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Week Number Calculator
Use this week number calculator to find the ISO week number of any date, along with the Monday that starts the week and the Sunday that ends it. Pick a date and the tool shows its week of the year, the full Monday-to-Sunday range, and the day of the year. You can also work in reverse: enter a year and a week number to see the exact dates of that week.
Weeks follow the ISO 8601 standard: every week starts on Monday, and week 1 is the week containing the first Thursday of the year.
How it works
Week numbers sound simple — just count the weeks — but the edges of the year make it tricky. A year is 365 days, which is 52 weeks plus one day, so the first and last days of a year rarely fall neatly on a Monday. The ISO 8601 standard solves this with two rules: every week starts on Monday and ends on Sunday, and week 1 of a year is the week that contains the first Thursday of that year. Since January 4 always falls in week 1, you can also remember it as the week that contains January 4.
Finding a date's week number starts by shifting the date to the Thursday of its week. Because Thursday is the middle of the Monday-to-Sunday week, the Thursday tells you which year the week belongs to — and that matters, because the first days of January can belong to the previous year's last week. Once the Thursday is found, the tool counts how many weeks have passed since the Thursday of week 1 of that year, and adds one. That count is the ISO week number.
The Monday-to-Sunday range comes from the weekday of your date. Monday is day zero in the ISO system, so the tool subtracts the number of days since Monday to find the week's start, then adds six days to find the Sunday. The same trick works in reverse: to find the dates of a given week number, the tool finds the Monday of week 1 — the Monday of the week containing January 4 — and adds seven days for each week after the first.
Long years with 53 weeks happen when the year starts on a Thursday, or when a leap year starts on a Wednesday. In those years the extra day pushes a 53rd week into the calendar. 2020 and 2026 are both 53-week years, while 2025 has 52. If you enter week 53 for a 52-week year, the tool will tell you that week does not exist in that year rather than showing a wrong date.
ISO weeks are the standard for business planning, payroll, school timetables, and project schedules, because every week is a full seven days and the numbering lines up from year to year. A simple "week 1 starts on January 1" count is easier to explain but leaves a short first week and drifts away from Mondays, which is why it is rarely used for real planning.
Worked example
What ISO week is Sunday, November 8, 2026, and what are the dates of that week?
Step 1: Find the Thursday of the week containing November 8, 2026. November 8 is a Sunday, and the Thursday of the same Monday-to-Sunday week is November 5, 2026.
Step 2: Find the Thursday of week 1 of 2026. January 4, 2026 is a Sunday, so the week containing it runs from Monday, December 29, 2025 to Sunday, January 4, 2026, and its Thursday is January 1, 2026.
Step 3: Count the weeks between those two Thursdays. From January 1 to November 5 is 308 days, which is exactly 44 weeks. Add one, and the week number is 45.
Step 4: Find the Monday of the week. November 8 is a Sunday, six days after Monday, so the Monday is November 8 minus 6 days: Monday, November 2, 2026. The Sunday is November 8 itself.
Step 5: Check the day of the year. November 8, 2026 is day 312 of 365.
Try it yourself: pick November 8, 2026 in the date box and press Calculate. The tool gives exactly the same answer — ISO week 45 of 2026, Monday, November 2, 2026 to Sunday, November 8, 2026 — and the calendar strip highlights Sunday the 8th as the last day of the week.
FAQ
What is an ISO week number?
An ISO week number counts weeks by the ISO 8601 standard: every week starts on Monday, and week 1 of a year is the week that contains the first Thursday of the year. That means week 1 is the week with January 4 in it. A year has 52 ISO weeks, except for long years with 53 weeks, which happen when January 1 is a Thursday, or in a leap year when January 1 is a Wednesday.
Why is January 1 sometimes in week 52 or 53 of the previous year?
Because ISO week 1 must contain the first Thursday of the new year. If January 1 falls on a Friday, Saturday, or Sunday, it belongs to the last week of the previous year instead. For example, January 1, 2023 was a Sunday, so it was part of ISO week 52 of 2022, and week 1 of 2023 started on Monday, January 2. This keeps every ISO week at a full seven days, Monday to Sunday.
Does every year have 53 weeks?
No. Most years have 52 ISO weeks, and a 53-week year comes around roughly every five or six years. A year has 53 ISO weeks when it starts on a Thursday, or when it is a leap year that starts on a Wednesday. Examples of 53-week years are 2020 and 2026, while 2025 has only 52 weeks.
What is the difference between ISO weeks and calendar weeks?
An ISO week always runs Monday to Sunday and always has seven days, so the year divides neatly into numbered weeks. A simple calendar week count just divides the year into chunks of seven days starting on January 1, which leaves a short first week in most years and does not line up with Mondays. Business planning, payroll, and school timetables usually use ISO weeks because the numbering is consistent from year to year.
How do I find the dates of a given week number?
Switch this calculator to year plus week number mode, enter the year and the week number, and it shows the Monday to Sunday date range for that week. Week 1 is the week containing January 4, so you can also count forward from there: the Monday of week 1 is the Monday of the week that holds January 4, and each later week starts seven days after the previous one.