How to find a week number
Pick a date, or press Today, and the calculator shows its ISO week number, the Monday and Sunday of that week, and how many weeks its year has. To go the other way, choose Dates of a week and enter a year and week number.
The ISO 8601 rules, often called the European week numbering system and used by Excel’s ISOWEEKNUM, are:
- Weeks start on Monday and end on Sunday.
- Week 1 is the week that contains the year’s first Thursday. The same week always contains 4 January, and it is the first week with at least four days in the new year.
- A year therefore has 52 or 53 whole weeks, and the few days around New Year can belong to the neighbouring year’s weeks.
The result also shows the US-style number: weeks start on Sunday and week 1 is whichever week contains 1 January, even if that is a single day. Excel’s WEEKNUM uses this by default. Check which convention your employer, school or software uses before relying on either.
Week number formula
week = ⌊(D(Thursday) − 1) ÷ 7⌋ + 1
- Thursday
- the Thursday in the same Monday-to-Sunday week as the date: date + (4 − weekday) days
- weekday
- 1 for Monday through 7 for Sunday
- D
- day of the year, with 1 January = 1
The year of that Thursday is the ISO week-year, which is the year the week number belongs to. Looking at the Thursday works because every week has exactly one, and a week belongs to the year that holds most of its days, which is the year its Thursday falls in.
Monday = (4 January − (weekday of 4 January − 1) days) + (n − 1) × 7 days
A year has 53 ISO weeks when it starts on a Thursday, or when it is a leap year starting on a Wednesday. In every other year it has 52.
week = ⌊(D − 1 + weekday of 1 January) ÷ 7⌋ + 1, with Sunday = 0
Worked examples
Wednesday 15 July 2026:
- Wednesday is weekday 3, so the Thursday of that week is 16 July 2026.
- 16 July is day 197 of 2026 (31 + 28 + 31 + 30 + 31 + 30 + 16).
- Week = ⌊(197 − 1) ÷ 7⌋ + 1 = 28 + 1 = 29, written 2026-W29-3. The week runs from Monday 13 July to Sunday 19 July.
- In the US system, 1 January 2026 is a Thursday (4 with Sunday = 0), and 15 July is day 196: ⌊(195 + 4) ÷ 7⌋ + 1 = 29 as well. The two systems agree for this date, but not always: in some years they are a week apart all year.
Around New Year they can disagree. Sunday 3 January 2021 has its Thursday on 31 December 2020, so it is 2020-W53-7, while the US count calls it week 2 of 2021. And because 2026 starts on a Thursday, it has 53 ISO weeks: 31 December 2026 and 1–3 January 2027 are all in 2026-W53. ISO week 1 of 2026 runs from Monday 29 December 2025 to Sunday 4 January 2026.
Other week numbering systems
- ISO 8601 (Monday start, first Thursday) is the international standard and the one shown first.
- US and Canada calendars and Excel’s default WEEKNUM start weeks on Sunday and count from the week containing 1 January, so the first and last weeks are often short.
- Fiscal and retail calendars (4-4-5 quarters, financial years that start in April or October) number weeks from their own start date. This calculator does not cover them.
- Dates use the Gregorian calendar for all years, extended backwards before 1582, which is the calendar ISO 8601 is built on. To count the days between two dates, use the days between dates calculator; to move a date by a number of weeks, the date calculator.
Frequently asked questions
Why is 1 January sometimes in week 52 or 53?
An ISO week belongs to the year that contains its Thursday. If 1 January falls on a Friday, Saturday or Sunday, that week’s Thursday is in December, so the days belong to the last week of the previous year.
Which years have 53 weeks?
Under ISO 8601, a year has 53 weeks when it starts on a Thursday, or when it is a leap year starting on a Wednesday. Between 2015 and 2040 that is 2015, 2020, 2026, 2032 and 2037.
Does the week start on Sunday or Monday?
ISO 8601 weeks start on Monday, the convention often called the European system. Calendars in the US and Canada usually show Sunday first, and Excel’s WEEKNUM uses a Sunday start unless told otherwise.
What does 2026-W29-3 mean?
It is an ISO week date: week-year 2026, week 29, day 3 of the week. Days are numbered from Monday = 1 to Sunday = 7, so it means the Wednesday of week 29, which is 15 July 2026.
How do I get the ISO week number in Excel?
Use ISOWEEKNUM(date), or WEEKNUM(date, 21). Plain WEEKNUM(date) uses the US convention with Sunday-start weeks and week 1 containing 1 January, so it can differ by one week or give week 1 to a date ISO puts in week 52 or 53.
Sources
Last reviewed September 19, 2026