ISO Week Calculator
Use this tool to convert a calendar date to an ISO week number, or convert an ISO year/week/day back to a date.
1) Date → ISO week
2) ISO week → Date
Working with weeks can be surprisingly tricky. Different countries, organizations, and software systems use different definitions of what “week 1” means. This ISO week calculator helps you avoid confusion by using the ISO 8601 week date standard—the most common international method for week numbering.
What is an ISO week?
An ISO week is part of the ISO 8601 date standard. In this system:
- Weeks start on Monday.
- Weekdays are numbered 1 to 7 (Monday to Sunday).
- Week 1 is the week that contains the first Thursday of the year (or equivalently, January 4).
This rule means that some dates near New Year can belong to the previous or next ISO year. For example, January 1 is not always in ISO week 1.
Why ISO week numbers matter
ISO weeks are useful whenever teams need consistent weekly reporting across regions and systems. Common use cases include:
- Business reporting dashboards and KPI summaries
- Payroll cycles and workforce scheduling
- Project planning and sprint management
- Manufacturing and logistics calendars
- Data analysis where week boundaries must be standardized
Without a shared week standard, two reports can show different week numbers for the same date. That creates avoidable mistakes in planning and communication.
How to use this ISO week calculator
Convert a date to week number
Pick a date in the first section and click Calculate ISO Week. You’ll get:
- ISO year
- ISO week number
- ISO weekday
- The start (Monday) and end (Sunday) date of that ISO week
Convert week number to date
In the second section, enter an ISO year, ISO week, and weekday. Click Convert ISO Week to Date to get the exact calendar date and the full week range.
Understanding years with 52 vs 53 weeks
Most ISO years have 52 weeks, but some have 53. A year gets ISO week 53 when the calendar alignment creates an extra partial week according to ISO rules. If you enter a week number that doesn’t exist in the selected year, this calculator flags it so your data remains accurate.
Practical examples
Example 1: Reporting deadline
Your team says a report is due in “week 10, Friday.” Enter ISO year, week 10, and weekday 5 to see the exact due date.
Example 2: Historical analysis
You have a transaction timestamp and need to group by ISO week for trend analysis. Convert each date to ISO year + ISO week for consistent aggregation.
Example 3: Cross-border project scheduling
When teams in different countries collaborate, ISO week numbers avoid local convention differences (for example, whether Sunday or Monday starts the week).
Tips for avoiding week-number errors
- Always store both ISO year and ISO week, not just week number alone.
- Confirm your BI tools and spreadsheets are configured for ISO 8601 weeks.
- Be extra careful around late December and early January.
- Use automated calculation instead of manual counting.
Frequently asked questions
Is ISO week the same as US week numbering?
Not always. Many US calendars start weeks on Sunday and define week 1 differently. ISO weeks always start Monday and follow ISO 8601 rules.
Can January 1 be in week 52 or 53?
Yes. If January 1 falls before ISO week 1 starts, it can belong to the final ISO week of the previous ISO year.
Why include ISO weekday?
Week number alone identifies a week, but weekday identifies a specific date within that week. ISO weekday 1–7 gives a universal mapping from Monday to Sunday.
Use this ISO week calculator anytime you need a fast, reliable week number calculator for planning, analytics, and scheduling. It keeps your dates aligned with the ISO 8601 standard so your weekly data stays consistent and trustworthy.