How to use Week Number
- The week number for today shows straight away.
- Pick another date to see its week number, the dates the week runs between and the day of the year.
- Read whether the year has 52 or 53 ISO weeks.
- Copy or share the week.
Why use Week Number
ISO weeks
Weeks start on Monday and week 1 holds the year's first Thursday, the rule used in planning, logistics and most of the world outside the US.
Week dates
The Monday and Sunday of the week, so a "week 37" deadline becomes real dates.
Day of the year
Day 270 of 365, with the days left in the year.
52 or 53
Years with 53 ISO weeks are flagged, since planners and payroll notice them.
About this tool
Week numbers run through project plans, delivery schedules, school terms, timesheets and factory calendars: "ship in week 42", "term ends week 51". The international rule (ISO 8601) starts weeks on Monday and calls the week with the year's first Thursday week 1, which means 1 January can fall in the last week of the previous year and some years have 53 weeks. Software such as Excel offers both this rule and a simpler one that starts on 1 January, and the two can differ by one.
This page shows the ISO week for today or any date, the Monday and Sunday it runs between, and the day of the year, which is handy for countdowns and for reading date stamps written as day numbers. Use it to turn a week number into dates, or dates into the week a plan refers to.
Frequently asked questions
What week number is it today?
The number at the top is today's ISO week, with the Monday and Sunday it runs between.
Why does my calendar show a different week number?
Some calendars (and Excel's WEEKNUM by default) start week 1 on 1 January and weeks on Sunday. The ISO rule used here starts weeks on Monday and counts the week with the first Thursday as week 1; the two can differ by one around the new year.
How many weeks are in a year?
52 in most years, 53 in years that start on a Thursday (or a Wednesday in a leap year), such as 2026.
What is the day of the year?
The day's number counted from 1 January: 1 March is day 60 (61 in a leap year). The result shows it with the days left.
Which day does a week start on?
ISO week numbers start on Monday, and week 1 is the week with the year's first Thursday. Some calendars in the US and the Middle East start weeks on Sunday or Saturday, which can shift the number.