Week Number Calculator

The ISO 8601 rules this calculator uses

• A week starts on Monday and ends on Sunday.
• Week 1 is the week containing the first Thursday of the year. Since January 4 is always in that week, "the week containing January 4" is the same rule.
• A year has 52 or 53 full weeks; there are no partial weeks.
• The week-based year can differ from the calendar year for a few days at each end. Python's documentation gives the classic case: 2004 began on a Thursday, so ISO week 1 of 2004 ran from Monday, December 29, 2003 to Sunday, January 4, 2004.

Alongside the week number the calculator shows the ISO week-year code, which matters around New Year: January 1, 2027 is a Friday and returns week 53 with the code 2026-W53, because it belongs to week 53 of 2026. The full ISO notation makes that explicit as 2026-W53-5 (year, week, weekday with Monday = 1). Internet date formats built on ISO 8601, such as the profile in RFC 3339, define week numbers as two digits from 01 to 52, or 53 depending on the year.

ISO vs US week numbers

Excel's WEEKNUM function documents both systems. With the default return type 1, weeks begin on Sunday and the week containing January 1 is week 1 (System 1). Return type 21, or the separate ISOWEEKNUM function, uses ISO 8601 (System 2). Google Sheets offers the same pair of functions.

How far apart the two systems are depends on the weekday of January 1. In 2026, January 1 falls on a Thursday, so ISO and US numbers agree from Monday to Saturday; only Sundays differ, because the US week has already rolled over:

In 2027, January 1 is a Friday. ISO week 1 does not start until Monday, January 4, while the US system has already counted a week 1, so the US number is one higher for most of the year:

When a partner quotes a week number, confirm which system they mean. Anyone in Europe, or any ERP or logistics system, almost certainly means ISO.

Which years have 53 ISO weeks?

A year has 53 ISO weeks when January 1 falls on a Thursday, or when it is a leap year that starts on a Wednesday. Both conditions mean the year contains 53 Thursdays. Recent and upcoming 53-week years:

The gap is usually five or six years. It matters for anything planned by week number: annual reports by week, weekly payroll counts, retail comparisons with the prior year and recurring "every week 10" schedules.

From a week number to dates

To find the Monday of ISO week w in year y:

• Find January 4 of year y and step back to the Monday of its week. That Monday starts week 1.
• Add 7 × (w − 1) days.

For week 27 of 2027: January 4, 2027 is itself a Monday, and 7 × 26 = 182 days later is Monday, July 5, 2027; the week ends on Sunday, July 11. For 2026, week 1 starts on December 29, 2025, so week 40 starts 273 days later, on Monday, September 28, 2026.

Week numbers vs payroll and fiscal weeks

A week number is a calendar label, not a pay period. Under the FLSA, an employer's workweek can begin on any day and at any hour, as long as it is fixed, so a Wednesday-to-Tuesday payroll week will straddle two ISO weeks. Company fiscal calendars that close on a set weekday can also number weeks from a fiscal year start. When a date is filed by week, note which of these schemes the number belongs to.

Week numbers in software

• Excel / Google Sheets: =ISOWEEKNUM(A1) for ISO; =WEEKNUM(A1) for the US Sunday-start count; =WEEKNUM(A1,21) also gives ISO.
• Python: date.isocalendar() returns the ISO year, week and weekday together, which avoids the year-boundary trap.
• strftime-style formats: %V is the ISO week (pair it with %G, the ISO year); %U counts Sunday-start weeks with days before the first Sunday in week 0, and %W does the same with Monday as the first day. %U and %W are therefore not the same as Excel's WEEKNUM either.

A common bug is pairing the ISO week with the calendar year, which labels December 29, 2025 as "2025-W01" instead of 2026-W01. Always take the year from the same ISO calculation as the week.