Week Number
Calculator
Find the ISO week number for any date
What week is…
How it works
ISO week numbers are a standard way to label weeks in the year, defined by ISO 8601. Instead of counting from 1 January, each week is numbered consistently so that week boundaries always fall on Monday through Sunday. Businesses, logistics companies, and project managers rely on ISO weeks for reporting, production schedules, and cross-border coordination because everyone agrees on which week a date belongs to.
Week 1 of a year is the week that contains the first Thursday of that year — equivalently, the week that contains 4 January. This means the first few days of January may belong to week 52 or 53 of the previous year, and the last few days of December may already belong to week 1 of the next. Clockr uses this ISO rule so your result matches what you would see in Excel, SAP, and most European business systems.
Most years have exactly 52 ISO weeks, but roughly one in five or six years has 53. This happens when 1 January falls on a Thursday, or on a Wednesday in a leap year — or when 31 December falls on a Thursday (or Wednesday in a leap year). This calculator determines the correct week count for the ISO year your date belongs to, so weeks remaining in the year are always accurate.
Frequently asked questions
What is an ISO week number?
An ISO week number is a label from 1 to 52 (or occasionally 53) assigned to each Monday–Sunday week under the ISO 8601 standard. It provides a consistent, internationally recognised way to refer to a specific week regardless of which month it spans. Many European countries and global industries use ISO weeks as their default week numbering system.
What week of the year is it today?
Click the "This week" button in the quick-reference section above, or simply load this page — the date field defaults to today and shows the current ISO week number instantly. The result includes the full Monday–Sunday range for this week and how many weeks are left in the ISO year.
How many weeks are there in a year?
In the ISO system, most years contain 52 weeks. However, some years — roughly one in five or six — contain 53 weeks because of how week 1 is defined relative to the calendar. A standard calendar year has 52 weeks plus one or two extra days, and those extra days occasionally create an additional full ISO week.
Why do some years have 53 weeks?
A year gains a 53rd ISO week when 1 January falls on a Thursday, or on a Wednesday in a leap year. The same applies when 31 December falls on a Thursday (or Wednesday in a leap year). In those cases an extra full Monday–Sunday week fits into the ISO year without being absorbed into week 1 of the following year.
Does the week start on Sunday or Monday?
Under ISO 8601, the week starts on Monday and ends on Sunday. This is the opposite of the convention used in the United States, where weeks are often considered to run Sunday through Saturday. This calculator follows the ISO Monday-start rule, which is the standard for business and manufacturing worldwide.
How is week 1 of the year determined?
ISO week 1 is the week that contains the first Thursday of the calendar year. A simpler equivalent rule: week 1 is the week containing 4 January. This means 1–3 January might belong to the last week of the previous ISO year, while 29–31 December might already be in week 1 of the next ISO year.
Why are week numbers used in business and manufacturing?
Week numbers simplify planning when work spans month boundaries. Production schedules, sales reports, payroll periods, and project milestones are often organised by week rather than by calendar month. ISO week numbers are unambiguous across countries and software systems, making them the preferred choice for supply chains, ERP systems, and international teams.