ISO 8601 Week Number Calculator

Q: Can a year have 53 weeks?

Yes, years that begin on a Thursday (or Wednesday in a leap year) have 53 ISO weeks instead of the usual 52.

Q: How do I find the date range for a specific week number?

Use our 'Find Dates for a Specific Week Number' section at the bottom of the tool to enter any year and week number.

Calculate the official ISO week number for any calendar date, view its Monday-to-Sunday span, and perform reverse week lookups.

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

Monday

ISO Day 1/7

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 an ISO 8601 week number?

ISO 8601 is the international standard for calendar dates and weeks. It defines Monday as the first day of the week, and Week 1 as the week with the year's first Thursday.

Can a year have 53 weeks?

How do I find the date range for a specific week number?