Week Number Calculator
The ISO week number for any date.
Input sheet
DIY at your own risk. Calcora's calculators and guides are general estimates and information only — not professional, engineering, legal, or safety advice. Always verify local building codes and permit requirements, and hire a licensed pro for electrical, gas, plumbing, structural, or any work you're not fully comfortable doing yourself.
Many businesses, manufacturers, and project schedules track progress by week number rather than by date, and the international standard for that is ISO 8601. This calculator returns the ISO week number for any date so a label like "Week 23" always points to the same stretch of the calendar.
How it works
ISO 8601 weeks start on Monday, and week 1 is the week containing the first Thursday of the year.
Under ISO 8601, weeks always begin on Monday, and week 1 is defined as the week that contains the year's first Thursday, which is equivalent to the week containing January 4. This rule means a date in early January can belong to the last week of the previous ISO year, and a late-December date can belong to week 1 of the next year.
The calculator also reports the ISO year, which may differ from the calendar year for those boundary dates, along with the day of the week and the day-of-year number.
ISO week = 1 + the number of whole weeks between the Thursday of the target week and the Thursday of week 1 (the week containing January 4).
Worked examples
June 3, 2026 (a Wednesday). → Week 23 of ISO year 2026
June 3 falls in the twenty-third Monday-to-Sunday week of 2026 under the ISO rule.
January 1, 2027 (a Friday). → Week 53 of ISO year 2026
Because the week containing January 1, 2027 has its first Thursday back in 2026, ISO assigns it to the prior ISO year.
Tips & gotchas
- ISO week numbering is not the same as simply dividing the day-of-year by 7; the Monday-start and first-Thursday rules shift the boundaries.
- Near New Year, check the ISO year as well as the week number, since the two can disagree with the calendar year.
- Most of Europe and many supply-chain and payroll systems use ISO weeks; the United States more often uses a Sunday-start week 1 scheme, which can differ.
- A year has either 52 or 53 ISO weeks; the long 53-week years occur roughly every five to six years.
FAQ
Why is January 1 sometimes shown as week 52 or 53?
ISO weeks belong to whichever year contains their Thursday. If the week holding January 1 has its Thursday in the previous year, the date is counted as the last week of that earlier ISO year.
What day does an ISO week start on?
Monday. Week 1 is the first week with a majority in January, defined as the week containing the year's first Thursday.
Can a year have 53 weeks?
Yes. Years whose first-Thursday rule pushes an extra week onto the calendar have 53 ISO weeks; most years have 52.
Related calculators
Add or Subtract Days Calculator · Business Days Calculator · Time Duration Calculator · Time Zone Offset Calculator · Countdown to Date Calculator