Week Number Calculator
Pick a date and get its ISO week number, the Monday-to-Sunday range of that week and the day of the year.
What this tool does
The ISO 8601 week number counts weeks starting on Monday, with week 1 being the week that contains the year's first Thursday. Pick a date and the calculator shows its week number, the dates the week runs from and to, the day of the year and the quarter. Early January and late December dates can belong to the neighbouring year's week.
Calculates the ISO week number and week-year of a date.
Shows the week's Monday and Sunday, the number of weeks in that year, the day of the year, days left and the quarter.
How to use it
- Choose a date (it starts on today).
- Read the week number at the top.
- Check the date range of that week.
- Use the day of the year and quarter if you need them.
Why use this calculator
Gives the ISO 8601 week number for any date, with the week's Monday-to-Sunday dates and the number of weeks in that year. Schedules, payroll, production plans and shipping often use week numbers, and the rules for week 1 and for the year boundary are easy to get wrong. Systems also disagree: ISO weeks start on Monday, while some countries number from Sunday.
Why we built it
We built the week number calculator to give the ISO 8601 week number for any date, with the Monday-to-Sunday dates of that week and the number of weeks in the year. The day of the week calculator gives the weekday, and the date difference calculator counts days between dates.
For a neighbouring job there is also the Year Planner and the Leap Year Calculator.
Handy alongside this: the Day of the Year Calculator.
Who this tool is for
- Planners and project managers using ISO weeks
- Developers and analysts needing week numbers
- Anyone reading a calendar that numbers its weeks
When to use it
- Project and production planning by week number.
- Reading a delivery date given as a week.
- Payroll and reporting periods.
- Converting a date into the week-based format used by a system.
How it works
ISO 8601: weeks run Monday to Sunday and week 1 is the week with the first Thursday of the year (equivalently, the week containing 4 January). A year has 52 or 53 weeks - 53 when it begins on a Thursday, or on a Wednesday in a leap year.
Example: 1 January 2021 is a Friday, so it belongs to week 53 of 2020; 30 December 2024 is a Monday in week 1 of 2025.
The US calendar often numbers weeks from Sunday with week 1 containing 1 January. That system gives different numbers for some dates.
A worked example
These are the values the calculator opens with, so you can see exactly what it does with them. Change any value on the page and the result updates; the figures below come from the same calculation.
| Input | Value |
|---|---|
| Date | 2026-03-15 |
With those inputs, the calculator returns:
- ISO week number: Week 11
- Week runs: Monday, 9 March 2026 to Sunday, 15 March 2026
- Weeks in that year: 53
- Day of the year: 74 of 365
- Days left in the year: 291
- Quarter: Q1
What you give it, and what you get back
| Item | Details |
|---|---|
| Date | Any calendar date. |
| Week number | The ISO week, with the year if it differs from the calendar year. |
| Week dates | The Monday and the Sunday of that week. |
| Extras | Weeks in the year, day of the year, days left and the quarter. |
Issues you may run into
| What you see | Why it happens | What to do |
|---|---|---|
| Week number differs from my calendar app | The app uses a Sunday-start or US numbering. | Compare with an ISO 8601 setting. |
| January date shows week 52 or 53 | It belongs to the previous year's last week. | This is correct under ISO 8601. |
| December date shows week 1 | It belongs to next year's first week. | This is correct under ISO 8601. |
| Year looks different | The week-year differs from the calendar year. | Use the "of YYYY" part shown. |
Assumptions and limitations
- ISO 8601 numbering only.
- Gregorian calendar dates only.
- Single date at a time.
Frequently asked questions
What week number is it?
Open this page: the date starts on today, and the week number is shown at the top.
How is the ISO week number defined?
Weeks start on Monday and week 1 is the one containing the year's first Thursday.
How many weeks are in a year?
Usually 52; some years have 53, as the calculator shows for the year you choose.
Why is 1 January in week 52 or 53?
If the first Thursday is later in January, the first days belong to the last week of the previous year.
Is the US week number the same?
Not always. US convention often starts weeks on Sunday and counts the week with 1 January as week 1.
Is it free and private?
Yes. It runs in your browser.