How does the calculator decide the ISO week-year?

It follows ISO 8601 exactly. The calculator resolves the ISO weekday, shifts to the Thursday in that week, and uses that Thursday to determine the ISO week-year.

Why can Jan 1 belong to the previous ISO week-year?

Because ISO week 1 is the week containing Jan 4, also described as the week containing the first Thursday. If Jan 1 lands before that anchor week, it stays in the previous ISO week-year.

Why can Dec 31 belong to the next ISO week-year?

For the same reason in reverse. If the Monday-to-Sunday week containing Dec 31 is also the week containing next year’s Jan 4 or first Thursday, that whole week becomes ISO week 1 of the next ISO week-year.

Does this follow local workweek, payroll, or holiday rules?

No. This calculator uses ISO week rules only: Monday-based weeks, ISO weekday numbering 1-7, and the Jan 4 / first-Thursday definition of week 1.

How is this different from Day of Week, Date Add/Subtract, and Weekdays Calculator?

This page answers one job: which ISO week does one exact date belong to? Day of Week labels a date, Date Add/Subtract transforms a date, and Weekdays Calculator counts days across a range.

ISO Week Number Calculator

Enter one exact date and get its ISO week label, ISO week-year, and Monday-to-Sunday bounds for timesheets, sprint plans, and delivery schedules.

How It Works

Formula

\text{isoWeekday} = ((\text{weekday}_{0-6} + 6) \bmod 7) + 1

\text{isoWeekYear} = \text{year}(\text{date} + (4 - \text{isoWeekday})\text{ days})

\text{isoWeekNumber} = 1 + \left\lfloor \dfrac{\text{weekStart} - \text{week1Start}}{7\text{ days}} \right\rfloor

Enter one exact date and the calculator applies ISO 8601 week rules only. The hero result shows the ISO week label, while the supporting rows show the ISO week-year and the exact Monday-to-Sunday bounds so you can use the answer immediately in planning or reporting.

ISO week lookup:

Parse the entered value strictly as one ISO calendar date.
Convert the Gregorian weekday into ISO weekday numbering, where Monday = 1 and Sunday = 7.
Shift that date to the Thursday of the same ISO week. The Gregorian year of that Thursday is the ISO week-year.
Find the Monday that starts ISO week 1 for that ISO week-year. ISO week 1 is the Monday-to-Sunday week containing Jan 4 / the first Thursday.
Count whole weeks from that ISO week-1 anchor to the entered date’s Monday to get the ISO week number.
Return the final ISO week label plus the exact Monday and Sunday dates for the containing ISO week.

Frequently Asked Questions

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ISO Week Number Calculator

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