About the Week Number Calculator
Tezmy's Week Number Calculator tells you which week of the year a date falls in and which dates a numbered week covers. It uses the ISO 8601 rule that most of Europe, Asia and many businesses use for “calendar week” or “KW”: weeks start on Monday and week 1 is the week containing the year's first Thursday. It also shows the US-style week number (weeks start on Sunday, week 1 contains 1 January), the day of the year, days left, the quarter and whether the year has 52 or 53 weeks. Pick a date to see its week, or enter a year and week number to see the Monday and Sunday it covers. Nothing is sent anywhere.
Updated September 2026
How to use the Week Number Calculator
- 1Choose “Week of a date” or “Dates of a week”.
- 2For a date, pick it from the calendar box or press Today.
- 3For a week, type the year and the week number.
- 4Read the week number, the Monday-to-Sunday range and the extra details.
Where week numbers are used
Many companies plan by calendar week: production schedules, delivery dates, payroll cycles, school timetables and project sprints are often written as “KW 40” or “week 40”. Because a week-based label is short and unambiguous, it is common in supply chains and in software release planning, and calendar apps often let you switch week numbers on.
The trouble is that not every country counts the same way, so “week 1” can mean different weeks. This calculator shows both ISO and US numbers so you can match whichever your organisation uses. Weeks are usually part of a larger plan. Use the Date Calculator to add or subtract time, Days Between Dates to count the gap between two weeks, the Working Days Calculator to skip weekends, the Age Calculator for birthdays, and the Countdown Timer for a deadline.
The rules in short
- ISO 8601: weeks start on Monday; week 1 contains the first Thursday of the year; a year has 52 or 53 weeks.
- US (Sunday start): weeks start on Sunday; week 1 is the week that holds 1 January; the last week may be short.
- Day of the year: 1 January is day 1, and 31 December is day 365 or 366 in a leap year.
- Leap years: divisible by 4, except century years that are not divisible by 400, so 2000 was a leap year and 1900 was not.
Tips for using week numbers
- When you share a week number across borders, write the year and the standard with it, for example 2026-W40 for ISO week 40 of 2026.
- Around New Year, check the week's year as well as its number. A date in late December may be week 1 of the next year.
- To plan a deadline by week, look up the Monday-to-Sunday range and pick the exact day.
- If your calendar app shows different numbers from these, it is probably set to a Sunday-start or 1 January-start rule. Compare it with the US week result.
Features
- ISO 8601 week number with the week's year
- Monday-to-Sunday dates for any week
- US week number for Sunday-start calendars
- Day of year, days left, quarter and weeks in the year
- Knows which years have 53 weeks
- Clear messages for impossible dates and weeks
Frequently asked questions
What week number is it today?
Open the tool and press Today. The calculator reads your device's date and shows the ISO week number, the dates the week covers and more. The page starts with today's date filled in.
How is the ISO week number defined?
Weeks run from Monday to Sunday and week 1 is the week that contains the first Thursday of the year, which is the same as the week containing 4 January. Every week belongs to the year that holds its Thursday.
Why does 1 January sometimes fall in week 52 or 53?
If 1 January is a Friday, Saturday or Sunday, its week has its Thursday in the previous year, so it counts as the last week of that year. For example, Sunday 3 January 2021 is in week 53 of 2020. Likewise 29 to 31 December can be week 1 of the next year.
Which years have 53 weeks?
Years that start on a Thursday, and leap years that start on a Wednesday. Recent examples are 2015, 2020 and 2026. The tool shows the number of weeks for the year of any date you enter.
What is the difference between ISO and US week numbers?
US numbering starts weeks on Sunday and calls the week containing 1 January week 1, even if it has only one day in the new year. ISO numbering starts on Monday and needs at least four days of the week in the new year. The two often differ by one, and US weeks can reach 53 in years that ISO counts as 52.
Can I convert a week number to a date?
Yes. Choose “Dates of a week”, enter the year and week number and you get the Monday and Sunday of that week. If the year has only 52 weeks, asking for week 53 gives an error.