📅 ISO Week Date Tool
Week Number Calculator
What Week Is It? Find ISO Week
Get the ISO 8601 week number for any date. Also shows week start (Monday) and end (Sunday) dates, plus year of the week.
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Calculator
📆
Week Number Calculator
Select a date → ISO week number & range
Select Date
ISO Week Number
—
week of the year
Week Year
—
ISO year (may differ from calendar year)
Month & Day
—
selected date
📅 Week Range (Monday → Sunday)
—
First day of week: Monday (ISO 8601). Week 1 is the week with the first Thursday of the year.
Reference
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Week Numbers per Month (Typical)
ISO week distribution across months
| Month | Week Numbers | Approx. Weeks |
|---|
Understanding ISO Week Dates
What is the ISO Week Number?
🔹 Weeks start on Monday.
🔹 Week 1 is the week containing the first Thursday of the year.
🔹 Each week belongs to the year of its Thursday (e.g., Dec 31, 2024 could be week 1 of 2025).
🔹 A year has 52 or 53 weeks.
🔹 Week 1 is the week containing the first Thursday of the year.
🔹 Each week belongs to the year of its Thursday (e.g., Dec 31, 2024 could be week 1 of 2025).
🔹 A year has 52 or 53 weeks.
Why week 53?
If the year starts on Thursday (common) or leap year starting on Wednesday, there are 53 weeks.
Week Year vs Calendar Year
January 1 can belong to week 52 or 53 of previous year. Our calculator shows the correct ISO year.
Global Standard
ISO 8601 is used worldwide for business and logistics planning.
FAQs
How do I calculate the week number of a date?+
ISO week algorithm: find the Thursday of the week containing the date. The week number is the number of Thursdays since the first Thursday of the year. Our calculator does this instantly.
What is the current week number?+
Click "Today" button to see the current ISO week number and week range. The week number updates based on your system date.
Can week numbers cross over into different years?+
Yes. For example, week 1 of 2025 starts on December 30, 2024. Our calculator shows the correct "week year" separate from the calendar year.
How many weeks are in a year?+
Most years have 52 weeks. A year has 53 weeks if it starts on Thursday (common year) or Wednesday in a leap year.