Week Number Calculator
Check the current ISO week, find the week number for any date, convert a week number into its Monday-to-Sunday range, and browse every week in a year.
Current Answer
Week 00 / 00
0000-W00
The selected week range appears here when the page loads.
This page calculates ISO week numbers in your browser, so the answer stays current without a rebuild.
Useful Copy
Week details appear here when the page loads.
Time zone used for “today”: Your device time zone
ISO Week Year
0000
US Week Number
00
Day of Year
0 / 0
Quarter
Q1
Week Start
Monday, January 1, 0000
Week End
Sunday, January 7, 0000
Weeks In ISO Year
52
Weeks Remaining
0
Find Week Number for a Date
Choose any local date and get its ISO week, ISO week year, week code, and full week range instantly.
The selected date will also update the week table below.
Find Dates for a Week Number
Enter an ISO week year and week number to jump directly to that week’s Monday-to-Sunday date range.
The page validates whether the selected ISO year has 52 or 53 weeks.
All Weeks in 0000
The selected ISO week year summary appears here when the page loads.
Week 1 Starts
Monday, January 1, 0000
Last ISO Week
Week 52
January 1 ISO Code
0000-W00
December 31 ISO Code
0000-W00
| Week | Week Code | Monday | Sunday | Month Span | Quarter |
|---|
Click any row to load that week into the calculator above. ISO weeks always run from Monday to Sunday.
Why Week Numbers Matter
Week numbers are a compact way to communicate schedules, deadlines, reporting windows, sprint cycles, and delivery plans. Saying “ship in week 20” is often faster and less ambiguous than listing a date range, especially when multiple teams work across countries.
This page is ISO-first because ISO 8601 is the most common neutral standard for software, logistics, manufacturing, and international planning. The page also shows the US week number for comparison so you can catch boundary differences near January and December.
How ISO 8601 Works
ISO weeks start on Monday, and week 1 is the week that contains January 4. That rule keeps early-January and late-December dates attached to complete weeks rather than broken partial ones.
Because ISO uses a week year as well as a calendar year, January 1 can belong to the last ISO week of the previous year, and December 31 can belong to week 1 of the next ISO year. That is normal, and this page shows both the week number and the ISO week year clearly.
Frequently Asked Questions
What week number is it today?
The answer at the top of this page is calculated from your device’s current local date when the page loads, so it stays correct even though the site is statically hosted.
Why is the ISO week year sometimes different from the calendar year?
ISO weeks are full Monday-to-Sunday blocks. That means dates near New Year’s can belong to the previous or next ISO week year if that produces a complete week.
Why do some years have 53 weeks?
Most ISO week years have 52 weeks, but some alignments create a 53rd full ISO week. This page calculates the correct total automatically for the year you choose.
What is the difference between ISO and US week numbers?
ISO weeks start on Monday and use the “first week with at least four days” rule. US week numbering usually starts on Sunday and counts the week containing January 1 as week 1, so the two systems can differ near year boundaries.