🗓️ Week Number Calculator
Enter a date to find its ISO 8601 week number (weeks start on Monday; week 1 is the week containing the year's first Thursday).
The Thursday rule
ISO 8601 weeks run Monday to Sunday, and a week belongs to whichever year contains its Thursday. The calculator applies that literally: it steps from your date to the Thursday of the same week, takes that Thursday's year as the week-year, then counts weeks from 1 January of it.
thursday = date + (4 - weekday) weekday: Mon=1 ... Sun=7
week = ceil(((thursday - 1 Jan of its year) / 1 day + 1) / 7)8 September 2026 is a Tuesday, its Thursday is 10 September, and the answer is week 37 of 2026. The arithmetic runs in UTC internally, so a daylight saving change cannot push a date into the wrong week.
When the week year is not the calendar year
- 1 January 2027 is a Friday, so its week's Thursday falls on 31 December 2026 - the tool reports week 53 of 2026, not week 1 of 2027. Week 1 of 2027 begins on Monday 4 January.
- That is why the ISO week year is shown separately - a week number alone is ambiguous at every year end.
- Most years have 52 weeks; a year gets 53 when it starts on a Thursday, or is a leap year starting on a Wednesday. 2026 starts on a Thursday, so it has 53 weeks.
Frequently asked questions
What week number is it today?
The date box opens on today, so the answer is on screen at load. 8 September 2026 falls in week 37.
Can a year have 53 weeks?
Yes, and 2026 does. It happens when the year starts on a Thursday, or on a Wednesday in a leap year, roughly every five to six years.
Why is 1 January sometimes in week 52 or 53 of the previous year?
Because the week keeps the year of its Thursday. If 1 January falls on a Friday, Saturday or Sunday, the whole week counts as the last week of the year before.