Convert any date to its ISO week number and ISO year — and convert an ISO week back into its Monday-to-Sunday dates — using the full ISO-8601 rules. Runs entirely in your browser — your numbers never leave this device.
ISO-8601 weeks start on Monday, and week 1 is the week containing the year's first Thursday (equivalently, the week containing 4 January). Dates near New Year can belong to the neighbouring ISO year — 1 January 2021, for instance, is in ISO week 2020-W53.
This tool converts in both directions. Give it a date and it returns the ISO-8601 week date — ISO year, week number and weekday (Monday = 1 to Sunday = 7). Give it an ISO year and week number and it returns the Monday and Sunday that bound that week, with every day in between listed. The ISO system is the one used in European business, logistics and most project planning, and its rules are stricter than 'count the Mondays': week 1 is defined as the week containing the year's first Thursday, which is the same as the week containing 4 January.
The consequence that surprises everyone is the year boundary. Because the ISO year is defined by the Thursday of each week, dates near New Year can belong to the neighbouring ISO year: 1 January 2021 is in ISO week 2020-W53, and 30 December 2019 is already 2020-W01. ISO years also vary in length — most have 52 weeks, but a year has 53 when it starts on a Thursday (or on a Wednesday in a leap year), as 2026 does. The converter validates against the real length of the ISO year you ask about, so requesting week 53 of a 52-week year gets an explanation rather than a wrong date.
Note that this is not the US week convention (Sunday start, week 1 = the week containing 1 January), which some spreadsheet WEEKNUM settings use. If a week number from an American source disagrees with this tool by one near New Year, that difference in definitions is almost always why.
It is the week numbering of ISO-8601, the international date standard: weeks run Monday to Sunday, and week 1 of an ISO year is the week containing that year's first Thursday. A full ISO week date looks like 2026-W44-7 — year 2026, week 44, day 7 (Sunday). Business Europe, airlines and logistics plan in these weeks.
Because the ISO year of a date is the calendar year of the Thursday of its week. The week containing 1 January 2021 has its Thursday on 31 December 2020, so the whole week — including the January days — belongs to ISO 2020, as its week 53. The same rule pulls 30 December 2019 forward into 2020-W01.
An ISO year has 53 weeks when the corresponding calendar year starts on a Thursday, or starts on a Wednesday in a leap year — roughly one year in five or six. 2026 qualifies (it starts on a Thursday), so 2026-W53 exists; 2025 starts on a Wednesday and is not a leap year, so it has only 52.
Probably a different convention. Excel's WEEKNUM default uses the US system (weeks start Sunday; week 1 contains 1 January), while its ISO.WEEKNUM function matches this tool. Near New Year the two can differ by one week, and sometimes by a whole year attribution — check which system your source used before treating either as wrong.
Monday 2 November to Sunday 8 November 2026. Use the week-to-dates mode for any other week: enter the ISO year and week and the full seven-day table is listed.
No. Conversions run in your browser; dates and week numbers are never uploaded or tracked.
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