Week Number Calculator – What Week Is It? (ISO Week Numbers)
Week Number Calculator
Quick answer
A week number tells you which week of the year a date falls in. Under the ISO 8601 standard, weeks start on Monday and week 1 is the week that contains the first Thursday of the year. 6 October 2026 is in ISO week 41, which runs from Monday 5 October to Sunday 11 October. 2026 has 53 ISO weeks.
Our free week number calculator shows the current week number, finds the week of any date and turns a week number back into its dates. It is useful for payroll, school timetables, shipping schedules, sprint planning and any calendar that is organised by week.
How ISO Week Numbers Work
Most of Europe and Asia, and most business software, use ISO 8601 week numbers. Three rules decide the number:
- Weeks start on Monday and end on Sunday.
- Week 1 is the week that contains the first Thursday of January. In other words, it is the first week with at least four days in the new year.
- A year has 52 or 53 weeks. It has 53 when it starts on a Thursday, or on a Wednesday in a leap year.
Because of these rules, the first or last few days of January and December can belong to a week of the neighbouring year. 1 January 2027 is a Friday, so it is still part of ISO week 53 of 2026.
| Date | ISO week | US week | Note |
|---|---|---|---|
| 1 Jan 2026 (Thu) | Week 1 of 2026 | Week 1 | Year starts on Thursday |
| 6 Oct 2026 (Tue) | Week 41 | Week 41 | Mon 5 Oct to Sun 11 Oct (ISO) |
| 31 Dec 2026 (Thu) | Week 53 of 2026 | Week 53 | 2026 has 53 ISO weeks |
| 1 Jan 2027 (Fri) | Week 53 of 2026 | Week 1 of 2027 | ISO and US systems differ |
| 4 Jan 2027 (Mon) | Week 1 of 2027 | Week 2 | First ISO week of 2027 |
How to Use the Week Number Calculator
Step 1: Pick a Date and Numbering System
Open the week number calculator and choose a date. It starts with today. Keep ISO 8601 for Monday-based weeks, or choose US if your calendar starts weeks on Sunday and counts the week with 1 January as week 1.
Step 2: Read the Week Number and Its Dates
The result shows the week number, for example Week 41 for 6 October 2026, with the full range from Monday 5 October to Sunday 11 October. The strip below lists all seven days, with your date highlighted.
Step 3: Turn a Week Number into Dates
Switch to Week number to dates, type a year and an ISO week number, and the calculator shows the Monday to Sunday dates. Week 53 of 2026 runs from 28 December 2026 to 3 January 2027.
Step 4: Move Between Weeks
The cards show the day of the year (279 for 6 October), the days left in the year, the number of ISO weeks in that year and the quarter. Use Previous week and Next week to step through the calendar, or This week to jump back to today.
ISO vs US Week Numbers
In the US system, weeks start on Sunday and week 1 is simply the week that contains 1 January, so it can have only one or two days. That is why the two systems sometimes give different numbers near the start and end of the year. In Excel, ISOWEEKNUM gives the ISO week, while WEEKNUM gives the US-style week by default.
Where Week Numbers Are Used
- Business and logistics: delivery dates, production plans and invoices are often written as week numbers, such as W41.
- Payroll and rotas: shift patterns and pay periods that repeat every week or two weeks.
- Agile sprints: two-week sprints are easy to label by week number.
- Schools: term planners and timetables that alternate between week A and week B.
Week Number Tips
- Write week numbers with the year, for example 2026-W41, so there is no confusion around New Year.
- When you share a week number with someone in another country, say whether it is ISO or US.
- Remember that some years have a week 53. 2026 does, while 2027 has 52 weeks.
Frequently Asked Questions
What week number is it today?
Open the calculator and it shows the current week straight away. For example, 6 October 2026 is ISO week 41.
How are ISO week numbers calculated?
Weeks run Monday to Sunday, and week 1 is the week that contains the first Thursday of the year. The calculator applies this rule for you.
How many weeks are in 2026?
2026 has 53 ISO weeks because it starts on a Thursday. Week 53 runs from 28 December 2026 to 3 January 2027.
Why is 1 January sometimes in week 52 or 53?
If 1 January falls on a Friday, Saturday or Sunday, those days belong to the last ISO week of the previous year.
What is the difference between ISO and US week numbers?
ISO weeks start on Monday and need four days in the new year to be week 1. US weeks start on Sunday and week 1 is the week with 1 January.
How do I find the dates of a week number?
Choose Week number to dates, enter the year and the week number, and the calculator lists the Monday to Sunday dates.
Is the week number calculator free?
Yes. It is free, needs no sign-up and runs in your browser.