Week Number Calculator

Find the week number for any date.

What Is a Week Number Calculator?

A week number calculator takes any calendar date and determines which numbered week of the year it falls in, according to the ISO 8601 international standard. ISO week numbering provides a consistent, unambiguous way to refer to specific weeks throughout the year. Instead of saying "the last week of March," you can say "Week 13," and anyone familiar with the standard will know the exact seven-day period you mean.

Week numbers are used extensively in business, manufacturing, project management, and logistics. Sprint planning in agile development, production schedules in manufacturing, shipping calendars in logistics, and fiscal reporting in accounting all benefit from the precision of numbered weeks. This calculator provides the ISO week number along with supplementary information including day of year, quarter, and a visual calendar of the full week.

How ISO Week Numbers Work

The ISO 8601 standard defines the week numbering system with three key rules.

Weeks start on Monday. Every ISO week runs from Monday through Sunday, regardless of the local convention for the first day of the week.

Week 1 contains the first Thursday of the year. This rule ensures that Week 1 always has at least four days in the new year. It also means that January 1, 2, or 3 might belong to the last week (52 or 53) of the previous ISO year if those days fall on a Friday, Saturday, or Sunday.

Most years have 52 weeks; some have 53. A 53rd week occurs when January 1 is a Thursday or when a leap year's January 1 is a Wednesday. In every 400-year cycle there are exactly 71 years with 53 ISO weeks.

The calculator implements these rules by finding the nearest Thursday to the given date, determining what year that Thursday belongs to, and computing the week number by counting how many weeks have elapsed since the start of that ISO year.

How to Use This Calculator

Select a date using the date picker input. The default is today's date.
Review the main result. The large week number appears at the top along with the full date, day name, and ISO year. Note that the ISO year may differ from the calendar year for dates in early January or late December.
Check the detail grid. Four panels show the day of year, days remaining in the year, the quarter (Q1 through Q4), and the total number of days in the year.
View the week calendar. A seven-day grid shows each day of the ISO week from Monday to Sunday, with the selected date highlighted. The full date range of the week is displayed below the grid.
Observe the year progress bar. The visual bar and percentage show how far through the calendar year the selected date falls.
Use the quick date buttons to load today's date, January 1, December 31, or July 4 of the current year for common reference points.

Worked Examples

Example 1: A Standard Mid-Year Date

July 15, 2026 falls on a Wednesday. Day of year is 196, which is approximately 53.7% through the year. ISO Week 29. The week runs from Monday July 13 to Sunday July 19. Quarter: Q3.

Example 2: New Year's Day

January 1, 2025 falls on a Wednesday. Since Wednesday is before Thursday, January 1 belongs to ISO Week 1 of 2025. The ISO year matches the calendar year. Day of year is 1, with 364 remaining days.

Example 3: Year-End Date Crossing

December 31, 2025 falls on a Wednesday. This date could belong to Week 1 of 2026 if the first Thursday of 2026 falls in the same Mon-Sun period. In this case, December 31 is ISO Week 1 of 2026 because January 1, 2026 is a Thursday, which anchors Week 1.

Example 4: Leap Year Date

February 29, 2028 falls on a Tuesday. Day of year is 60 out of 366 (since 2028 is a leap year). ISO Week 9. The year progress is 16.4%. Quarter: Q1.

Common Use Cases

Agile development: Plan and reference two-week sprints by ISO week numbers, ensuring all team members share the same understanding of when each sprint starts and ends.
Manufacturing schedules: Coordinate production runs, maintenance windows, and shipping timelines using week numbers that are consistent across international facilities.
Financial reporting: Identify fiscal weeks for revenue tracking, payroll processing, and compliance reporting, especially in organizations that operate on weekly cycles.
Academic planning: Align course schedules, assignment deadlines, and exam periods with specific week numbers in the academic calendar.
Logistics and supply chain: Schedule deliveries, inventory checks, and warehouse operations using numbered weeks for precise coordination.
Personal planning: Track how far through the year you are, set weekly goals, and plan vacations or events relative to numbered weeks.

Tips and Common Mistakes

Check the ISO year, not just the week number. For dates near the boundary between years, the ISO year may differ from the calendar year. December 31 can be in Week 1 of the following year, and January 1 can be in Week 52 or 53 of the previous year. Always verify the ISO year shown in the result.

Remember that ISO weeks start on Monday. If you are accustomed to weeks starting on Sunday (common in the United States), the ISO week boundary may differ from your expectation. A Sunday in your mental model may belong to the previous ISO week.

Do not assume every year has the same number of weeks. Most years have 52 ISO weeks, but some have 53. Plans and schedules that assume exactly 52 weeks per year will eventually misalign with the calendar.

Use week numbers for precision in team communication. Saying "due in Week 15" is more precise than "due mid-April" because everyone can look up the exact dates of Week 15. This reduces ambiguity in deadline setting and project tracking.

Verify week numbers when working across regional standards. While ISO 8601 is the international standard, some systems and locales use different week numbering conventions. Confirm that all parties are using ISO week numbers before relying on them for scheduling.

Frequently Asked Questions

What is an ISO week number?

An ISO week number is defined by the ISO 8601 international standard. Weeks start on Monday and end on Sunday. Week 1 of a year is the week that contains the first Thursday of that year. This means January 1 sometimes falls in the last week of the previous ISO year. Each ISO year has either 52 or 53 weeks depending on how the calendar days align.

Why does January 1 sometimes show as week 52 or 53?

Under ISO 8601, if January 1 falls on a Friday, Saturday, or Sunday, it belongs to the last week of the previous year. For example, January 1, 2027 is a Friday, so it falls in ISO week 53 of 2026. The ISO year displayed in the results may differ from the calendar year for dates near the year boundary.

How many weeks are in a year?

Most years have 52 ISO weeks. Occasionally a year has 53 weeks, which happens when January 1 is a Thursday or when January 1 is a Wednesday and the year is a leap year. Years with 53 ISO weeks occur approximately every 5 to 6 years. The last 53-week year was 2020, and the next is 2026.

What is the day of year shown in the results?

The day of year is a sequential count from January 1 (day 1) through December 31 (day 365 or 366 in a leap year). It provides a simple linear measure of how far into the year a date falls. February 15 is day 46, July 4 is day 185, and October 31 is day 304. This number is useful for scheduling and project planning.

How is the week range calculated?

The week range shows Monday through Sunday of the ISO week containing the selected date. The calculator finds the Monday by subtracting enough days to reach the start of the week, then adds six days to find Sunday. The selected date is highlighted within the range so you can see its position in the week.

What does the year progress bar represent?

The year progress bar shows what percentage of the calendar year has elapsed as of the selected date. It divides the day of year by the total number of days in the year and displays the result as a visual bar and a percentage. On July 1 of a non-leap year, approximately 49.6 percent of the year has passed.

Can I use week numbers for project scheduling?

Yes. ISO week numbers are widely used in business and project management to plan sprints, track deliverables, and set milestones. Referring to 'Week 12 deliverable' is unambiguous across the team, unlike month-based references that span different day ranges. Many project management tools and calendars display ISO week numbers for this reason.

Do all countries use the same week numbering system?

Most countries that follow international standards use the ISO 8601 system where weeks start on Monday. However, in some regions including the United States, weeks traditionally start on Sunday, which can shift the week number by one in certain cases. This calculator uses the ISO standard for consistency and international compatibility.