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Calendar Dates

ISO 8601 Week Number Calculator

Q: Can a year have 53 ISO weeks instead of 52?

Yes. Years that begin on a Thursday (or leap years starting on Wednesday) contain 53 full ISO calendar weeks.

Q: Can I look up the exact date range for a specific week number?

Yes. Select any week number and year to view the exact Monday start date and Sunday end date for that designated period.

Q: Does this tool support both US and international week systems?

The calculator defaults to the international ISO 8601 standard (Monday-Sunday) while providing reference for Sunday-start systems.

Find the current ISO 8601 week number, determine start and end dates for any calendar week, and look up which week any date belongs to. Essential for corporate planning, production schedules, and agile sprints directly inside your web browser with complete privacy.

ISO 8601 Week Number Calculator

Find the international standard ISO week number, date span, and yearly progress

Select Calendar Date

ISO 8601 Result

Week 37 of 2026

Week Number

W37

Monday, 2026-09-07 — Sunday, 2026-09-13

Selected Weekday

Tuesday

ISO Day 2/7

Total Weeks in Year

53 Weeks

Year 2026

Year Progress

69.8%

Passed so far

Remaining Weeks

16 Weeks

Left in year

According to international standard ISO 8601, weeks begin on Monday and are numbered from 1 to 52 (or 53). The first week of the year is the one containing the first Thursday of the year.

Find Dates for a Specific Week Number

Enter a year and week number to calculate its exact Monday to Sunday calendar dates.

Year:

Week Number:

Week 1 of 2026 runs from 2025-12-29 (Monday) to 2026-01-04 (Sunday).

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Step-by-Step Guide: How to Use Week Number Calculator Online

1Pick any calendar date (or keep today's date).
2View the ISO week number (e.g. Week 36) and its Monday-to-Sunday span.
3Review year progress percentage and total weeks in the year.
4Use the reverse lookup tool below if you need dates for a future week.

Week Number Calculator Features & Capabilities

Compliant with international standard ISO 8601 calendar weeks
Shows exact week start (Monday) and end (Sunday) dates
Calculates percentage of year passed and remaining weeks in the year
Reverse lookup: Convert any Year + Week number into its exact date range
One-click 'Current Week' button for instant today lookup

Technical Specifications & Privacy Standards

Supported Formats	Calendar Dates
Execution Architecture	100% Client-Side WebAssembly & HTML5 Canvas (Local CPU/GPU)
Server File Transfer	0 Bytes Transferred (Zero Cloud Uploads)
Output Quality	Lossless Vector & Original Resolution Maintained
Compatibility	Cross-Platform (Windows, macOS, Linux, iOS, Android)

Local Client-Side Processing Guarantee

Your files are processed locally in your browser memory and are not uploaded to our file-processing servers.

Week Number Calculator FAQ

What is the ISO 8601 standard for week numbers?

Under ISO 8601, weeks begin on Monday, and Week 1 of the year is the week containing the first Thursday of that calendar year (or January 4).

Can a year have 53 ISO weeks instead of 52?

Can I look up the exact date range for a specific week number?

Does this tool support both US and international week systems?

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