Week number
Look up the week for a date, or go the other way and see which dates a week covers.
Which week is a date in?
Week number
40
Week 40 of 2026 · Day 277 of the year
Which dates does a week cover?
2026 has 53 weeks.
- Monday2026-09-28
- Sunday2026-10-04
- The week runs from 28 September 2026 to 4 October 2026.
Week numbers follow ISO 8601, the European standard with Monday as the first day.
How it works
Europe numbers its weeks according to ISO 8601. Week 1 is the week containing the first Thursday of the year, which is the same as the week where most days fall in the new year.
The week runs Monday to Sunday. That differs from the North American convention, where the week starts on Sunday and week 1 always contains 1 January.
As a result the first days of January can belong to week 52 or 53 of the previous year, and the last days of December can belong to week 1 of the next.
Most years have 52 weeks, but every fifth or sixth year has 53. That happens when the year starts on a Thursday, or on a Wednesday in a leap year.
How the tool finds the week
The tool moves the date to the Thursday of the same Monday-to-Sunday week and reads off the year that Thursday falls in. That year is the week’s year, and the week number is the count of whole weeks since the first Thursday of that year, plus one. That is why the answer gives both week and year: Friday 1 January 2027 belongs to week 53 of 2026, because the Thursday of that week is 31 December 2026.
Going the other way, the tool starts from the week containing 4 January, which is always week 1, and adds seven days for each week. If the Monday it lands on has a different week number, the week does not exist that year and the tool says so. The year can be anything from 1900 to 2200.
Example: week 53 of 2026
2026 began on a Thursday, so week 1 ran from Monday 29 December 2025 to Sunday 4 January 2026. Add 52 weeks and week 53 starts on Monday 28 December 2026 and ends on Sunday 3 January 2027. Monday 4 January 2027 opens week 1, and 2027 has 52 weeks. The next year with a week 53 is 2032. Running the tool’s code over a full 400-year Gregorian cycle, 2001 to 2400, gives 71 years with 53 weeks.
Payroll weeks and spreadsheets
UK payroll does not use ISO weeks. Under regulation 2 of the Income Tax (PAYE) Regulations 2003, tax week 1 runs from 6 to 12 April and each following period of seven days is the next tax week. Monday 6 April 2026 is therefore tax week 1 but ISO week 15. Because 52 weeks come to 364 days, a tax week 53 is left over: just the last day of the tax year, or the last two days in a leap year. In the 2026–27 tax year that is 5 April 2027.
Spreadsheets need care too. Excel’s WEEKNUM uses return type 1 by default, where weeks start on Sunday and the week containing 1 January is always week 1, so it gives week 1 for 1 January 2027 where ISO 8601 gives week 53. ISOWEEKNUM, or WEEKNUM with return type 21, counts as this tool does. The full ISO form, as set out in RFC 3339, writes the week’s year, a W, the week and the weekday from 1 for Monday to 7 for Sunday: 2026-W53-5 for 1 January 2027.