Week Number Calculator
See which week a date falls in, using ISO 8601 week numbers, with the Monday and Sunday of that week.
Week
41
Of the year
2026
Monday
5 October 2026
Sunday
11 October 2026
Weeks in that year
53
Please note: The result is an estimate and has been rounded. Actual costs and values may differ, and no rights can be derived from this calculation.
How this number is calculated
The blocks use your input. Change a value above and they update with it.
Step 1 · Day of the week (1 = Monday)
Step 2 · Thursday of this week
Step 3 · 1 January of that year
Step 4 · Week
How the calculation works
What week is it? Pick a date and see its week number, with the Monday and Sunday of that week.
Week numbers follow the international standard ISO 8601, used in the UK and across Europe. A week starts on Monday. Week 1 is the week with the first Thursday of the year, which is also the week with 4 January.
So 1 January can still belong to the last week of the year before, and the last days of December can already be in week 1 of the new year. A year has 52 or 53 weeks: 53 when it starts on a Thursday, or when it is a leap year starting on a Wednesday.
Formula
Take the Thursday of the same week. The week number is the number of days from 1 January to that Thursday, divided by 7, rounded down, plus 1.
Assumptions
- Weeks follow ISO 8601 and run from Monday to Sunday.
- In the United States weeks start on Sunday and are numbered differently; this result does not apply there.
Example calculations
The button fills in the calculator above, so you can carry on from there.
-
An ordinary day
Week
41
Date 9 October 2026
-
New Year's Day in week 53
Week
53
Date 1 January 2027
-
Late December already week 1
Week
1
Date 30 December 2024
Frequently asked questions
Why is 1 January sometimes week 53?
Week 1 is the week with the first Thursday of the year. If 1 January falls on a Friday, Saturday or Sunday, it still belongs to the last week of the previous year.
Which years have 53 weeks?
Years that start on a Thursday, and leap years that start on a Wednesday. 2026 has 53 weeks; 2027 has 52.
Sources and checks
Sources
How we check it
This calculator has 6 fixed checks: 6 calculations with a result that is fixed in advance, and 0 cases of invalid input that must be rejected. They all run automatically every time we publish; if one fails, the calculator does not go live.
See the calculations
- Date 9 October 2026 → Week 41
- Date 1 January 2027 → Week 53
- Date 30 December 2024 → Week 1
- Date 3 January 2021 → Week 53
- Date 1 January 2026 → Week 1
- Date 29 February 2020 → Week 9
Last reviewed: · How we build and check our calculators
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