Under the ISO 8601 standard used across the UK and most of Europe, weeks start on Monday and week 1 is the week that contains the first Thursday of the year, which is the same as the week containing 4 January. That means 1 January can fall in week 52 or 53 of the previous year. For example, Friday 1 January 2027 is in week 53 of 2026, and week 1 of 2027 begins on Monday 4 January.
Note: week numbering is a convention, and systems differ. Check which one a payroll, school, retailer or software system uses before relying on a week number for a deadline, and see GOV.UK or your employer for tax weeks.
The two rules that define an ISO week
ISO 8601 sets out two rules, and everything else follows from them:
- Weeks run from Monday to Sunday. Monday is day 1 and Sunday is day 7.
- Week 1 is the week with the year’s first Thursday in it. An equivalent way to say that is that week 1 is the week that contains 4 January, or the first week with at least four days in the new year.
Because a week is attached to the year that holds its Thursday, a week can straddle two calendar years while belonging to just one. The days in the straddling week keep the same week number, even though some of them are in a different calendar month and year. The year the week belongs to is called the ISO week-year, and it can differ from the calendar year for a few days at each end.
The House of Calculator week number tool shows this directly. Enter Friday 1 January 2027 and it returns week 53, with an ISO week-year of 2026 that differs from the calendar year.
Week 1 of 2027: a worked example
To find week 1 of 2027, locate the first Thursday of the year. In 2027, 1 January is a Friday, so the first Thursday is 7 January. The Monday of that week is 4 January, so ISO week 1 of 2027 runs from Monday 4 January to Sunday 10 January 2027. The tool gives the same dates.
The days before that, Friday 1 to Sunday 3 January 2027, are the end of the previous ISO year, so they belong to week 53 of 2026. Week 53 of 2026 ran from Monday 28 December 2026 to Sunday 3 January 2027. A document dated 1 January 2027 with the “week 53” label is correct, and a calendar that shows “week 1” on 1 January is using a different system.
Now look at the start of 2026. 1 January 2026 was a Thursday, so it is itself in week 1, and week 1 of 2026 began on Monday 29 December 2025. That means 29, 30 and 31 December 2025 are in week 1 of 2026, which can surprise anyone who sees “Week 1” printed on a December date.
Edge dates around the year end
These are the dates people most often get wrong. All of them are checked against the tool:
| Date | Weekday | ISO week | ISO week-year |
|---|---|---|---|
| 28 December 2025 | Sunday | 52 | 2025 |
| 29 December 2025 | Monday | 1 | 2026 |
| 1 January 2026 | Thursday | 1 | 2026 |
| 25 December 2026 | Friday | 52 | 2026 |
| 31 December 2026 | Thursday | 53 | 2026 |
| 1 January 2027 | Friday | 53 | 2026 |
| 4 January 2027 | Monday | 1 | 2027 |
| 5 April 2027 | Monday | 14 | 2027 |
| 31 December 2027 | Friday | 52 | 2027 |
| 1 January 2028 | Saturday | 52 | 2027 |
You can see the pattern: the early days of January can be in the old year’s last week, and the late days of December can be in the next year’s week 1. The week-year is the one that matters whenever you combine a week number with a year, such as a reference like “2027-W01”.
Which years have 53 weeks?
An ISO year has either 52 or 53 weeks. A year has 53 weeks if 1 January is a Thursday, or if it is a leap year and 1 January is a Wednesday. Put another way, a year has 53 weeks when it has 53 Thursdays.
The result is that 53-week years come around every five or six years. Recent and upcoming ones include 2015, 2020, 2026 and 2032, and the next after that is 2037. The tool confirms that 2026 has 53 weeks and 2027 has 52. Because a normal year is 364 days when counted as 52 whole weeks, with one extra day or two left over, those spare days accumulate until a 53rd week is needed.
This matters in planning. If you have a 52-week reporting calendar and a 53-week year turns up, one period is longer than usual and year-on-year comparisons for that period are not like for like. Businesses that report by week usually have a rule to deal with it, and you should check theirs.
Using week numbers in planning
Week numbers are handy for project plans, production schedules, school terms, delivery slots and rotas. They avoid ambiguity about “the week of the 5th”, and they sort easily. A few practical tips:
- Write the year with the week. “Week 14” is ambiguous. “Week 14 of 2027” is not, and the compact form is 2027-W14. You can add a day too: 2027-W14-1 means Monday of that week, which is 5 April 2027.
- Count across the year end carefully. Suppose a 14-week project starts on Monday 5 October 2026, which is ISO week 41. Fourteen weeks later is Monday 11 January 2027, so the project’s last day is Sunday 10 January 2027, which is week 1 of 2027. The project runs through week 53 of 2026, so the numbering is 41, 42 … 52, 53, then 1.
- Do not assume week numbers add up neatly. From week 41 of 2026, adding 12 weeks takes you to week 53 of 2026 (Monday 28 December 2026), not to week 1 of 2027. In a 52-week year, the same sum would have landed on week 1.
