How is the ISO 8601 week number calculated?

The ISO 8601 standard specifies that each week starts on Monday, and the first week of the year is the one containing the first Thursday of that year. Therefore, the first week of a year may start in December of the previous year, and the last week may extend into January of the following year.

What is the difference between ISO week number and simple week number?

ISO week numbers start each week on Monday, and the first week must contain at least 4 days belonging to that year. Simple week numbers simply divide the day of the year by 7 and round up, treating the week containing January 1 as the first week. The two may differ by 1 week.

📅 Week Number Calculator

Calculate which week of the year a date falls on

Select Date

ISO 8601 Week Number

Week 22

2026 Fri · ISO Year 2026

ISO Week

Week 22

Simple Week

Week 22

Day of Year

149

Total Weeks in Year

53 Weeks

Date Details

Date: 2026-05-29 (Fri)

Year: 2026 (365 days)

ISO Week: Week 22 of 2026

Year Progress: 149 / 365 days (40.8%)

Week Number Calculator Guide

This calculator determines which week of the year a given date falls into, using the ISO 8601 international standard. Enter a date to see its corresponding week number.

ISO 8601 standard: Each week starts on Monday, and the first week of the year is the one containing the first Thursday of that year. This is widely used in project management, production planning, and international business.

ISO Week Number Rules

ISO 8601 week number rules:

1. Each week runs Monday through Sunday 2. Week 1 is the week containing the year's first Thursday 3. A year has 52 or 53 weeks

Special cases: - If January 1 falls on Friday, Saturday, or Sunday, it belongs to the previous year's last week - If December 31 falls on Monday, Tuesday, or Wednesday, it belongs to the next year's first week

Examples: - January 1 is Wednesday → Week 1 - January 1 is Saturday → Belongs to previous year's week 52 or 53

Applications: 1. Project scheduling: Manage progress by week number 2. Production planning: Weekly production schedules 3. Financial reporting: Weekly statistics

This calculator is for reference only. Verify against actual calendar.

FAQ

Ad Slot

Related Calculators

🎲Random Number Generator 🔢Number Base Converter 🔢Prime Number Checker 🎂Age Calculator ⏱️Detailed Age Calculator ⏳Countdown Calculator

Week Number Calculator Instructions

The week number calculator allows you to view the complete weekly calendar for any year, including the start and end dates of each week, the current week number, and how many weeks the year has in total (usually 52 or 53). Enter a year to generate a calendar view.

The standardized definition of week numbers follows the ISO 8601 standard: the first week of each year is the one containing the first Thursday of that year. This means January 1 may belong to the last week of the previous year, and December 31 may belong to the first week of the following year. This definition ensures that every week has a complete 7 days and that cross-year boundaries are handled consistently.

ISO 8601 Week Number Standard

【ISO 8601 Rules】
- Monday is the first day of each week, Sunday is the last
- Week 1 of each year = the week containing the first Thursday of that year
- A year may have 52 or 53 weeks
- If January 1 falls on Thursday, Friday, Saturday, or Sunday → the year has 53 weeks
- Otherwise, it has 52 weeks

【Determination Method】
January 4 of the year is always in Week 1
December 28 of the year is always in the last week

【Differences from Other Standards】
US convention: Sunday is the first day of the week
Chinese convention: Monday starts the week (consistent with ISO)

Calculation Examples

📋

【Example 1】Week number distribution for 2026 January 1, 2026 is a Thursday According to ISO standard: It belongs to the last week (Week 53) of 2025 Week 1 of 2026 starts from December 29, 2025 (Monday) 2026 has 52 weeks in total (since January 4 falls within Week 1) 【Example 2】Finding today's week number Date: May 13, 2026 Week 1 of 2026: 12/29 - 1/4 Calculation: May 13 falls around Week 20 【Example 3】Years with 53 weeks 2020: January 1 is a Wednesday → 53 weeks (the week containing January 1 has a Thursday) 2026: January 1 is a Thursday → 53 weeks? Verification: January 1 is Thursday → the week contains the first Thursday of 2026 → it is indeed Week 1 But whether the entire year has 52 or 53 weeks depends on December's situation

Important Notes

💡

- The ISO 8601 standard differs slightly from the "natural week" concept commonly used in daily life in China - A fiscal year's "fiscal weeks" may differ from calendar weeks (e.g., a new fiscal year starting in April) - Different programming languages may implement getWeek differently (JavaScript's toLocaleString supports ISO weeks) - Cross-border businesses need to confirm which week number definition the other party uses - Some industries (such as retail) treat Sunday as the first day of the week

Application Scenarios

- Project management: Track Sprint progress by week, weekly report time labeling
- Financial accounting: Weekly/bi-weekly report period identification, payroll settlement cycles
- Logistics and transportation: Weekly-numbered shipping batches, delivery cycle planning
- Healthcare: Pregnancy week calculation (weeks from last menstrual period), vaccination scheduling
- Education: Semester teaching week plans, exam week scheduling

History and Applications of Week Number Systems

The concept of the "week" originated from Babylonian civilization, using a 7-day cycle corresponding to the five visible planets plus the Sun and the Moon. The ISO 8601 standard was published in 1988, unifying international norms for representing dates and times, with the week number definition being one of the most frequently cited parts.

In practice, many systems need a custom "fiscal week" concept. For example, the US federal government's fiscal year starts on October 1, so its W1 is completely different from the calendar W1. Enterprise Resource Planning (ERP) systems typically allow configuring multiple week calendars to meet compliance requirements in different regions.

Ad Slot