How to Use This Tool
Pick a Date
Select any date using the date input. The current date is pre-filled by default.
Read the Week Numbers
The tool instantly shows both the ISO week number and the US week number for your date.
See Additional Info
Day of the year, quarter, and days remaining in the year are all shown alongside the week numbers.
Change Dates Freely
Pick a different date and all values update instantly. No button to click.
Frequently Asked Questions
Is this tool really free?
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Yes, completely free with no limits and no sign-up. Everything runs in your browser.
What is an ISO week number?
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ISO 8601 defines weeks starting on Monday. Week 1 is the week containing the first Thursday of the year. This is the standard used in most of Europe and in international business.
What is a US week number?
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In the US system, weeks start on Sunday and week 1 begins on January 1 regardless of the day. This can differ from the ISO week number by one or two weeks.
Why are ISO and US week numbers different?
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They use different rules for when weeks start and how week 1 is defined. The ISO system starts weeks on Monday; the US system starts on Sunday.
What is Day of Year?
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The day number counting from January 1. For example, February 1 is day 32. It goes up to 365 (or 366 in a leap year).
What is the Quarter?
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The year is divided into 4 quarters: Q1 (Jan-Mar), Q2 (Apr-Jun), Q3 (Jul-Sep), Q4 (Oct-Dec).
What does Days Remaining mean?
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The number of days left in the year after the selected date, including that day.
Can I check any date?
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Yes. Pick any date in the past, present, or future using the date input.
Is my data sent to a server?
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No. All calculations happen directly in your browser. Nothing you enter is ever sent anywhere.
Does it update instantly?
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Yes. Change the date and all values recalculate immediately.
What Is Week Number Calculator?
Week Number Calculator is a free tool that shows the ISO and US week number, day of the year, quarter, and days remaining for any date you pick. Nothing you enter is ever sent anywhere - everything runs locally in your browser.
Both the ISO and US week numbering systems are shown side by side so you can use whichever your region or organization follows. All values update instantly as you change the date.
Features Explained
ISO Week Number
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The international standard week number (ISO 8601). Weeks start on Monday, and week 1 is the week containing the first Thursday of the year.
US Week Number
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The US convention where weeks start on Sunday and week 1 begins on January 1. Shown alongside ISO for easy comparison.
Day of Year
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The day number counting from January 1 (1 to 365 or 366). Useful for Julian date references and planning.
Quarter
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Which quarter of the year the date falls in: Q1 (Jan-Mar), Q2 (Apr-Jun), Q3 (Jul-Sep), Q4 (Oct-Dec).
Days Remaining
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How many days are left in the year after the selected date. Helpful for year-end planning and countdowns.
Instant Results
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All values update immediately when you change the date. No button to click.
Any Date Supported
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Check dates in the past, present, or future. The tool works for any valid date.
Pre-Filled Today
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The current date is pre-filled when you open the tool so you can see today's week number right away.
Clear Layout
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All information is displayed in a clean card layout so you can read every value at a glance.
100% Browser-Based
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All calculations run directly in your browser. Nothing you enter is ever sent to any server.
Who Is This Tool For?
Project Managers
Find the week number for sprint planning, milestone tracking, and weekly reporting.
Accountants
Determine which fiscal quarter and week a transaction falls in for financial reporting.
HR Professionals
Calculate payroll weeks, leave balances, and benefits enrollment periods by week number.
Students
Find the week number for school schedules, assignment deadlines, and semester planning.
Teachers
Plan lessons and track the school calendar by week number.
Supply Chain Workers
Reference week numbers for shipping schedules, production planning, and inventory cycles.
Marketers
Plan campaigns by calendar week and track weekly performance metrics.
Journalists
Reference week numbers in editorial calendars and publishing schedules.
Developers
Check ISO week numbers for software sprint tracking and weekly build numbering.
Event Planners
Identify which week an event falls in for coordination and scheduling.
Fitness Enthusiasts
Track training weeks and plan workout cycles by week number.
Farmers
Plan planting, harvesting, and crop rotation by calendar week.
Retailers
Plan promotions and inventory by retail calendar week.
Researchers
Reference week numbers in longitudinal studies and data collection schedules.
Government Workers
Track reporting periods and compliance deadlines by week number.
Sales Teams
Track weekly quotas, pipeline reviews, and forecast periods.
Logistics Coordinators
Schedule deliveries and track shipments by calendar week.
Healthcare Workers
Plan shift rotations and patient scheduling by week number.
Bloggers
Plan content calendars and publishing schedules by week.
Non-Profit Workers
Track fundraising campaigns and program milestones by week.
Consultants
Track billable weeks and project phases for client reporting.
Athletes
Plan training periodization and competition schedules by week number.
Parents
Track school terms, vacation weeks, and activity schedules.
Anyone
Quickly find the week number for any date for any reason.
Tips for Best Results
ISO vs US weeks
ISO weeks start on Monday and are used in Europe and international business. US weeks start on Sunday. The tool shows both.
Week 1 differs between systems
In ISO, week 1 contains the first Thursday. In the US system, week 1 always starts on January 1. This can cause a difference of 1-2 weeks.
Day of Year for quick reference
Day 1 is January 1, day 365 is December 31 (or 366 in a leap year). Useful for Julian date references.
Quarter for fiscal planning
Q1 is Jan-Mar, Q2 is Apr-Jun, Q3 is Jul-Sep, Q4 is Oct-Dec. Great for financial and business planning.
Days remaining for countdowns
See how many days are left in the year at a glance. Useful for year-end deadlines and goal tracking.
Pre-filled with today
The tool opens with today's date already filled in, so you can see the current week number immediately.
Change dates freely
Pick any date and all values update instantly. Check past dates, future dates, or jump between them.
Use for sprint planning
Many software teams plan in weekly sprints. Use the ISO week number to align your sprint calendar.
Results are instant
All values recalculate the moment you change the date. No button to click.
Privacy is built in
Nothing you enter is ever sent to any server. All calculations run in your browser.
Privacy & Security
This tool runs 100% in your browser. Your files stay entirely on your own device. Nothing is uploaded, nothing is shared, and no server, advertiser, or third party has access to your files.
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