How ISO week numbers work
ISO 8601 is the international standard, and it is the week number most of Europe, logistics and software use. Its rules are short:
- Weeks run Monday to Sunday.
- Week 1 is the week containing the year’s first Thursday, which is also the week containing January 4.
- A year therefore has 52 or 53 full weeks, and no week is ever split across two week-years.
The Thursday rule means a week belongs to whichever year holds most of its days (4 or more of the 7). The cost is that the edges of the year can disagree with the calendar. In 2026, week 1 began on Monday, December 29, 2025, and week 53 ends on Sunday, January 3, 2027.
Why some years have 53 weeks
52 weeks are 364 days, one or two short of a year. The spare days add up, and roughly every five or six years the calendar gains a 53rd ISO week. That happens when the year starts on a Thursday, or when a leap year starts on a Wednesday. 2026 has 53 weeks and 2027 has 52. Payroll and budgeting run on weeks: a business paying weekly makes 53 payments in some years, and a fixed annual budget split into 52 weekly amounts falls one week short in those years.
The US system, and why spreadsheets disagree
Many US calendars, and Excel’s default WEEKNUM(), count weeks from Sunday and call the week containing January 1 “week 1”, even if it is only a single day. That system can have 53 or even 54 numbered weeks, and from early January it often runs one ahead of ISO. The lookup above shows both. In a spreadsheet, use ISOWEEKNUM(date)in Excel or Google Sheets for the ISO number. In code, Python’s date.isocalendar() and the %V format specifier both return ISO weeks.
To count the days or working days between two weeks, use the days between dates calculator or the business days calculator. To jump a set number of weeks forward or back, use the date calculator.