Week Number Calculator
Find the current week number, convert any date to ISO 8601 or US week numbers, reverse-lookup the dates for a week number, and browse every week of the year with its start and end dates. Free, instant, and entirely in your browser.
Convert Date to Week Number
Find Dates from a Week Number
All Weeks in a Year
| Week # | ISO start (Mon) | ISO end (Sun) | US start (Sun) | US end (Sat) |
|---|
What does this Week Number Calculator do?
This free online week number calculator tells you the week number for any date, in both the ISO 8601 standard used across most of the world and the US week numbering system used in the United States and a few other countries. It also works in reverse: enter a year and week number, and it shows you the exact start and end dates of that week.
Week numbers are used everywhere — from project planning and payroll to school schedules and ISO-formatted business reports. But because different countries use different conventions for when a week starts and which week is "week 1," the same date can have different week numbers depending on where you are. This calculator shows both systems side by side, so there's no confusion.
Everything runs locally in your browser. No server, no signup, no tracking. Your dates never leave your device.
How to use the Week Number Calculator
- See the current week: The top banner shows the current ISO week number, the total weeks in the current year, and the start and end dates of the week you're in right now.
- Convert any date to a week number: Pick any date in the date field. The result grid shows the ISO week number, the US week number, the day of the year, and the day of the week — all instant.
- Compare ISO vs US: Below the results, two cards show how the same date maps to each system, with the exact start and end dates of the week in each convention.
- Reverse lookup: Enter a year and week number, choose ISO or US, and click "Find Dates" to get the start and end dates of that specific week.
- Browse the whole year: The table at the bottom lists every week of the selected year with its ISO and US start/end dates. The current week is highlighted automatically.
Why use this Week Number Calculator?
Instant, real-time results
Change the date and everything updates immediately — no button press needed for the main converter.
ISO and US systems
Shows both the ISO 8601 week number and the US week number side by side, so you never have to guess which convention you need.
Bidirectional
Convert date → week number and week number → date. Both directions are fully supported.
Full year week table
Browse every week of any year with start and end dates in both systems. The current week is highlighted.
100% private
Everything runs in your browser. No server, no logs, no tracking. Your dates never leave your device.
Works offline
Once the page loads, the calculator works without an internet connection. Perfect for travel or remote areas.
How ISO 8601 week numbers work
ISO 8601 is the international standard for date and time representation. Its week-numbering rules are used by most countries in Europe, Asia, and Oceania, as well as by many international organisations and software systems.
The three rules
- Weeks start on Monday and end on Sunday.
- Week 1 is the week that contains the first Thursday of January — equivalently, the week that contains January 4th.
- A year has 52 or 53 weeks. The 53rd week appears when January 1 falls on a Thursday (common year) or on a Wednesday (leap year).
Because of the Thursday rule, the first few days of January sometimes belong to week 52 or 53 of the previous year, and the last few days of December sometimes belong to week 1 of the next year. This is what makes ISO week numbers occasionally counter-intuitive — but it keeps weeks aligned with the calendar year in a way that US week numbers don't.
| Year type | Jan 1 falls on | Weeks in year | Example |
|---|---|---|---|
| Common year (365 days) | Thursday | 53 weeks | 2026, 2037, 2043 |
| Leap year (366 days) | Wednesday | 53 weeks | 2020, 2048, 2076 |
| All other years | Any other day | 52 weeks | 2024, 2025, 2027 |
ISO vs US week numbers: what's the difference?
The biggest difference between the two systems is which day a week starts on and which week counts as week 1.
| Feature | ISO 8601 | US (WEEKNUM) |
|---|---|---|
| Week starts on | Monday | Sunday |
| Week ends on | Sunday | Saturday |
| Week 1 contains | First Thursday of January (≈ Jan 4) | January 1 |
| Range of week numbers | 1–52 or 1–53 | 1–53 or 1–54 |
| Used by | Europe, Asia, Oceania, ISO-standard reports | United States, Canada, some Middle Eastern countries |
Because of these differences, the same date can have different week numbers in the two systems. For example, December 31, 2025, is week 1 of 2026 under ISO 8601 (because it falls in the same week as Jan 1, 2026, which is a Thursday), but it is week 53 of 2025 under the US system.
Common week number use cases
Business and project planning
- Scheduling weekly team meetings and sprint cycles
- Formatting ISO-standard reports and invoices that use week numbers
- Tracking deliverables against weekly targets
Payroll and HR
- Calculating weekly pay periods and overtime
- Planning annual leave and holidays by week number
- Aligning shift schedules across regions that use different week conventions
Education
- Planning semester timetables by academic week number
- Tracking assignment deadlines across a term
- Coordinating with international students or partner institutions
Personal planning
- Tracking New Year's resolutions by week
- Planning fitness, reading, or habit goals week by week
- Working out which week a holiday falls in for travel bookings
Why some years have 53 weeks
Most years have 52 ISO weeks, but some have 53. A year has 53 weeks when one of these is true:
- It is a common year (365 days) and January 1 falls on a Thursday.
- It is a leap year (366 days) and January 1 falls on a Wednesday.
This happens because 365 days is exactly 52 weeks plus 1 day, and 366 days is 52 weeks plus 2 days. Under the ISO Thursday rule, that extra day (or two) can push the year into a 53rd week when it lands at the right spot in the week.
The pattern of 53-week years repeats roughly every 5–6 years, but the exact spacing depends on leap year placement. Well-known 53-week years include 2020, 2026, 2032, 2037, and 2043.