Week Number Calculator
Date & TimeISO week, day of year and year progress
Quick Answer
Week Number Calculator: ISO week, day of year and year progress. Week starts Monday; Week 1 contains Jan 4 (first Thursday).
2026-02-10
Week 7
2026 ISO week-year · Tue
Day of year41
Week starts2026-02-09
Week ends2026-02-15
Weeks left46 / 53
Calculation steps
- 1ISO 8601: weeks start Monday; week 1 is the week containing the year's first Thursday (equivalently, Jan 4).
- 22026-02-10 is a Tue; its Thursday week belongs to ISO year 2026.
- 3Week number = 7; that week runs 2026-02-09 → 2026-02-15.
Week systems compared
| System | Week 1 definition | Used by |
|---|---|---|
| ISO 8601 | Week with first Thursday (Jan 4) | Europe, business, this tool |
| US | Week containing Jan 1, weeks start Sunday | US calendars |
| %U strftime | First Sunday starts week 1 | Unix/C |
💡 Edge case trap: Dec 29 – Jan 4 can belong to a DIFFERENT ISO week-year than the calendar year — e.g. 2021-01-01 is ISO week 53 of 2020! Years starting on Thursday (or leap years starting Wednesday) have 53 weeks.
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How to Use
- Pick dates or a mode: select start/end dates or switch modes in the Week Number Calculator.
- Instant calculation: results appear immediately with day/week/month equivalents.
- Solve both ways: change any field and everything updates — forward or reverse.
- Check the context: read the solar-term/holiday/rule notes on this page.
Key Points
- Week Number Calculator is a free online date & time tool — open and use, no sign-up.
- All calculations run locally in your browser; your data never leaves the device.
- Key capability: ISO week, day of year and year progress.
- Results come with formulas and reference tables so you can verify every number.
Formula & How It Works
ISO 8601 week date
Week starts Monday; Week 1 contains Jan 4 (first Thursday)
Jan 1 may belong to last year's W52/53 · years with 53 weeks exist
Day of year: Jan 1 = 1, Dec 31 = 365/366
FAQ
How do I use the Week Number Calculator?
Open the page and fill in the input fields — results update automatically. ISO week, day of year and year progress.
Are the results accurate?
Yes. Standard formulas and conversion factors are used and verified.
Is it free? Do I need to sign up?
Completely free. No registration, login or download required.
Does it work on mobile?
Yes. The responsive design works on phones, tablets and desktops.
Is my data saved?
No. Everything runs locally in your browser.
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Source & Updates
Based on standard calendar arithmetic. Last updated: 2025-07
Authoritative sources:
Last updated:
Formulas and content on this page are reviewed by the YiJianSuan team. For reference only — official sources prevail.
Maintained and periodically reviewed by the YiJianSuan editorial team — review process in ourEditorial Policy.
Cite this page: YiJianSuan. Week Number Calculator[EB/OL]. https://www.yijiansuan.com/en/c/date/week-number
Results provided by this tool are for reference only and do not constitute professional advice.