Find the ISO 8601 week number for any date. Enter a year, month, and day to instantly see which week of the year it falls in.
The Week Number Calculator determines the ISO 8601 week number for any given date. ISO 8601 defines week numbering as starting on Monday, with week 1 being the week that contains the year's first Thursday (equivalently, the week containing January 4).
Week numbers are widely used in business planning, project management, manufacturing scheduling, and European business communications. "Delivery in week 15" or "report due CW 22" are common references that require knowing the exact week number for a given date.
This calculator implements the full ISO 8601 week-date algorithm, handling the edge cases where dates near year boundaries may belong to week 52 or 53 of the previous year, or week 1 of the following year. For example, December 31 can sometimes fall in week 1 of the next year, and January 1 can fall in week 52 or 53 of the previous year.
Understanding this metric in precise terms allows professionals to set achievable targets, measure progress objectively, and continuously refine their approach to time and task management.
Week numbers are the standard way to reference time periods in European business, manufacturing, and project management. This calculator gives you the exact ISO 8601 week number for any date, handling the tricky edge cases near year boundaries correctly. Consistent measurement creates a reliable baseline for evaluating personal efficiency and identifying the habits and practices that contribute most to achieving professional goals.
ISO 8601 Week Number algorithm: 1. Find the ordinal day of the year (1–366) 2. Find the day of the week (Mon=1 through Sun=7) 3. Week = floor((ordinal + 10 − weekday) / 7) 4. Adjust for year boundaries: if week = 0, it belongs to the last week of the previous year; if week = 53 and certain conditions, it belongs to week 1 of the next year.
Result: Week 6 of 2026
February 8, 2026 is a Sunday. The ISO 8601 week containing this date is week 6, starting Monday February 2 and ending Sunday February 8. Since the date is in February, the ISO year matches the calendar year.
The ISO 8601 week numbering system provides an unambiguous way to reference weeks within a year. It is based on three rules: weeks start on Monday, the first week contains the first Thursday of January, and years have either 52 or 53 weeks.
The most confusing aspect of ISO week numbering is the year boundary. December 29–31 may belong to week 1 of the following year, and January 1–3 may belong to week 52 or 53 of the previous year. This is why the "ISO week year" concept exists—it may differ from the calendar year.
ISO 8601 week numbering is the standard in most European countries, widely used in international business, and supported by most programming languages and spreadsheet applications. In the US, a different convention (where weeks start on Sunday and week 1 contains January 1) is sometimes used but is less standardized.
ISO 8601 defines a standard for week numbering where weeks start on Monday, the first week of the year contains the first Thursday of January, and week numbers range from 1 to 52 or 53. This system ensures consistency in international business communication.
Yes. If January 1 falls on a Friday, Saturday, or Sunday, it belongs to the last week (52 or 53) of the previous year under ISO 8601 rules. For example, January 1, 2027 falls on a Friday, so it's in week 53 of 2026.
Most years have 52 weeks. Years where January 1 is a Thursday (or Thursday/Friday in a leap year) have 53 weeks. These "long years" occur roughly every 5–7 years.
The ISO standard reflects the convention in most of the world where Monday is the first day of the work week. This differs from the US convention where Sunday is often considered the first day of the week, but ISO 8601 has been widely adopted for consistency.
Week numbers provide a compact way to reference time periods. Instead of saying "the week of February 2–8," you can simply say "week 6." This is common in manufacturing schedules, project timelines, delivery planning, and European business communications.
The ISO week year can differ from the calendar year for dates near the year boundary. Days in early January might have an ISO week year of the previous year, and days in late December might have an ISO week year of the next year. The calculator shows both.