Week Number Calculator
Which ISO week a date falls in — and which dates make up a given week.
Look up a week
Week numbers by year
How it works
The week number follows ISO 8601, the standard most of the world and nearly all software uses. Weeks begin on Monday, and week 1 is the week containing the first Thursday of the year — equivalently, the week containing 4 January. The Thursday rule exists so that a week is never split between two years: whichever year holds most of the week owns all of it.
That has a consequence people find surprising. If 1 January falls on a Friday, Saturday or Sunday, it belongs to the last week of the previous year, so early January can be week 52 or 53. Equally, late December can already be week 1 of the year ahead.
Most years have 52 weeks; a year has 53 when it starts on a Thursday, or is a leap year starting on a Wednesday. Note that North American business and broadcast calendars often number weeks from Sunday instead, which does not match ISO and will disagree by one in some years.
Examples
Finding the week for a date. Enter any date and the tool gives the ISO week, the year that week belongs to, and the Monday-to-Sunday range it covers.
Reconciling with a colleague. If someone says "week 1" and means the week containing 1 January, they may be a week out from ISO. Comparing the actual date ranges settles it faster than comparing numbers.
Planning a 53-week year. Payroll and retail calendars built on 52 weeks need an explicit plan for the years that have 53, and the tool shows which those are.
Practical notes
The commonest source of confusion is that "week 1" means different things in different places. ISO 8601 defines it as the week containing the first Thursday of the year; several North American business and broadcast calendars simply call the week containing 1 January week 1. In some years those differ by one, which is enough to misalign a whole reporting calendar.
Because ISO weeks never split across years, the week number carries its own year. Late December can be week 1 of the following year, and 1 January can be week 52 or 53 of the previous one. Always quote the two together.
Years with 53 weeks arrive irregularly, and any plan built on a fixed 52 needs an explicit answer for them. Retail and payroll calendars are where this usually surfaces.
Frequently asked questions
What week is it now?
The form opens on today’s date, so the first result card is the current ISO week.
Why does 1 January sometimes fall in week 53?
Because ISO weeks never split: if 1 January is a Friday, Saturday or Sunday, that week has more days in the outgoing year and belongs to it.
Which years have 53 weeks?
Years that start on a Thursday, and leap years that start on a Wednesday. The result card reports the count for the year you enter.
Why is 1 January sometimes in week 52 of last year?
Because ISO weeks are never split across years. A week belongs to the year containing its Thursday, so a January that starts late in the week is counted with the outgoing year.
Do all countries number weeks the same way?
No. ISO 8601 starts weeks on Monday and is standard across Europe and in most software. Several North American conventions start on Sunday and number differently, so always confirm which is meant.
Can a year have 53 weeks?
Yes, when the year starts on a Thursday, or is a leap year starting on a Wednesday. Any plan built on a fixed 52 needs an answer for those years.
Do weeks start on Sunday anywhere?
Commonly, in the United States, Canada and Japan among others. ISO weeks start on Monday, so the two conventions disagree in some years.
Which week is a given date in?
The ISO week that contains its Thursday. Weeks run Monday to Sunday and week 1 is the week with the year’s first Thursday, so 1 January can fall in week 53 of the previous year and 29 to 31 December in week 1 of the next. Enter a date above to see its week, its week-numbering year and the Monday-to-Sunday range.
What week of the year is Christmas in?
Usually week 52, occasionally week 51 or 53 depending on which weekday 25 December falls on and how the year began.
Do all industries use ISO weeks?
No. Retail and broadcast calendars often use their own week structures with different start days and year boundaries, so confirm which is meant before reconciling numbers.