Thursday of the week
T = D + (4 − d)
d is the weekday of D, from Monday (1) to Sunday (7).
ISO Week Number Calculator
Enter a date to get its ISO 8601 week number, the ISO year it belongs to, the Monday-to-Sunday period and the number of weeks in that year.
Enter a valid date.
ISO week
Week
ISO year
Monday to Sunday:
Weeks in this ISO year:
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How does this tool work?
Production schedules, school calendars, deliveries and many business systems talk about “week 41” rather than dates. That number follows ISO 8601: weeks start on Monday and are numbered from 1 to 52, or 53 in some years. This calculator gives the week number of a date, the ISO year that week belongs to, its first and last day, and how many weeks the year has. It removes the classic New Year trap, where 1 January may still belong to the last week of the previous year.
The ISO 8601 rule fits in one sentence: week 1 is the week that contains the first Thursday of the year, which is also the week that contains 4 January. A week therefore belongs entirely to the year that holds its Thursday.
For a given date the tool moves to the Thursday of the same week (Monday = 1, Sunday = 7), reads the year of that Thursday (the ISO year), then counts how many weeks separate that Thursday from 1 January of that year. The Monday and Sunday are found by stepping back and forward from the date you entered.
An ISO year has 53 weeks when 28 December falls in week 53, which happens when the year starts on a Thursday, or on a Wednesday in a leap year. 2020 and 2026 are such years; 2025 and 2027 are not.
The calculation works on Gregorian calendar dates with no time or time zone: the date is used exactly as entered, and the Today button uses your device’s date.
How to use this tool
Pick the date
Today’s date is filled in by default; enter another date to see its week.
Read the week and ISO year
The number comes with its ISO year, which can differ from the calendar year around 1 January.
Note the period and the year length
The Monday and Sunday bound the week, and the week count tells you whether a week 53 exists.
Share the calculation
The calculation link keeps the date so a colleague sees the same result.
Computing the ISO week number
Move the date to the Thursday of its week, then count weeks from 1 January of that Thursday’s year.
Week number
W = ⌊(n(T) − 1) / 7⌋ + 1
n(T) is the day of the year of T (1 for 1 January).
Week 53
53 weeks if 28 December is in week 53
True for years starting on a Thursday, or on a Wednesday in leap years.
- D
- date entered
- d
- weekday of D (Monday = 1 … Sunday = 7)
- T
- Thursday of the same week; its year is the ISO year
- W
- ISO week number
Use cases
Production or delivery planning
Turn an order or delivery date into a week number for an industrial or logistics plan that works in “W41”.
Leave and rota planning
Find the Monday and Sunday of a week announced only by its number in a team or on-call calendar.
Checking a spreadsheet formula
Verify ISOWEEKNUM or a programming-language date function on the tricky dates at the end and start of a year.
Naming weekly files and reports
Use the 2026-W41 notation with the correct ISO year so late-December reports sort in the right place.
Examples
An ordinary date
Friday 9 October has Thursday 8 October, day 281 of 2026: ⌊280 / 7⌋ + 1 = 41.
Input
2026-10-09Result
Week 41 of 2026, 2026-10-05 to 2026-10-11New Year in week 53
1 January 2027 is a Friday, so its Thursday is 31 December 2026. 2026 starts on a Thursday and therefore has 53 weeks.
Input
2027-01-01Result
Week 53 of 2026Late December already in week 1
The Thursday of that week is 2 January 2025, so Monday 30 December belongs to ISO year 2025.
Input
2024-12-30Result
Week 1 of 2025, 2024-12-30 to 2025-01-05Tips and best practices
- Write the ISO year with the week number (2026-W53): “week 53” alone is ambiguous around New Year.
- In the United States and in several apps, weeks start on Sunday and week 1 contains 1 January, so their numbers can differ by one from ISO numbers.
- In Excel and Google Sheets, ISOWEEKNUM applies the same rule; WEEKNUM without a second argument follows the US convention.
- 4 January and 28 December are always in the first and in the last ISO week of their year, which makes them handy checks.
- To get the day number within the year rather than the week, use the day of the year calculator.
Sources and references
- ISO — ISO 8601 date and time format (opens in a new tab)Page in English
- Claus Tøndering, Week-related questions (opens in a new tab), Calendar FAQPage in English
Frequently asked questions
What is the rule for week 1?
Week 1 is the Monday-to-Sunday week containing the first Thursday of January, and therefore 4 January. January days before it belong to the last week of the previous year.
Why is 1 January sometimes in week 52 or 53?
When 1 January is a Friday, Saturday or Sunday, the Thursday of its week is still in December, so the whole week belongs to the previous year. For example, 1 January 2023 is in week 52 of 2022.
Which years have 53 weeks?
Years starting on a Thursday, or on a Wednesday in leap years: 2015, 2020, 2026 and 2032, for example. That is 71 years in each 400-year Gregorian cycle.
Can a week span two years?
Its dates can, its number cannot: 30 December 2024 to 5 January 2025 is entirely week 1 of 2025.
Why does my calendar app show a different number?
It probably uses another convention, such as weeks starting on Sunday or week 1 being the week of 1 January. Check its week numbering setting.
What week is it today?
The tool shows the week of today’s date when the page opens; the Today button brings you back after looking up another date.
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