Week Number
Find the ISO week number for any date with week start, end, and year context. Free week number calculator for planning, payroll, and reporting.
How to Use
- Select the date you want to evaluate-past, present, or future.
- Click Get week number to run the ISO week lookup.
- Read the week number and ISO week-year in the result.
- Review week start (Monday) and week end (Sunday) in the detail section.
- Use Copy result when documenting reports or tickets.
Input Field Guide
- Date
- Any valid calendar date. The tool computes which ISO week contains that day.
Understanding Your Results
The primary result is the ISO week number (1-53) and the ISO week-year, which may differ from the calendar year near January and December boundaries. Detail chips list the Monday that opens the week and the Sunday that closes it, matching ISO-8601 week dating used in business and government reporting.
Features
- ISO 8601 week number with correct week-year
- Monday-through-Sunday week boundary display
- Handles week 53 and year-boundary edge cases
- Instant lookup from any calendar date
- Copy-friendly output
- Private in-browser calculation
Formula & Calculation Logic
ISO week: find the Thursday of the date week; its calendar year is the ISO week-year. Week 1 is the week containing the first Thursday of that ISO year. Week starts Monday.
Real-World Use Cases
- Payroll and timesheet week alignment
- Sprint and agile reporting by week index
- Supply chain and production weekly KPIs
- Academic week-of-term scheduling
- API and database filters using yyyy-Www
Practical Examples
13 Jul 2026 -> ISO week 29 of 2026 with Monday 6 Jul through Sunday 12 Jul boundaries shown.
1 Jan 2026 -> may map to week 1 of 2026 or week 52/53 of 2025 depending on weekday-tool shows the correct ISO pair.
Benefits
- Eliminates manual ISO week tables
- Clarifies week-year vs calendar-year near holidays
- Fast verification before filing weekly reports
- No signup required
Tips & Best Practices
- Use Monday start dates when building weekly reports to match ISO boundaries.
- Cross-check December dates-calendar year labels can mislead.
- Pair with the calendar view to browse months with week numbers visible.
Common Mistakes to Avoid
- Assuming calendar year equals ISO week-year in early January.
- Using Sunday-start weeks while reporting ISO numbers.
- Confusing US week-of-year conventions with ISO week numbering.
Frequently Asked Questions
What standard does the week number calculator use?
ISO 8601: weeks start Monday, week 1 contains the first Thursday of the ISO week-year.
Why can January dates show the previous ISO year?
Days before the first ISO week of the year belong to week 52 or 53 of the prior ISO week-year.
Does week 53 exist every year?
No. Week 53 appears only in ISO years where Thursday-based counting yields 53 weeks.
What are the week start and end dates?
ISO weeks run Monday 00:00 through Sunday end-of-day in local calendar terms.
Is this the US week-of-year system?
No. US locale week-of-year can differ. This tool follows ISO 8601.
Can I find the week number for a future date?
Yes. Any valid calendar date works.
How does this relate to the calendar tool?
The calendar can overlay ISO week numbers on a month grid; this tool gives a precise lookup for one date.
Is week data sent to a server?
No. Lookups run locally in your browser.