When to use this calculator
Find ISO week-year and week number for a chosen date. An ISO week-year can differ from the calendar year near New Year.
Explore the calendar dates & age calculators for other questions in this topic. For a related calculation, use the Leap Year Calculator: Check whether a Gregorian calendar year has 366 days.
PUTTING IT INTO PRACTICE
ISO Week Number: a worked example
The values below are an illustrative example, not a recommendation. Replace them with measurements or assumptions that match your question.
| Input | Example value |
|---|---|
| Calendar date | 2026-10-17 |
| Result | 2026-W42 |
- Check the input labels and enter the example values above.
- Apply the relationship: ISO weeks start Monday; week one contains the year’s first Thursday..
- The result is 2026-W42 , rounded for display.
How to interpret your result
An ISO week-year can differ from the calendar year near New Year.
Choose whether an interval includes the start or end boundary. Month and year additions use explicitly stated clamping rules when a destination day does not exist. Date-only arithmetic is kept independent of local daylight-saving hour shifts.
Frequently asked questions
Does adding one month always add the same number of days?
No. Calendar months vary in length. A day-count addition and a calendar-month addition solve different questions.
When is the iso week number calculator useful?
Find ISO week-year and week number for a chosen date. An ISO week-year can differ from the calendar year near New Year.
How do I use the iso week number calculator?
Enter calendar date. The result updates automatically. Use the units shown beside each input; the example values illustrate the method.
Which formula does this calculator use?
ISO weeks start Monday; week one contains the year’s first Thursday..
What should I check before using the result?
An ISO week-year can differ from the calendar year near New Year.
How are results rounded?
Scalar numbers display up to six decimal places, with scientific notation for very large or small values. Text, fraction, date and schedule outputs use the format described on the tool. Intermediate calculations use JavaScript floating-point arithmetic. Keep sensible precision for your measurements.
Further reading
For background on this topic, see MDN: date values and calendar handling ↗. This is a topic reference, not an external review or endorsement of this tool.
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