ISO Week Date Converter
How to use iso week date converter
- Enter an ISO week-year, week number and weekday.
- Use Monday as one and Sunday as seven.
- Calculate; invalid week 53 values are rejected.
Convert an ISO week-year, week number and Monday-to-Sunday weekday to a calendar date.
How the result is calculated
The calculator validates the required dates, units and range before producing a result. Keep the inputs and the selected convention with the output so another person can reproduce it.
Read all calculation conventionsHow to read your result
Keep the inputs and conventions beside the answer. These values explain what the result measures.
| Result field | Meaning |
|---|---|
| Calendar date | The actual date belonging to the validated ISO components. |
| ISO week date | Week-year, week number and Monday-one/Sunday-seven weekday together. |
| Ordinal day | The result’s position within its calendar year, which can differ from the ISO week-year. |
ISO dates can cross the calendar-year boundary
2026-W53-5 maps to January 1, 2027. The ISO week-year is 2026 even though the calendar date is in 2027.
Try the example in the calculator above. Change one input at a time and compare the target, interval and selected rules. For a longer explanation read Convert ISO Week Dates and Ordinal Days to Calendar Dates.
Conventions to keep with your result
An ISO week-year contains 52 or 53 full weeks. The converter validates the week against its entered week-year instead of rolling an invalid week into the next year.
ISO Week Date Converter: frequently asked questions
Is January 1 always ISO week one?
No. Week one contains January 4 and starts Monday.
Is Sunday weekday zero?
No. This ISO converter uses Monday one through Sunday seven.
Can every year have week 53?
No. A nonexistent week 53 is rejected.
Learn the method
All guidesISO Week Numbers: Week 1, Week-Years and 53-Week Years
Find the ISO week-year near New Year and understand why January dates can belong to the preceding week-year.
Convert ISO Week Dates and Ordinal Days to Calendar Dates
Convert week-year/week/weekday values and year/day-number values correctly. Examples cover week 53, New Year boundaries, leap days and invalid inputs.
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