In everyday work and project management, week-number calculation is a common need. Especially around New Year, people often wonder: “Which week is 29 Dec 2025?” The answer is Week 1 of 2026! That surprising result comes from the international ISO 8601 rule.
If you need an accurate and handy week-number calculator, try the WeekNumber — a free online tool that supports ISO 8601 week lookup, cross-year calculation, and date-range display.
What Is ISO 8601 Week Number & Why Does It Matter?
ISO 8601 is the international standard for date and time. Its week-number part says:
- A week starts on Monday and ends on Sunday.
- Week 1 is the week that contains the year's first Thursday (equivalently, 4 January).
- The year of a week is decided by the year that owns its Thursday.
- Most years have 52 weeks; some have 53 (2026 will have 53).
- Year-boundary days at the end of December may belong to Week 1 of the next year, and early January days may belong to the last week of the previous year.
This standard is widely used in Europe, China (GB/T 7408 is equivalent), international trade, project management, financial reporting, and software development, eliminating ambiguity caused by local customs.
Common Cross-Year Examples (2025-2026)
Real calculations under ISO 8601:
| Date | ISO Week | Remark |
|---|---|---|
| Mon 29 Dec 2025 | 2026-W01 | Day 1 of Week 1, 2026 |
| Wed 31 Dec 2025 | 2026-W01 | Day 3 of Week 1, 2026 |
| Thu 1 Jan 2026 | 2026-W01 | Day 4 of Week 1, 2026 |
| Sun 4 Jan 2026 | 2026-W01 | Day 7 of Week 1, 2026 |
| 1 Jan 2025 | 2025-W01 | Day 3 of Week 1, 2025 |
| 31 Dec 2030 | 2031-W01 | Crosses into 2031 |
As shown, the last days of 2025 are “borrowed” by Week 1 of 2026 — common in payroll, scheduling, and reporting.
How to Use the WeekNumber Calculator
WeekNumber is simple:
- Enter any date (YYYY-MM-DD).
- It instantly shows the ISO week number (e.g. 2026-W01), the Monday-Sunday range, and the day-of-week.
- Batch query, current-week display, and historical years are supported.
- 100 % free, no ads, mobile-friendly.
Compared with Excel WEEKNUM or Python isocalendar(), this online tool needs no install and is ready for project managers, HR, and developers to verify week numbers on the fly.
Why Prefer ISO Week Numbers Over Other Standards?
- US convention: week starts on Sunday; Week 1 contains 1 Jan.
- Simple method: divide year days by 7 from 1 Jan.
- ISO 8601 is the internationally recommended standard and gives the highest consistency across countries.
If you deal with weekly reports, Sprint planning, or global projects, this tool helps you avoid week-number mistakes.
Pro Tip
Remember the key ISO rule: Week 1 always contains 4 January. This single fact lets you quickly assign any date to its ISO week.