ISO 8601 Week Calculator
Convert any date to its ISO week number (and back) — get the Monday-start / Sunday-end of that week, ISO year, day-of-year, and how many weeks the year has.
- ISO week42
- ISO year2026
- Week start2026-10-12 (Mon)
- Week end2026-10-18 (Sun)
- ISO weekday7 (Sun)
- Day of year291
- Weeks in ISO year53
Weeks start on Monday. Week 1 is the week that contains January 4th.
How to use
- Date → Week: pick a date to see its ISO year, week, weekday, and the full week range.
- Week → Date: enter an ISO year + week number to see the seven dates that fall in that week.
- Check the weeks-in-year figure before you plan against it: an ISO year has 52 weeks, except when it has 53.
About this tool
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ISO 8601 weeks start on Monday and the year that owns each week is the one that owns its Thursday — so the first week of the year is the one containing 4 January. This tool implements that exactly. Useful for fiscal calendars, project scheduling, or any pipeline that reports by ISO week (most European business systems, etc.). The reverse direction lets you pick a year + week to see exactly which seven dates it spans.
The Thursday rule is what makes the week number stable, and it has a consequence people meet once a year: the ISO week-year is not always the calendar year. 1 January 2027 falls in week 53 of 2026, and 31 December 2029 falls in week 1 of 2030. Reporting a week as 2026-W53 rather than W53 is not pedantry; dropping the year is how a report ends up attributing a week to the wrong side of a boundary.
Most years have 52 weeks and some have 53, which is the other thing that breaks naive arithmetic. A year is a 53-week year when it starts on a Thursday, or when it is a leap year starting on a Wednesday. Year-on-year comparisons by week number therefore compare unequal spans roughly every fifth or sixth year, and a dashboard that divides an annual figure by 52 is quietly wrong in those years.
ISO weeks are not the only week numbering in use. North American calendars commonly start the week on Sunday and count week 1 as the one containing 1 January, which produces different numbers for the same dates, and spreadsheet functions default to that convention unless told otherwise. A number copied between the two systems without conversion is off by one more often than not.
The seven dates shown for a given week are the ISO ones, Monday through Sunday, regardless of which day your locale considers the start.
Frequently asked questions
Why does Jan 1 sometimes belong to the previous year?
An ISO week belongs to the year that holds at least four of its seven days — the same as the year its Thursday falls in. If the year starts on a Friday/Saturday/Sunday, Jan 1 sits inside the last week of the previous ISO year — that's why the ISO year and the calendar year can differ for early-January and late-December dates.
When does a year have 53 ISO weeks?
When the calendar year starts on a Thursday, or is a leap year that starts on a Wednesday. Otherwise it has 52 weeks.
Why does my spreadsheet report a different week number?
Because most spreadsheets default to a non-ISO scheme in which week 1 is simply the week containing 1 January, and the week may start on Sunday. That can sit one number away from the ISO week for much of the year. Use the ISO-specific function, or set the mode explicitly, whenever the number will be compared against another system.
Which year does a week at the boundary belong to?
The year that owns its Thursday, which is why the ISO year and the calendar year disagree for a few days each January and December — 1 January can fall in the last week of the previous ISO year. Always write the pair together, as 2027-W01 rather than "week 1", because a bare week number is ambiguous in exactly the place where it matters.
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