Week Number Calculator — Find the ISO Week Number

How to Use the Week Number Calculator

1. Select a date: Use the date picker to choose any date you want to look up. It defaults to today's date. Navigate through months and years using the calendar controls or type the date directly. The calculator works for any valid Gregorian calendar date.
2. Read the ISO week number: The large number at the top shows the ISO 8601 week number for your selected date, ranging from 1 to 53. Below it, the ISO week year is displayed, which may differ from the calendar year for dates near January 1 or December 31.
3. Check week boundary dates: Two info cards show the Monday (week start) and Sunday (week end) for the week containing your selected date. These boundaries are essential for weekly reporting periods, payroll cycles, and scheduling.
4. View the ISO format: The ISO 8601 week format (e.g., "2026-W08") is shown in a monospace font for easy copy-paste into reports, spreadsheets, and data systems. This format is internationally unambiguous and automatically sortable.

The calculator is particularly useful for business reporting, project management, payroll processing, and any context where ISO week numbering is the standard. Results update instantly when you change the date.

ISO Week Number Formula

Variables Explained

• Ordinal Day: The day-of-year number (1-366) for the target date. January 1 = 1, February 1 = 32, etc.
• Weekday: The ISO day number (Monday = 1, Sunday = 7) for the target date.
• Week Year: The year the ISO week belongs to. For most dates this equals the calendar year, but dates in the first few days of January may belong to the previous year's last week, and dates in the last few days of December may belong to the next year's first week.
• Week 1 Definition: The first week of the year that contains at least 4 days of the new year. Equivalently, the week containing January 4, or the week that has Thursday in the new year.
• 53-Week Years: Occur when January 1 is Thursday (common years) or Wednesday/Thursday (leap years). The Gregorian calendar has a 400-year cycle with 71 years having 53 weeks.

Step-by-Step Example

Find the ISO week number for February 20, 2026 (Friday):

1. February 20, 2026 is day 51 of the year (31 + 20 = 51)
2. February 20, 2026 is a Friday, ISO weekday = 5
3. Find the Thursday of the same week: Feb 20 - (5-4) = Feb 19 (Thursday)
4. Feb 19 = day 50 of the year
5. Week number = ceil(50 / 7) = ceil(7.14) = Week 8
6. Week year: 2026 (Feb 19 is clearly in 2026)
7. ISO format: 2026-W08
8. Week starts: Monday, Feb 16 | Week ends: Sunday, Feb 22

Practical Examples

Example 1: Elena's Weekly Sales Report

Elena manages a European retail chain that uses ISO week numbering for all reporting. She needs to prepare the sales report for the week containing February 20, 2026:

• February 20, 2026 falls in ISO Week 8 (W08)
• Week 8 runs from Monday, February 16 to Sunday, February 22
• Report header: "Weekly Sales Report — 2026-W08"
• Data collection period: Feb 16 00:00 to Feb 22 23:59

Elena uses the week boundaries to query her point-of-sale database for the exact date range. The ISO week format "2026-W08" serves as the file name and report identifier, ensuring consistency across all 300+ store locations. She compares this week with the same ISO week from previous years (2025-W08, 2024-W08) for year-over-year trending, which is more accurate than comparing calendar dates because ISO weeks always start on Monday. For analyzing data patterns, the time card calculator helps track employee hours within each reporting week.

Example 2: Marcus's Sprint Numbering

Marcus is a scrum master who names sprints using ISO week numbers. His team runs 2-week sprints and he needs to plan Sprint naming for Q1 2026:

• January 5, 2026 (Monday) = Week 2 → Sprint "W02-W03"
• January 19, 2026 = Week 4 → Sprint "W04-W05"
• February 2, 2026 = Week 6 → Sprint "W06-W07"
• February 16, 2026 = Week 8 → Sprint "W08-W09"
• March 2, 2026 = Week 10 → Sprint "W10-W11"
• March 16, 2026 = Week 12 → Sprint "W12-W13"

Using ISO week numbers for sprint naming provides an unambiguous, universally understood reference. When Marcus says "Sprint W08-W09," every team member globally knows the exact dates (Feb 16 - Mar 1). This is cleaner than date-range naming and avoids the confusion of arbitrary sprint numbers. For planning longer timelines, the average calculator helps compute mean velocity across sprints.

Example 3: Anna's Payroll Processing

Anna works in payroll for a German company that processes wages on an ISO weekly basis. She needs to determine which ISO week covers a specific time sheet entry dated February 20, 2026:

• February 20 = ISO Week 8 (2026-W08)
• Week period: Feb 16 (Monday) to Feb 22 (Sunday)
• Payroll processing day: Following Wednesday (Feb 25)
• Payment date: Following Friday (Feb 27)

Anna assigns the time sheet entry to payroll batch "2026-W08." German labor law and accounting standards use ISO weeks for reporting, making this categorization legally compliant. The week boundaries define the exact overtime thresholds (hours worked within Monday-Sunday) and shift differentials. Using ISO weeks eliminates the ambiguity that can arise when a work week crosses a month boundary — the week belongs to one ISO week regardless of which months the days fall in.

ISO Week Numbers for 2026

Tips and Complete Guide

Understanding the ISO Week System

The ISO 8601 week numbering system was designed to provide a consistent, unambiguous way to refer to weeks that works across all countries and languages. By always starting on Monday and using a fixed rule for Week 1 (must contain the first Thursday), the system ensures every day belongs to exactly one week and one week year. This eliminates the confusion that arises from different cultural conventions about week start days (Sunday in the US, Monday in Europe, Saturday in some Middle Eastern countries).

Using Week Numbers in Business Communication

In European business contexts, referring to "Week 8" is as natural as saying "February" in American contexts. When scheduling across international teams, using ISO week numbers avoids date format confusion (02/03 could mean February 3 or March 2 depending on the country). Instead of writing "the week of February 16-22," you can simply say "W08." Calendar applications like Google Calendar and Outlook can display ISO week numbers — enable this setting for easier cross-referencing with international colleagues.

Week Numbers in Data Analysis

When analyzing time-series data, grouping by ISO week provides consistent 7-day bins that are more reliable than monthly grouping. Months vary from 28 to 31 days, making monthly comparisons uneven. ISO weeks are always exactly 7 days, ensuring fair comparison. In SQL databases, functions like YEARWEEK() or EXTRACT(WEEK FROM date) return the ISO week number. In spreadsheet applications, ISOWEEKNUM() provides the same result. This consistent bucketing is essential for accurate trend analysis, forecasting, and reporting.

Common Mistakes to Avoid

• Assuming Week 1 always starts on January 1: Week 1 is defined by the first Thursday rule, not by January 1. January 1 can fall in the last week of the previous year. In 2026, January 1 is a Thursday, so it falls in Week 1 of 2026.
• Mixing US and ISO week numbering: The US system starts weeks on Sunday and counts January 1 as always being in Week 1. The ISO system starts on Monday with a different Week 1 definition. Make sure you know which system your organization uses before referencing week numbers.
• Ignoring the week year: Near year boundaries, the ISO week year can differ from the calendar year. December 31 might be in Week 1 of the next year, or January 1 might be in Week 52 of the previous year. Always check the week year, not just the week number.
• Assuming 52 weeks per year: While most years have 52 ISO weeks, some have 53. Planning that assumes exactly 52 weekly cycles per year will miss one week in 53-week years. Check the ISO week count for your specific year when creating annual schedules.

Frequently Asked Questions