- Agree the week start. If a colleague’s week begins on Sunday, their numbers will not match yours.
For counting days between two dates, rather than weeks, How to count down to an event or deadline explains the method, including whether to count the first and last day.
ISO weeks versus other systems
Week numbering is not universal. The main alternatives are:
- US-style numbering. Weeks often start on Sunday, and week 1 is the week that contains 1 January. Under that rule, 1 January 2027 is in week 1, where ISO says week 53 of 2026. Many US calendars and some software use this.
- Spreadsheets. Excel’s WEEKNUM function uses a Sunday-start, 1 January system by default, with options for other starts. Excel and Google Sheets also provide an ISOWEEKNUM function that follows ISO 8601. If your week numbers do not match another team’s, a different function may be the cause.
- Programming languages. Many languages and libraries have ISO week functions, such as the isocalendar method in Python. Some others default to local conventions, so check the documentation.
- Retail and fiscal calendars. Some organisations use 4-4-5 or similar week patterns, and fiscal years that start in a month other than January. Their week 1 can fall at a very different time of year.
The practical rule is to state which system you mean. In the UK, ISO weeks are the sensible default for general planning, but payroll and tax follow their own calendars.
UK tax weeks are not ISO weeks
If you work with payroll, you will meet tax weeks and tax months. The UK tax year runs from 6 April to 5 April, and PAYE weekly pay uses tax weeks that start on 6 April. Tax week 1 is the first seven days from 6 April, and tax month 1 runs from 6 April to 5 May. So a payslip showing “week 14” is counting from 6 April, not from the ISO year, and it will be a different date range from ISO week 14.
For example, ISO week 14 of 2027 starts on Monday 5 April 2027, which is the last day of the 2026/27 tax year. The first tax week of 2027/28 begins the next day, on Tuesday 6 April 2027. The two numbering systems do not line up, and mixing them can cause errors in payroll or deadlines. Check with GOV.UK or your payroll provider for how tax weeks are counted in a given year, particularly for the odd days at the end of the tax year.
Other UK calendars also have their own numbering: school terms, university teaching weeks and academic years all use week counts that start at term start, not in January. If a deadline refers to “week 5”, ask what week 1 is.
Common mistakes
- Assuming 1 January is always in week 1. In ISO numbering it can be week 52 or 53 of the previous year.
- Assuming every year has 52 weeks. 2026 has 53.
- Mixing ISO weeks with US weeks. The same date can have different week numbers in different systems.
- Using the calendar year with the ISO week. The right pair is the ISO week-year and the week number. Late December dates can belong to the next week-year.
- Treating tax weeks as ISO weeks. They start on 6 April.
- Forgetting public holidays and working days. A week number tells you nothing about how many working days it has. In England and Wales, bank holidays reduce some weeks to four days, and UK bank holidays and how to count working days explains how to count working days.
A short method for finding any week number by hand
If you do not have a tool to hand, you can work out the ISO week of a date like this:
- Find the weekday of 4 January in that year. Week 1 is the week with that date in it.
- Find the Monday of that week. This is the start of week 1.
- Count the days between that Monday and your date, divide by 7 and round down, then add 1.
For 5 October 2026: 4 January 2026 was a Sunday, so week 1 began Monday 29 December 2025. The days from 29 December 2025 to 5 October 2026 are 280, and 280 ÷ 7 = 40, so the week number is 40 + 1 = 41. That matches the tool, which gives week 41 for Monday 5 October 2026.
Be careful with dates in early January and late December, where the date can belong to a neighbouring year. If the result is zero or negative, the date belongs to the previous year’s last week, and if it is past 52, check whether the year has a week 53. For time-zone questions, such as a deadline stated in a different zone, see Time zones explained: how to convert times without mistakes.
Try the calculator
Enter a date in the Week Number Calculator to see its ISO week number, weekday, day of the year and the Monday-to-Sunday dates of that week, or choose a year and week to see the dates it covers.
Frequently asked questions
What is the ISO week number?
It is the week number defined by ISO 8601. Weeks start on Monday, and week 1 is the week containing the first Thursday of the year, which is the week with 4 January in it. A year has 52 or 53 weeks.
Why is 1 January sometimes in week 52 or 53?
Because week 1 must contain at least four days of the new year. If 1 January falls on a Friday, Saturday or Sunday, it belongs to the last week of the previous year. For example, 1 January 2027 is in week 53 of 2026.
How many weeks are in a year?
An ISO year has 52 weeks, or 53 in some years. Years with 53 weeks include 2020, 2026 and 2032. The tool shows how many weeks are in the ISO year of any date you enter.
Is the UK week number the same as the US week number?
Not always. The UK usually follows ISO weeks, which start on Monday. US systems often start the week on Sunday and make the week with 1 January week 1, so numbers can differ by one or more at the start and end of the year.
How do tax weeks work in the UK?
PAYE tax weeks are counted from 6 April, the start of the tax year, so tax week 1 is the first seven days from that date. They do not match ISO weeks, so check GOV.UK or your payroll provider.