Week Number Calculator
Find the ISO week number for any date — see what week of the year it is right now.
Week numbers provide a simple, universal way to refer to a specific seven-day period within a year. They are defined by the ISO 8601 international standard, which is used across industries including manufacturing, logistics, payroll, project management, and academic scheduling. Under ISO 8601, weeks always begin on Monday and end on Sunday, and week 1 is defined as the first week of the year that contains a Thursday. This means that some years have 53 weeks instead of the usual 52, and that the first few days of January may technically belong to the last week of the previous year.
Our week number calculator instantly shows you the current ISO week number and lets you look up the week number for any date in history or the future. You can also view a full year table showing every week with its start and end dates, making it easy to plan schedules, align reporting periods, or verify payroll cycles. Whether you are coordinating international shipments, scheduling sprints in agile development, or simply need to know which week it is for a form, this tool gives you the answer in seconds. All calculations run entirely in your browser with nothing sent to any server.
How to Find the Week Number for Any Date
Knowing the week number of a given date is essential in many professional and personal contexts. International businesses coordinate schedules by week number, payroll departments process salaries on a weekly cycle, and project managers track progress using sprint numbers that align with ISO weeks. While calendars often display week numbers, looking up the week for a specific date in the past or future usually requires a dedicated tool.
This week number calculator handles all of that instantly. Enter any date and get the ISO week number, or browse a full-year table to see every week at a glance. The tool correctly handles edge cases such as years with 53 weeks and dates at the boundary between December and January where the ISO week may belong to the adjacent year.
How to Use This Tool
- Check today's week — the current ISO week number is displayed automatically at the top of the page as soon as you load it, along with the date range for the current week.
- Look up any date — select a date using the date picker and click "Find Week Number" to see which ISO week that date falls in, along with the week's start and end dates and the day of the year.
- Browse a full year — choose a year from the dropdown to display a table of all 52 or 53 weeks in that year, with Monday start dates and Sunday end dates. The current week is highlighted if you are viewing the current year.
- Copy the result — click "Copy Result" to copy the week number and date details to your clipboard for use in emails, reports, or spreadsheets.
How It Works Behind the Scenes
The calculator implements the ISO 8601 week numbering algorithm. For any given date, it first determines the day of the week (where Monday is 1 and Sunday is 7). It then adjusts the date forward to the nearest Thursday, because the ISO standard defines the week by which year its Thursday falls in. From this adjusted Thursday, it calculates the number of days elapsed since January 1 of that year, divides by 7, and rounds up to produce the week number.
This approach correctly handles the boundary cases at the start and end of each year. For example, if January 1 falls on a Friday, Saturday, or Sunday, it belongs to the last week of the previous year (week 52 or 53). Conversely, if December 31 falls on a Monday, Tuesday, or Wednesday, it belongs to week 1 of the following year. The year table is generated by iterating from the Monday of week 1 through every subsequent Monday, computing the week number for each, and stopping when the week number resets or exceeds the year boundary.
Frequently Asked Questions
What week number is it today?
The current ISO week number is shown at the top of this page and updates automatically each day. ISO week numbers follow the ISO 8601 standard, where weeks start on Monday and week 1 is the first week of the year containing a Thursday. You can also verify the week number for any specific date by entering it in the date picker above.
How is the ISO week number calculated?
The ISO 8601 standard defines week 1 as the week that contains the first Thursday of January. All weeks start on Monday and end on Sunday. The algorithm works by shifting any given date to its nearest Thursday, then counting how many days have elapsed since January 1 of that Thursday's year, and dividing by 7. This ensures a consistent and unambiguous week numbering system used internationally in business, government, and technology.
Can a year have 53 weeks?
Yes. A year has 53 ISO weeks if January 1 falls on a Thursday, or if it is a leap year and January 1 falls on a Wednesday. This happens approximately every 5 to 6 years. For example, 2004, 2009, 2015, 2020, and 2026 all have 53 weeks. In a 53-week year, the last week (week 53) typically includes only the final few days of December and possibly the first days of the following January.
Do all countries start the week on Monday?
No. The ISO 8601 standard starts weeks on Monday, and this convention is followed throughout most of Europe, much of Asia, Australia, and South America. However, the United States, Canada, Japan, and several other countries traditionally start the week on Sunday. Some Middle Eastern countries start on Saturday. This calculator follows the ISO 8601 Monday-start convention, which is the standard used in international business and most software systems.
Why does my calendar show a different week number?
Different calendar applications and systems may use different week numbering conventions. The most common alternative is the US system, where week 1 always begins on January 1 regardless of what day it falls on. The ISO system used by this calculator defines week 1 as the week containing the first Thursday of the year, which sometimes means January 1 belongs to week 52 or 53 of the previous year. Check your calendar app's settings to see which system it uses.
What are week numbers used for in business?
Week numbers are used extensively in payroll processing, manufacturing schedules, supply chain logistics, sprint planning in agile software development, academic term planning, and financial reporting. They provide a compact, unambiguous way to reference a seven-day period without listing specific dates, which simplifies communication across teams, departments, and time zones. Many ERP and project management systems use ISO week numbers as their primary scheduling unit.
