Week Number Calculator
Pick a date to see which week and day of the year it falls on.
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Production plans, delivery schedules and much European correspondence refer to weeks by number — “week 38”. The numbering follows the international ISO 8601 standard.
Under that standard weeks start on Monday, and week 1 is the week containing the year’s first Thursday. That is why the first days of January sometimes belong to week 52 or 53 of the previous year.
How to calculate Week number
- Choose a date; today is filled in for you.
- Read the week number and the Monday–Sunday dates of that week.
- See the day of the year and how much of the year is left.
Week number formula
Week number = whole part of ((day-of-year of that week’s Thursday − 1) ÷ 7) + 1Week 1 = the week containing the first Thursday of the year
Week number example
Sunday, 4 October 2026 is in week 40; the week starts on Monday, 28 September. It is day 277 of the year with 88 days left. The year 2026 has 53 weeks.
Week number: common questions
How many weeks are in a year?
Most years have 52. Years that start on a Thursday, and leap years that start on a Wednesday, have 53.
Why is the week number different in the US?
In the US weeks start on Sunday and the week containing 1 January is week 1, so the number can be one off from ISO.
How do I count the weeks between two dates?
Use the days between dates calculator; it shows the result in weeks and days as well.