ISO Week Number Calculator
Look up the current ISO-8601 week number, calendar week schedule, and date ranges.
Current Week Number
39
Week 39 of ISO Year 2026 (53 weeks total)
Week Start (Monday)Monday, Sep 21, 2026
Week End (Sunday)Sunday, Sep 27, 2026
Based on the international standard ISO-8601, where Monday is the first day of the week.
Free ISO Week Number Calculator & Calendar Week Finder
AlarmOnline.net provides a free online ISO week number calculator to instantly look up the current calendar week or find the exact ISO week number, ISO week-numbering year, and Monday-to-Sunday boundaries for any date.
How to Use This Tool
- 1View Current Week Number — The calculator automatically displays today’s active ISO-8601 calendar week upon opening the page.
- 2Check Week Date Range — Inspect the exact starting Monday date and concluding Sunday date for the active week.
- 3Select Any Date — Use the date picker under "Select Another Date" to look up the week number for any upcoming or historical date.
- 4Return to Today — Click the "Today" button at any time to reset the calculator to your current local date.
Key Features & Capabilities
Standard ISO-8601 Compliance
Complies with international ISO-8601 rules where Monday is day 1 and Week 1 contains the first Thursday of the year.
ISO Week-Numbering Year Tracking
Accurately distinguishes the ISO week-numbering year from the Gregorian calendar year during New Year transition weeks.
Monday to Sunday Date Range
Displays the full calendar start (Monday) and end (Sunday) dates for the selected week.
52 vs 53-Week Detection
Accurately indicates whether the selected ISO year contains 52 or 53 total weeks.
Any Date Lookup
Calculate week numbers for past milestones, current projects, or future scheduling dates.
Integrated Date & Time Tools
Need other date or scheduling tools? Calculate elapsed days between dates with our Date Calculator, check exact chronological age with the Age Calculator, track milestones with the Countdown Timer, convert international meeting times with the Time Zone Converter, or set browser alarms with the Online Alarm Clock.
Popular Use Cases
Agile & Scrum Sprints
Align two-week software engineering sprints with official calendar weeks across distributed development teams.
Supply Chain & Manufacturing
Coordinate delivery schedules, production batches, and freight logistics using standardized work week numbers.
Project Management & Timelines
Organize project milestones, Gantt charts, and deliverables by work week for clear cross-department reporting.
Internal Schedules & Timesheets
Organize internal team rotas, shifts, and weekly timesheet periods by standardized calendar week numbers.
Quarterly & Fiscal Reporting
Track business performance and revenue metrics within 13-week quarterly reporting blocks.
Academic & School Schedules
Plan syllabus milestones, assignment deadlines, and semester calendars by week number.
Frequently Asked Questions (FAQ)
What is an ISO week number?
An ISO week number is a standardized calendar week number defined by the international ISO-8601 standard. Each week begins on Monday (Day 1) and ends on Sunday (Day 7), numbered from Week 1 to Week 52 or 53.
How is the current week number determined?
The current week number is calculated automatically from your device current local calendar date according to ISO-8601 rules, where Week 1 is the week containing the first Thursday of the year.
When does an ISO week start and end?
Under the ISO-8601 standard, every calendar week starts on Monday at 00:00:00 and concludes on Sunday at 23:59:59.
How is ISO Week 1 of the year calculated?
ISO Week 1 is defined as the week containing the year’s first Thursday, which is also the week that contains January 4th. If January 1st falls on a Friday, Saturday, or Sunday, it belongs to the final week (Week 52 or 53) of the previous ISO year.
Why can December 31 belong to Week 1 of the next year?
If December 31st falls on a Monday, Tuesday, or Wednesday, more than half of that week’s days fall in the new year (including Thursday), meaning the entire week is classified as Week 1 of the following ISO week-numbering year.
Why can January 1 belong to Week 52 or Week 53 of the previous year?
If January 1st falls on a Friday, Saturday, or Sunday, the majority of that week occurred in December of the prior year, so it belongs to the final week of the preceding ISO year.
Can a calendar year have 53 ISO weeks?
Yes. Any year starting on a Thursday, or a leap year starting on a Wednesday, contains 53 full ISO weeks instead of the standard 52 weeks.
How do I find the week number for a specific past or future date?
Select any past or upcoming date in the "Select Another Date" calendar input. The calculator immediately displays the exact ISO week number, ISO year, and starting/ending week dates.
Is this week number calculator free to use?
Yes. The week number calculator on AlarmOnline.net is 100% free with no registration, subscriptions, or downloads required.
Does this calculator require an active internet connection?
No. All week calculations execute client-side directly in your browser, meaning it functions seamlessly even offline once loaded.
What is the difference between the Week Number Calculator and the Date Calculator?
The Week Number Calculator identifies the standardized ISO-8601 week number and Monday-to-Sunday date range for any date, whereas the Date Calculator computes general differences between two dates or adds/subtracts arbitrary time units.