Many businesses plan by week number, and the rules for numbering weeks differ between countries. This tool gives the ISO week number of any date (weeks start on Monday and week 1 contains 4 January), plus the day of the year, quarter and days remaining. Enter a year and week number instead and it gives you that week's Monday and Sunday.
How to use it
- Choose a date.
- Read the week number and other details.
- Or enter a year and a week to see its dates.
- Copy what you need.
When it helps
- Planning by week in business and logistics.
- Reading week numbers on schedules and calendars.
- Finding the dates of a given week.
- Checking what day of the year a date is.
Good to know
- Late December can belong to week 1 of the next year, and early January to week 52 or 53 of the last.
- If your calendar disagrees, it probably numbers weeks differently, as the US system does.
- Most years have 52 ISO weeks, some have 53.
The ISO week rules
ISO 8601 defines weeks as starting on Monday. Week 1 is the week with the year's first Thursday, which is the same as the week containing 4 January. The consequence is that a few days at the start of January can belong to week 52 or 53 of the previous year, and a few days at the end of December to week 1 of the next.
Most years have 52 weeks, and years in which 1 January falls on a Thursday, or on a Wednesday in a leap year, have 53.
Worked example: planning by week
Enter 13 October 2026 and the tool says week 42. Planning "week 42" in a spreadsheet is easier than juggling dates. Enter year 2026 and week 1 instead and you get Monday 29 December 2025 to Sunday 4 January 2026, which shows that week 1 starts in the previous year.
That explains why some calendars show week numbers that don't match your own: they use different rules.
Different systems around the world
The United States commonly counts weeks from Sunday, and week 1 contains 1 January. Some countries in the Middle East start the week on Saturday. Retail calendars such as the 4-4-5 pattern divide the year into fixed blocks.
When you share week numbers with other organisations, state which system you use, or give the dates as well.
Why week numbers are popular
Week numbers are short, unambiguous and easy to compare. Manufacturers use them in production codes, schools use them in term plans, and project managers use them to build timelines. A reference such as week 42 is clearer than a date range in a table.
They're also useful for recurring work: a task due every second week can be scheduled on even-numbered weeks.
Questions
What is week 1?
The week containing the first Thursday of the year, which is the same as the week containing 4 January.
Why does my calendar show another number?
It may use Sunday-start weeks.