ISO week date converter
Convert between ISO 8601 week dates and calendar dates: turn 2026-W41-3 into a date, get the start and end of that week and its ISO week year, look up the week date of any date, and list every week of a year.
Runs in your browserEvery computation happens in your browser — your data never leaves this device.
Input
ISO 8601: weeks start on Monday and week 1 contains the first Thursday of the year; a week date is written YYYY-Www-D, where YYYY is the ISO week year and may differ from the calendar year.
Result
Calendar date → ISO week date2021-01-012020535 · Friday2020-12-28 ~ 2021-01-0353Every week of the chosen ISO year, starting on Monday; the first and last week may cross the year boundary.
W01 2025-12-29 ~ 2026-01-04 W02 2026-01-05 ~ 2026-01-11 W03 2026-01-12 ~ 2026-01-18 W04 2026-01-19 ~ 2026-01-25 W05 2026-01-26 ~ 2026-02-01 W06 2026-02-02 ~ 2026-02-08 W07 2026-02-09 ~ 2026-02-15 W08 2026-02-16 ~ 2026-02-22 W09 2026-02-23 ~ 2026-03-01 W10 2026-03-02 ~ 2026-03-08 W11 2026-03-09 ~ 2026-03-15 W12 2026-03-16 ~ 2026-03-22 W13 2026-03-23 ~ 2026-03-29 W14 2026-03-30 ~ 2026-04-05 W15 2026-04-06 ~ 2026-04-12 W16 2026-04-13 ~ 2026-04-19 W17 2026-04-20 ~ 2026-04-26 W18 2026-04-27 ~ 2026-05-03 W19 2026-05-04 ~ 2026-05-10 W20 2026-05-11 ~ 2026-05-17 W21 2026-05-18 ~ 2026-05-24 W22 2026-05-25 ~ 2026-05-31 W23 2026-06-01 ~ 2026-06-07 W24 2026-06-08 ~ 2026-06-14 W25 2026-06-15 ~ 2026-06-21 W26 2026-06-22 ~ 2026-06-28 W27 2026-06-29 ~ 2026-07-05 W28 2026-07-06 ~ 2026-07-12 W29 2026-07-13 ~ 2026-07-19 W30 2026-07-20 ~ 2026-07-26 W31 2026-07-27 ~ 2026-08-02 W32 2026-08-03 ~ 2026-08-09 W33 2026-08-10 ~ 2026-08-16 W34 2026-08-17 ~ 2026-08-23 W35 2026-08-24 ~ 2026-08-30 W36 2026-08-31 ~ 2026-09-06 W37 2026-09-07 ~ 2026-09-13 W38 2026-09-14 ~ 2026-09-20 W39 2026-09-21 ~ 2026-09-27 W40 2026-09-28 ~ 2026-10-04 W41 2026-10-05 ~ 2026-10-11 W42 2026-10-12 ~ 2026-10-18 W43 2026-10-19 ~ 2026-10-25 W44 2026-10-26 ~ 2026-11-01 W45 2026-11-02 ~ 2026-11-08 W46 2026-11-09 ~ 2026-11-15 W47 2026-11-16 ~ 2026-11-22 W48 2026-11-23 ~ 2026-11-29 W49 2026-11-30 ~ 2026-12-06 W50 2026-12-07 ~ 2026-12-13 W51 2026-12-14 ~ 2026-12-20 W52 2026-12-21 ~ 2026-12-27 W53 2026-12-28 ~ 2027-01-03
What this tool does
- Translate between ISO 8601 week dates such as 2026-W41-3 and ordinary calendar dates when talking to planning or reporting systems.
- Confirm which days a sprint or meeting week covers: the result gives the Monday and the Sunday, and cross-year weeks stay correct.
- Check the ISO week year and week number of any date for weekly reports and statistics — 2021-01-01 correctly lands on 2020-W53-5.
- Export the whole year: every week with its start and end date, ready to copy into a spreadsheet for weekly planning.
Example
Input
2026-W41-3
Output
Direction ISO week date → calendar date; ISO week date 2026-W41-3; calendar date 2026-10-07; ISO week year 2026; week number 41; day of week 3 (Wednesday); week range 2026-10-05 ~ 2026-10-11; weeks in that year 53
Feeding in 2021-01-01 instead returns 2020-W53-5 — the ISO week year differs from the calendar year, which is exactly where cross-year weeks go wrong.
Frequently asked questions
Why does the year in an ISO week date differ from the date itself?
ISO 8601 defines week 1 as the week containing the first Thursday of the year, and the whole week (Monday to Sunday) belongs to that year. In the week from 2020-12-28 to 2021-01-03 the Thursday is 2020-12-31, so the week belongs to 2020 and 2021-01-01 is written 2020-W53-5.
Which input formats are accepted?
Dates may be written 2026-10-07, 2026/10/07, 07.10.2026 or 10/07/2026; week dates may be 2026-W41-3, 2026W413 or 2026-W41, where an omitted day of week defaults to Monday (1). The tool detects which of the two you entered.
How many ISO weeks can a year have?
52 or 53. A year has 53 weeks when 1 January is a Thursday, or when it is a Wednesday in a leap year; 2020 and 2026 both have 53 while 2021 has 52. A week number outside that range is reported as an error rather than silently clamped.
How is the day of the week numbered?
Monday is 1 through Sunday is 7, matching ISO 8601. The result shows both the number and the localised weekday name so you can double-check it.
Does the conversion depend on my time zone?
No. Everything is computed from integer day numbers rather than Date objects, so the same input gives the same answer in every time zone and season, and a daylight saving change cannot shift a week.
Keywords：iso weekiso 8601week dateweek numbercalendar weekiso 周周期计算周历周数转换跨年周iso week dateweek date converter