ganak.appWeek Number Calculator
Get the ISO week number for any date.
Pick a date to find its week number
What Is an ISO Week Number?
The international standard for numbering weeks of the year
The ISO week number (defined by ISO 8601) is the internationally standardized way to number weeks. It eliminates confusion caused by different countries starting weeks on different days and using different numbering rules.
ISO week numbering is widely used in business reporting, sprint planning, payroll systems, manufacturing, and supply chain management. When someone says "deliver by Week 14," ISO week numbers ensure everyone is on the same page regardless of location.
ISO 8601 Week Rules
Weeks start on Monday (not Sunday)
Week 1 is the week containing the first Thursday of January (or equivalently, the week containing January 4th)
Years have 52 or 53 weeks (53-week years occur when Jan 1 is Thursday, or Thu/Fri in leap years)
ISO Week Number Examples
See how dates map to ISO week numbers
| Date | Week # | ISO Year | Note |
|---|---|---|---|
| 1 Jan 2025 | Week 1 | 2025 | Wednesday — starts in Week 1 |
| 31 Mar 2025 | Week 14 | 2025 | End of Q1 |
| 30 Jun 2025 | Week 27 | 2025 | Mid-year / End of Q2 |
| 30 Sep 2025 | Week 40 | 2025 | End of Q3 |
| 25 Dec 2025 | Week 52 | 2025 | Christmas 2025 |
| 31 Dec 2025 | Week 1 | 2026 | Belongs to ISO year 2026! |
| 1 Jan 2024 | Week 1 | 2024 | Monday — perfect ISO week start |
| 29 Dec 2024 | Week 52 | 2024 | Sunday — last ISO week of 2024 |
Notice: December 31, 2025 belongs to ISO Year 2026, Week 1 — not Week 53 of 2025. This is because the majority of that week's days fall in 2026.
Quarters & ISO Week Ranges
Map business quarters to ISO week numbers for planning
January – March
~91 days
April – June
~91 days
July – September
~92 days
October – December
~92 days
* Quarter-to-week mapping is approximate. Exact ISO week boundaries may vary by 1-2 weeks depending on the year's starting day.
ISO vs US Week Numbering Systems
Understanding the two main week numbering standards
ISO 8601 (International)
- Weeks start on Monday
- Week 1 contains the first Thursday of January
- Used in Europe, Asia, most of the world
- 52 or 53 weeks per year
- Format:
2025-W14
US / North American System
- Weeks start on Sunday
- Week 1 contains January 1st
- Used in US, Canada, parts of Latin America
- 52 or 54 weeks per year
- No standard format notation
This calculator uses ISO 8601 — the international standard used by most business software, ERP systems, and global organizations. In Excel, use ISOWEEKNUM() for ISO or WEEKNUM() for the US system.
When Do You Need a Week Number Calculator?
Professional and personal uses for ISO week numbers
Agile Sprint Planning
Map sprints to ISO week numbers — 'Sprint 7 runs from Week 14 to Week 15.' Consistent numbering prevents confusion across distributed teams.
Weekly Business Reports
Align sales, marketing, and financial reports to ISO weeks for clean week-over-week comparisons. 'Revenue was up 12% in W35 vs W34.'
Payroll & HR
Biweekly and weekly payroll cycles reference ISO week numbers. Timesheets, overtime, and leave calculations use week numbers for consistency.
Manufacturing & Supply Chain
Production schedules, delivery windows, and inventory planning reference ISO weeks. 'Ship by W22' is unambiguous across global facilities.
Academic Calendars
Universities and schools organize semesters by week numbers. 'Exams start in W48' provides a clear reference for all stakeholders.
Personal Goal Tracking
Track habits, fitness, and projects by week number. 'I've completed 14 out of 52 weeks of marathon training' gives clear progress visibility.
ISO Week Number Calculation Method
How the algorithm determines week numbers
The ISO week number is computed using the ISO 8601 algorithm:
- Step 1: Find the Thursday of the same ISO week as the given date (since Thursday determines which year the week belongs to)
- Step 2: The year of that Thursday is the ISO year
- Step 3: Find January 4th of that ISO year (always in Week 1)
- Step 4: Count the number of weeks between January 4th's Monday and the given date's Monday
- Step 5: The result + 1 is the ISO week number
This produces results identical to Excel's ISOWEEKNUM(), Python's date.isocalendar(), and JavaScript's date libraries like date-fns/getISOWeek.
Frequently Asked Questions
Common questions about ISO week numbers and weekly calendar systems
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Last updated Mar 17, 2026