Week Number Calculator - ISO 8601 & US Standards

Calculate week numbers using ISO 8601 or US standards. Find the current week, lookup specific dates, or reverse-search to find all dates in a week. Perfect for project planning, scheduling, and business operations.

Select any date to find its week number
Week start day
ISO 8601 Week Number

Week 2 of 2026

Wednesday, January 07, 2026 is Wednesday (Day 3 of the week)

Mon
5
Tue
6
Wed
7
Thu
8
Fri
9
Sat
10
Sun
11
Week Range
January 05, 2026 - January 11, 2026
Day of Year
Day 7 of 365
Year Progress
Week 2 of 53 3.8%
Q1 Quarter
2 Week of Q1
51 Weeks Left
53 Total Weeks

How to Use the Week Number Calculator

Find Week Number for a Date

  1. Select your date - Use the date picker or enter a date manually
  2. Choose week standard - ISO 8601 (Monday start) or US (Sunday start)
  3. Click Calculate - Get complete week information including range, quarter, and year progress

Find Dates in a Specific Week

  1. Switch to reverse lookup - Click "Find Dates in Week" radio button
  2. Enter year and week number - For example, Week 15 of 2025
  3. Choose your standard - ISO 8601 or US
  4. Click Calculate - See all 7 dates in that week with visual calendar

Back to Calculator

Understanding Week Number Standards

ISO 8601 Standard

International standard used by most of the world, especially in business, manufacturing, and international organizations.

  • Weeks start: Monday
  • Week 1: First week containing January 4th
  • Rule: First week with at least 4 days in the new year
  • Year: Can have 52 or 53 weeks
  • Used by: EU, most of Asia, manufacturing, logistics

US Standard

Common in North America, particularly for retail, school calendars, and consumer applications.

  • Weeks start: Sunday
  • Week 1: Week containing January 1st
  • Rule: First partial week is week 1
  • Year: Always starts with a partial or full week 1
  • Used by: US retail, schools, consumer calendars

Why It Matters

Using the correct standard is crucial for international business, project management software, ERP systems, and coordinating across different regions. ISO 8601 is recommended for professional and international use to ensure consistency across borders.

When to Use the Week Number Calculator

Business & Project Management

Sprint planning: Align development sprints with ISO week numbers. Know exactly which week your 2-week sprints fall in for better planning and reporting.

Production schedules: Manufacturing often uses ISO week numbers for production planning, inventory management, and delivery schedules across multiple facilities.

Retail & Sales

Sales reporting: Retail businesses often report sales by week number for year-over-year comparisons and seasonal trend analysis.

Education

Semester planning: Universities and schools use week numbers to organize syllabi, track curriculum progress, and schedule exams.

Healthcare

Pregnancy tracking: Medical professionals track pregnancies by week number for consistency across different calendar years.

Frequently Asked Questions

What is ISO 8601 week numbering?
ISO 8601 is the international standard for week numbering. Weeks start on Monday, and Week 1 is the first week containing January 4th. This standard is widely used in business and manufacturing.
Can a year have 53 weeks?
Yes, in ISO 8601, a year can have either 52 or 53 weeks. A year has 53 weeks if January 1st falls on a Thursday, or if it's a leap year and January 1st falls on a Wednesday.
Which standard should I use?
For international business and manufacturing, use ISO 8601. For US retail or consumer applications, the US standard may be more appropriate.

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