Week Number
Payroll cycles, manufacturing reports, academic timetables, and sprint planning often reference week numbers instead of month names-especially in Europe where ISO week dating is standard. This week number calculator takes any calendar date and returns its ISO 8601 week number along with the Monday-start week boundaries that define that week.
ISO rules can surprise you: 1 January sometimes belongs to week 52 or 53 of the previous year, and late December dates may already be week 1 of the next year. Rather than memorizing those edge cases, pick a date and read the authoritative week index plus the exact start and end dates of that week. The detail panel helps when you need to align spreadsheets, Gantt charts, or API filters that expect yyyy-Www formatting. All lookups run in your browser without logging your schedule. Copy the week label into filenames that must sort chronologically across year boundaries in shared drives.
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.
What is the Week Number?
Payroll cycles, manufacturing reports, academic timetables, and sprint planning often reference week numbers instead of month names-especially in Europe where ISO week dating is standard. Run it free in your browser on TimerClock.
When should you use the Week Number?
Open this page when you need Payroll and timesheet week alignment, Sprint and agile reporting by week index, or Supply chain and production weekly KPIs. If that is not your goal, try a related tool linked below.
Troubleshooting the Week Number
- No sound: Allow audio for this site, unmute the tab, and raise system volume. Some browsers block sound until you interact with the page.
- Timer seems paused in another tab: Keep the Week Number tab visible for critical sessions, or install TimerClock as a PWA for better background behavior.
- Wrong time shown: Check the device clock and timezone. World and countdown tools also depend on the target zone you selected.
- Settings disappeared: Preferences are stored locally in this browser. Clearing site data removes saved alarms, themes, and recent durations.
About the Week Number page
Written and updated by the TimerClock team. The live tool stays at the top; the notes below cover setup limits, sound issues, and related pages. Timing uses your device clock. Preferences stay in this browser. See the privacy policy, terms, and contact page if you need help.
Related reading for the Week Number
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.