Week Number Calculator
ISO 8601 week numbers, including the year-boundary cases.
Work out Week Number. ISO 8601 week numbers, including the year-boundary cases. States the assumption it rests on, rather than hiding it.
ISO week number
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How the Week Number Calculator works
The ISO 8601 week number for any date. Weeks start on Monday and week 1 is the one containing the first Thursday of the year, which produces the case that catches everyone out: the 1st of January 2021 belongs to week 53 of 2020, not week 1 of 2021. The ISO year is reported separately for that reason.
Also known as: what week is it · iso week number calculator · current week of the year · week of the year calculator
The calculation itself
ISO 8601 defines weeks as running Monday to Sunday, with week 1 being the week that contains the first Thursday of the year. Equivalently, and more usefully for computing it: the week containing the 4th of January.
The Thursday rule exists to make weeks indivisible. A week always belongs entirely to one year, which means no week is ever split across a year boundary and every week has exactly seven days in the year it belongs to.
The case that catches everyone
The 1st of January 2021 falls in ISO week 53 of 2020, not week 1 of 2021. The 30th of December 2019 falls in week 1 of 2020.
This is why the ISO year is reported as its own figure rather than assumed from the date. A scheduling system that files a 1 January date under its calendar year puts it in the wrong week entirely — an off-by-one that surfaces every few years and is genuinely hard to spot when it does.
The 2020 example is the clearest: that ISO year began on the 30th of December 2019 and ran 53 weeks.
52 or 53 weeks
A year has 53 ISO weeks when it begins on a Thursday, or on a Wednesday in a leap year. Otherwise it has 52.
The reason is that 52 weeks is 364 days, one short of a normal year and two short of a leap year. Those spare days accumulate, and roughly every five or six years they add up to a whole extra week.
Not everyone uses this
ISO weeks are standard across European business, payroll and manufacturing planning, where 'week 32' is a normal way to name a date range.
A common North American convention differs on both counts: weeks start on Sunday, and week 1 is whichever week contains the 1st of January. It can produce a different number for the same date, which is worth confirming before two systems exchange week numbers.
Frequently asked questions
What week number is it?
Enter a date above — it defaults to today. The result is the ISO 8601 week, which is the standard used across European business, payroll and manufacturing scheduling.
How is the ISO week defined?
Weeks run Monday to Sunday, and week 1 is the week containing the first Thursday of the January in question. Equivalently, it is the week containing the 4th of January.
Why does 1 January sometimes fall in week 52 or 53?
Because ISO weeks never split across years — a week belongs entirely to whichever year holds its Thursday. If January starts late in the week, those first days belong to the previous year's final week. The 1st of January 2021 was week 53 of 2020.
How many weeks are in a year?
52 in most years and 53 in some. A year has 53 ISO weeks when it starts on a Thursday, or on a Wednesday in a leap year. 2020 had 53.
What is the ISO year?
The year the week belongs to, which is not always the calendar year of the date. Reporting a 1 January date under its calendar year rather than its ISO year puts it in the wrong week entirely — a common source of off-by-one errors in scheduling systems.
Does the US use ISO week numbers?
Often not. A common North American convention starts weeks on Sunday and counts the week containing 1 January as week 1, which gives different numbers. This page computes the ISO standard.
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