Week Number Calculator
Need to know which ISO week a date falls in? Convert dates to week numbers or find all dates in a specific week. Perfect for project planning and team coordination.
Quick example
Say it's March 15, 2025. That's ISO Week 11 — running from Monday, March 10 to Sunday, March 16.
Or flip it around: Week 1 of 2025 starts on Monday, December 30, 2024 (yes, it crosses into the new year!) and ends on Sunday, January 5, 2025.
Some years like 2020 and 2026 have 53 weeks instead of the usual 52. The calculator handles this automatically.
How to Use This Calculator
- 1Choose between two modes: convert a date to its ISO week number, or find all dates in a specific week.
- 2For Date → Week Number: pick any date (or tap Today), then hit Calculate to see which ISO week it belongs to.
- 3For Week Number → Dates: select a year, type in a week number (1–52 or 53 for special years), and the tool shows you all seven days from Monday to Sunday.
- 4Watch the visual progress bar fill up to see how far through the year you've traveled by weeks.
- 5Use Clear to reset and run a new calculation whenever you need.
Where This Helps
Sprint planning & Agile teams
Align two-week sprints with ISO week numbers so everyone knows exactly which weeks cover each sprint cycle.
Payroll & billing cycles
Run weekly payroll or invoicing? Week numbers make it easier to track cycles that don't line up with calendar months.
Cross-timezone coordination
When your team spans multiple cities or continents, referencing ISO Week 23 keeps everyone on the same page.
Sales & performance reports
Compare week-over-week sales or KPIs using consistent week numbers instead of variable month lengths.
What to try next
Browse full week number calendar
See the entire year's worth of week numbers with start and end dates in a table format.
ExploreCheck day numbers
Pair week numbers with day numbers for complete time tracking across the year.
ExploreGet week number code snippets
Access Excel, SQL, and JavaScript formulas for ISO week calculations in your tools.
ExploreFrequently Asked Questions
What is an ISO week number?
ISO week numbers follow the ISO 8601 standard: weeks start on Monday, and Week 1 is the week containing January 4th. This means weeks stay consistent across years, even if they cross calendar year boundaries.
Why does Week 1 sometimes start in December?
If January 1st falls on a Friday, Saturday, or Sunday, the first few days of January belong to the last week of the previous year. Week 1 always contains January 4th, so it might start as early as December 29.
How does the calculator handle 53-week years?
Years with 53 weeks happen when January 1st is a Thursday, or when it's a leap year and January 1st is a Wednesday. The calculator detects these years automatically and adjusts the valid week range to 1-53.
What's the difference between ISO weeks and regular calendar weeks?
ISO weeks always start on Monday and follow strict rules about which week is Week 1. Regular calendar weeks might start on Sunday (US) or Monday (EU), but without the January 4th rule.
Can I use this for planning across multiple years?
Yes. The calculator supports years from 2020 to 2030 for the Week Number → Dates mode. For Date → Week Number, you can pick any date your browser allows.
How can I share the results with my team?
Screenshot the result cards or copy the week range manually. We're building shareable links and export features based on user feedback.