How is an ISO week number defined?

Under ISO 8601, weeks start on Monday. Week 01 of any year is the first week with at least four days in the new year (i.e. the week containing the first Thursday of January).

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Week Number Calculator

Calculate ISO 8601 week number, start/end dates of any week, and total weeks in a year.

How to Use Week Number Calculator

1Select any date or click 'Current Week' for today.
2View the ISO week number (e.g. Week 36 of 2026).
3Inspect the calendar date range for that week (Monday to Sunday).
4See year completion percentage and remaining weeks.

Key Features & Benefits

Strict ISO 8601 week date standard compliance
Monday-to-Sunday date range display for any week
Year progress bar and remaining weeks counter
Supports all calendar years with 52-week or 53-week leap detection

About Week Number Calculator

Calculate the exact ISO 8601 week number (Week 1 through 52/53) for any date. View week start (Monday) and week end (Sunday) dates, total weeks in the selected year, and percentage of year completed.

Unlike traditional web tools that send your personal data, uploaded photos, or sensitive text to remote cloud servers, our Week Number Calculator operates strictly inside your client browser sandbox. This guarantees high processing speeds without network throttling, and ensures your data privacy is completely preserved.

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Week Number Calculator

Calculate ISO 8601 week number, start/end dates of any week, and total weeks in a year.

Date Selector

Calculate official ISO 8601 calendar week number and week boundaries.

ISO 8601 Week NumberWeek 37

Monday through Sunday: Sep 7, 2026 – Sep 13, 2026

Calendar Year: 2026

Year Progress: 71%

Week 37 of 52 Weeks15 weeks remaining in 2026

How to Use Week Number Calculator

1Select any date or click 'Current Week' for today.
2View the ISO week number (e.g. Week 36 of 2026).
3Inspect the calendar date range for that week (Monday to Sunday).
4See year completion percentage and remaining weeks.

Key Features & Benefits

Strict ISO 8601 week date standard compliance
Monday-to-Sunday date range display for any week
Year progress bar and remaining weeks counter
Supports all calendar years with 52-week or 53-week leap detection

About Week Number Calculator

Calculate the exact ISO 8601 week number (Week 1 through 52/53) for any date. View week start (Monday) and week end (Sunday) dates, total weeks in the selected year, and percentage of year completed.