Week Number Calculator
Find the ISO 8601 and US week number for any date, plus week start/ end dates and a full year list. 100% client-side.
ℹ️ Two conventions: ISO 8601 (weeks start Monday, week 1 contains first Thursday) is the international standard. US (weeks start Sunday, week 1 contains Jan 1) is used in the United States. They often differ by 1.
ISO 8601 Week
W41
Year 2026
Start (Monday)Mon, Oct 05, 2026
End (Sunday)Sun, Oct 11, 2026
Total ISO weeks in 202653 weeks
US Week (Sunday start)
W42
Year 2026
Start (Sunday)Sun, Oct 11, 2026
End (Saturday)Sat, Oct 17, 2026
Total US weeks in 202653 weeks
📊 Date details
Date
2026-10-11
Day of year
284 of 365/366
Day of week
Sunday
Year-week (ISO)
2026-W41
📆 Week list for a year
Click "Show list" to see all ISO weeks in 2026 with their start and end dates.
🔒 100% private: All calculation happens in your browser. Your dates never leave your device.
About Week Number Calculator
Week numbers are used in many contexts: payroll periods, project sprints, academic calendars, ISO reporting, and year-week tracking. However, there are multiple conventions, and they do not always agree. The most common is ISO 8601: weeks start on Monday, week 1 is the week containing the first Thursday of the year (equivalently, the week containing January 4). The US convention often uses Sunday as the start of the week and counts week 1 as the week containing January 1. As a result, the same date can have different week numbers in different conventions. This tool shows both, along with the week's start and end date and a full year view.
How It Works
- Enter any date (defaults to today).
- See the ISO week number, the US week number, and the week start/end dates.
- Optionally enter a year to see the full list of weeks with their date ranges.
- Copy any value with one click.
✨ Key Features
- ✓ISO 8601 week number (weeks start Monday)
- ✓US week number (weeks start Sunday)
- ✓Week start and end dates for any date
- ✓Day of week and day of year
- ✓Full list of all weeks in any year (ISO + US)
- ✓Highlight the current week
- ✓Handles leap years and edge cases (week 53)
- ✓Copy any value
- ✓Quick jump to today
- ✓100% client-side — no data sent to any server
🎯 Common Use Cases
- →Finding the ISO week number for payroll or invoicing
- →Project sprint tracking (Scrum uses week numbers)
- →Academic week numbering (semester weeks)
- →ISO 8601 reporting (year-week format)
- →Finding the week number for a specific date
- →Scheduling recurring meetings by week number
- →Understanding the difference between ISO and US week conventions
- →Verifying week-based data in spreadsheets
💡 Advanced Tips & Pro Insights
- ▸ISO 8601 week numbering is the international standard: weeks start on Monday, and week 1 is the week containing the first Thursday of the year. This means some years have 52 weeks and others 53.
- ▸US convention: weeks start on Sunday, and week 1 is the week containing January 1. This often results in different numbers than ISO — usually by 1.
- ▸Week 53 exists in ISO years where January 1 is a Thursday (or a Wednesday in a leap year). About 71% of years have 52 ISO weeks; the rest have 53.
- ▸The year of a week is not always the same as the calendar year. December 31 might be in ISO week 1 of the next year; January 1 might be in ISO week 52 or 53 of the previous year.
- ▸For payroll and financial reporting, always specify which convention you use (ISO or US) — they do not agree on the same date.
Frequently Asked Questions
Is this week number calculator free?▾
Yes, this week number calculator is completely free with no signup, email, or usage limits.
Is my data safe?▾
Yes, 100% safe. All calculation happens in your browser. Your dates are never sent to any server.
What is the difference between ISO and US week numbers?▾
ISO 8601 weeks start on Monday, and week 1 contains the first Thursday of the year. US weeks start on Sunday, and week 1 contains January 1. As a result, the same date can have different week numbers in the two conventions.
What is ISO 8601 week numbering?▾
ISO 8601 is the international standard for date and time formats. It defines weeks as starting on Monday, with week 1 being the week containing the first Thursday of the year (equivalently, the week containing January 4).
Why do some years have 53 weeks?▾
In ISO 8601, a year has 53 weeks if January 1 is a Thursday, OR if it is a leap year and January 1 is a Wednesday. This happens roughly once every 5–6 years.
Which week number should I use?▾
For international business, payroll, and ISO contexts, use ISO 8601. For US-based companies without specific requirements, use the US convention. Always specify which convention you use when sharing dates.
Is "week 1" always the first week of January?▾
Not necessarily. In ISO 8601, week 1 might start in late December of the previous year. The first few days of January might belong to week 52 or 53 of the previous ISO year.
What is the week number today?▾
The current week number is shown when you open the tool — it defaults to today and displays both ISO and US week numbers instantly.