About the Week Number Calculator
This week number calculator tells you which week of the year a date falls in, using either the ISO 8601 standard (weeks start on Monday and week 1 contains the year’s first Thursday) or the US convention (weeks start on Sunday and week 1 contains January 1). You can also do the reverse: pick a year and week number to get the exact Monday-to-Sunday (or Sunday-to-Saturday) dates.
Week numbers are used for payroll periods, production and shipping schedules, sprint planning, school timetables and European business calendars, where “see you in week 42” is normal. The full list of weeks for the year underneath makes it easy to plan ahead or print a week-number reference.
Note that the ISO week-year can differ from the calendar year around New Year: 1 January can belong to week 52 or 53 of the previous year, and 29–31 December can fall in week 1 of the next year. The calculator shows the week-year alongside the week number so you never mislabel those days.
With the default inputs, the week number is 40. Change any value above to recalculate instantly.
How to use the week number calculator
- 1Choose whether you want the week number of a date or the dates of a week.
- 2Enter the date, or the year and week number.
- 3Pick ISO 8601 (Monday start) or the US Sunday-start system.
- 4Read the week number, week-year and the week’s start and end dates.
- 5Scroll the full-year list to plan or print every week of the year.
Formula and method
ISO 8601 weeks run Monday to Sunday and each week belongs to the year that contains its Thursday. To number a date, move to the Thursday of its week, take that Thursday’s day of the year, and divide by 7. This is why week 1 is the week with 4 January in it, and why a year has 53 weeks when it starts on a Thursday (or on a Wednesday in a leap year).
The US convention starts weeks on Sunday and simply counts the week that contains 1 January as week 1, so the first and last weeks are usually partial and a year can have 53 or even 54 week numbers. Reverse lookups add (week − 1) × 7 days to the first day of week 1.
- day of year
- Ordinal day (1 = January 1)
- weekday of Jan 1
- 0 = Sunday … 6 = Saturday
Worked examples
ISO week of 29 September 2026
Tuesday 29 September 2026 is in the week running Monday 28 September to Sunday 4 October. That week’s Thursday is 1 October, day 274, so it is ISO week 40. 2026 starts on a Thursday, so it has 53 ISO weeks.
New Year’s Day 2027 belongs to the previous year
1 January 2027 is a Friday. Its Monday-to-Sunday week starts on 28 December 2026 and its Thursday is 31 December 2026, so the day is in ISO week 53 of 2026.
Dates of ISO week 1 of 2027
ISO week 1 is the week containing 4 January. In 2027 that date is a Monday, so week 1 runs 4–10 January 2027, and 2027 has 52 ISO weeks.
US week number of 29 September 2026
1 January 2026 is a Thursday (weekday 4) and 29 September is day 272, so the US week is ⌊(271 + 4) ÷ 7⌋ + 1 = 40, running Sunday 27 September to Saturday 3 October.
Frequently asked questions
What is the ISO week number?+
ISO 8601 numbers weeks from Monday to Sunday. Week 1 is the week containing the year’s first Thursday (equivalently, 4 January). Most years have 52 weeks; years starting on Thursday, or leap years starting on Wednesday, have 53.
Why is 1 January sometimes in week 52 or 53?+
Under ISO rules a week belongs to the year that holds its Thursday. If 1 January falls on a Friday, Saturday or Sunday, its week’s Thursday is in December, so the day is counted in the last week of the previous year.
Do the US and Europe number weeks differently?+
Often, yes. The US system starts weeks on Sunday and makes the week containing 1 January week 1, while ISO starts on Monday and uses the first Thursday. The two can differ by one week for many dates, so confirm which system a partner uses.
How many weeks are in a year?+
A year has 52 weeks and 1 day (52 weeks and 2 days in a leap year). That extra time means ISO years have either 52 or 53 numbered weeks, and US-style calendars show 53 or occasionally 54 partial week rows.
How do I get the week number in Excel?+
Use =ISOWEEKNUM(A1) for the ISO 8601 week, or =WEEKNUM(A1) for the US Sunday-start system. =WEEKNUM(A1,21) also returns the ISO week.