What week of the year is it right now?

The current week number is calculated automatically based on today's date using ISO-8601 rules (Monday through Sunday) and US calendar rules (Sunday through Saturday).

How is the ISO-8601 week number calculated?

Under the ISO-8601 standard, weeks start on Monday, and Week 1 of any year is defined as the week that contains the first Thursday of the year (or contains January 4th). A year has either 52 or 53 weeks.

What is the difference between ISO week and US week?

ISO weeks always begin on Monday and follow the first-Thursday rule. US weeks begin on Sunday and consider whichever week contains January 1st as Week 1.

Time

Week Number Calculator

Find the exact ISO-8601 and US calendar week number for any date in the year.

Quick Answer & Summary

The week number identifies the chronological week (1 to 52 or 53) of the year. ISO-8601 weeks start on Monday (Week 1 contains the first Thursday of the year), while US calendar weeks start on Sunday (Week 1 contains January 1st).

ISO-8601 Week Standard

Week 1 = Week containing first Thursday of January | Total weeks in year = 52 or 53 (leap week years)

Complies with ISO 8601 international business and fiscal calendar standards.

Select Date

ISO-8601 Week (International)

Standard International Week

US / North America Week

US Sunday-Based Week

ISO-8601 Week: Starts on Monday; Week 1 is the week with the first Thursday of the year.
US Week: Starts on Sunday; Week 1 begins on the week containing January 1st.

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Updated: August 2026

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Documentation & Guide

How to Use Week Number Calculator

Select Any Date

Pick today's date or select any calendar date.

View Week Numbers

Compare the international ISO-8601 week number alongside the North American (US) week number.

Learn Week Definition Rules

Understand whether your project or accounting uses Monday-start (ISO) or Sunday-start (US) numbering.

Pro Tips

Standard in corporate agile sprints, supply chain, and fiscal reporting

Practical Examples & Conversions

Input

Jan 1, 2026

Output

ISO Week 1 / US Week 1

Frequently Asked Questions (PAA)

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Authoritative Standards & Citations

Calculations and algorithms on this page are implemented and verified in strict accordance with the following official technical specifications:

International Organization for Standardization

ISO 8601: Date and Time Formats

Internationally accepted standard for representing dates and times.

Internet Assigned Numbers Authority

IANA Time Zone Database (TZDB)

Authoritative historical and current daylight saving time (DST) database.