Week Number Calculator

The week number shown here follows ISO 8601, the international standard used by most European businesses, logistics companies and manufacturing plans. Under ISO rules a week starts on Monday, and week 1 of a year is the week containing the first Thursday of January (equivalently, the week containing 4 January). This means the first few days of January can belong to week 52 or 53 of the previous year, and 29–31 December can belong to week 1 of the next. A year has 52 weeks, or 53 when it starts on a Thursday (or on a Wednesday in a leap year).

The page shows today's ISO week, the week for any date you enter, and a full-year calendar with week numbers down the side. The copy button gives you the week in the form 2026-W36 used in ISO date strings.

North America commonly uses a different convention: weeks start on Sunday and week 1 is the week containing 1 January. Under that system a date can be up to a week apart from its ISO number. Microsoft Outlook, Google Calendar and phone calendars let you choose which rule to use, and the default depends on the regional settings, which is why two colleagues can disagree about "week 36". When a document or a shipment references a week number, check which system it means; ISO is the safe assumption for anything European or international.

• Manufacturing and supply chains. Production schedules, delivery slots and quality batches are commonly planned by week ("shipping in week 38"). Car parts, electronics and many food products carry a week code.
• Retail and finance. Weekly sales comparisons and 52/53-week fiscal years.
• Project planning. Gantt charts and sprint calendars in week units; the sprint timer counts a two-week cycle.
• Pregnancy. Counted in weeks from the last period, which is a different calculation; see the pregnancy calculator.
• Scandinavia, Germany, the Netherlands. Everyday use: people book holidays and dentist appointments by week number.

For the number of days or weeks between two dates, the date calculator; for today's full date and day of the year, the current time page.