friday and sits in week 52 as day 5. The Saturday and Sunday that follow it fall in January 2028 but still count as 2027 week 52.
Rotas, delivery schedules and project plans that run across the new year are easier to read by week number than by calendar year. A shift on Sunday 2 January 2028, for instance, belongs to week 52 of 2027 in ISO terms, not to the first week of 2028.
ISO weeks and the UK tax year
ISO week numbers are not the same as UK tax weeks. The UK tax year 2027 to 2028 starts on 6 April 2027, and tax week 1 runs from 6 to 12 April 2027.
ISO week 1 of 2027, by contrast, runs from 4 Jan to 10 Jan 2027. On the ISO calendar, 6 April 2027 falls in week 14, which runs from 5 Apr to 11 Apr 2027.
A payslip that says week 1 and a project plan that says week 1 can therefore point to quite different dates. Look at which system a document uses before comparing its week numbers with the table below.
Using the calculator and the table
The calculator above gives the ISO week number for any date in 2027, along with the Monday to Sunday range of that week. Pick a date and the result shows both.
The table below is handy for planning further ahead. A summer project booked for week 26 would run from 28 Jun to 4 Jul 2027. A team working in week 45 would cover 8 Nov to 14 Nov 2027, and week 51 covers 20 Dec to 26 Dec 2027.
For counts of working days between two dates, use the working days calculator. For the gap between any two dates, use the date calculator, and for the countdown to Christmas, see the days until Christmas page.
Frequently Asked Questions
How many weeks are there in 2027? There are 52 ISO weeks in 2027. Week 1 starts on Monday 4 January 2027 and week 52 ends on Sunday 2 January 2028.
When does week 1 of 2027 start? Week 1 of 2027 starts on Monday 4 January 2027 and ends on Sunday 10 January 2027. Under ISO 8601, week 1 is the week that contains 4 January.
What week number is 1 January 2027? 1 January 2027 is a Friday in week 53 of 2026. It is day 5 of that ISO week, because 2026 has 53 ISO weeks.
What week number is 31 December 2027? 31 December 2027 is a Friday in week 52 of 2027. That week runs from 27 Dec 2027 to 2 Jan 2028.
Is ISO week 1 the same as tax week 1? No. ISO week 1 of 2027 runs from 4 Jan to 10 Jan 2027, while the UK tax year 2027 to 2028 starts on 6 April 2027 and tax week 1 runs from 6 to 12 April 2027.
Related Calculators
Week Number Calculator | Working Days Calculator | Date Calculator | Days Until Christmas
Official Sources
Figures are based on 2026/27 published UK government rates. This guide is general information only and not financial advice. Last checked September 2026.
Reuse of these figures requires credit and a followed link to this page, see our AI reuse policy.